Category: TUBE AMPS

The Big 3!

#TUBEAMPS #Marshall #EL34 #Fender #6L6GC #Vox #AC30 #EL84 #COMBOs

Fender Prosonic Re-Revisited: Reunited

And it feels so good!

My never ending quest for that Fender ‘sparkle’ may just have ended. For the Fender Prosonic, anyway.

After a couple of years of mixing and matching speakers, i discovered the best speakers to make the Prosonic sound like a TRUE Fender amp were the 25W Greenbacks in tandem.  I’m quite aware that it’s underpowered compared to the amp, but man if you got it, you gotta get it. For me it’s all about tone. I hardly crank the amp so there’s hardly any fear of blowing the Greenbacks.

Now by Fender clean sparkle i mean the bell-like tones you get, especially when you play a Strat in positions 2 and 4. I’ve heard this distinct Strat tone on a Bassman, and on a Twin Reverb, hell even on a Princeton and a Champ.

I knew the Prosonic can sound like a true Fender amp, and the Celestion Greenbacks simply has it. It disproves the critics saying the amp is too hot to be a Fender. It still is hot with the Greenbacks, but it finally has the identifiable clean Fender sound.

But this configuration also has a disadvantage- there are no cases for a 2×12 Fender Prosonic. You can have it custom made, but it’s expensive. So i re-housed the chassis in an empty Twin Reverb case. Worked well, but its not aesthetically pleasing. The gap on either ends of the front panel bothered me.

The original case held 2×10 ‘vintage 30 flavored’ speakers, but i was able to make a baffle that held a 1×12 speaker. I originally had a WGS Veteran 30 in it and it sounded good, but it lacked that Fender clean sound. A single Greenback is way underpowered (25W vs the Prosonic’s 60W). Even set to 30W the amp is still too much for the Greenback. I needed a beefier 1×12 speaker that sounds like the  2 greenbacks, but does it exist?

Yes. Yes, it does.
And it comes in the form of the 75W G12H-75 Creamback.

I FINALLY can stop the nagging feeling about the amp not looking presentable. It’s superficial, but having a mismatched chassis and case just bugged the heck out of me. I could have saved myself a lot of trouble and time if i had just simply studied and compared the Frequency Response of the 25W Greenback G12M versus the other 12″ Celestions.

Courtesy of Celestion’s website, the charts below show the 75W Creamback’s frequency response approaching 20KHz held at above 70dB, meaning there’s more emphasis on those frequencies and they’re essentially louder compared to the Greenback which slopes downward below 70dB:

Greenback G12M
G12MGreenback

Creamback G12H-75G12H-75CREAMBACK

superimposed (Greenback is appropriately in GREEN):
G12H75-G12M-merged

From around 100Hz is when the frequencies coincide, but you can see the higher frequencies on the 75W Creamback stays above 70dB, in contrast to the Greenback. This would explain the initial icepick-y impression i had, coming from a greenback setting (read on for my fix for it below).

I had the G12m-65 Creamback before, and though it sounded normal, it didn’t have the sweet Fender clean sparkle. Check out the difference between that and the 75W Creamback (65W in Blue):

G12H75-G12M-65-merged
The most important factor were the high frequencies. You can see the drop off of the 65W just before 20KHz, while the 75W retains it above 70dB.

You can also see a drop in the original Greenback, but not as drastic (compared to the 65W Creamback):

G12m-G12M-65-merged
Though the Greenback and the 65W Creamback almost looks identical, that dissimilar high frequencies emphasis  made all the difference in getting the Fender clean tone.

Just a disclaimer, its not just the speaker change that made this Prosonic finally sound like it’s venerated Fender tube amp ancestors (the Prosonic was made in the mid 90s). Stock, the Prosonic is a hot-lava-spewing high-gain beast. I had to experiment with tube swaps in the preamp and the amp stages to raise the headroom for it, as well as a couple of tried-and-true well-publicized (online) mods coming from the original Prosonic designer himself Bruce Zinky. But if there was just one change that could have the most effect on it, it was the speaker change.

Also, when it comes to the dirty tones, the 75W Creamback seems to have a much tighter response than the Greenback. There’s more boomy bottom end on the greenbacks (possibly because there are 2 of them?), and sounds more loose, almost a sponge-y feel to it. Though the tight response may also be due to the Creamback being brand new and hardly broken in yet. That the creamback already sounds great out of the box is a good indication it can only get better as it’s played on.

The highs are also more apparent on the Creamback (in high gain), which the graphs earlier confirms. On the same settings as the greenbacks, the Creamback is almost icepick-y, but this is easily tamed. Instead of rolling back on the Treble (i maxed it out), i roll back a bit on the Bass (was at 7 on the greenbacks, now at 4), and to my own surprise, by increasing the mids (sounds great between 3 and 7). Sounds counter-intuitive, though it does make sense to do this. Instead of subtracting the levels of higher frequencies, other frequencies are tweaked to achieve the balance that provides the most harmony in your ear.
Also, the Creamback is perceptibly louder on the same greenback settings. Either clean or on high gain, i found myself rolling back a bit on the volume to achieve the same dB level i had on the greenbacks.

Well, THE PROSONIC IS HOME. Looks as it should be, far less bulkier. Relatively small, it’s compact and heavy. Deceptive in sound as well, it looks and sounds like a quintessential Fender amp when played clean, but kick in to high gain, and you swear it’s NOT a Fender amp – it’s a hellacious fire-breathing monster!

Last thing i need to do now is to apply the Fender logo. It’s like pinning a medal on a rookie that struggled and prevailed and came out on top, more versatile than ever before. It’s now ready for the big leagues. It’s now earned the name of the badge itself.

2005 VOX AC30 CC2X

This is IT.
Save for the Prosonic, this amp has made every other amp i own (and still want to own) almost irrelevant. Not that i don’t want anymore amps (i still would like a Marshall!), but it makes me feel like i dont NEED them as much.
The AC30 pretty much rounds up the tube tone i personally have been looking for. There’s not one tube amp that can nail EVERY tube sound authentically. Each amp specializes itself to a particular sound and tone. Though the Prosonic grabs the top prize for unparalleled versatility (goes from Fender CLEAN to hot bluesy tone to scorching red hot shred machine), the AC30 CC2X takes the niche of the sweet clean chime and the un-imitatable Top Boost tone. Think about it – almost every amp manufactured these days either imitates or clones a Fender or a Marshall (which by the way, as every tube amp afficionado knows, Marshall amps originally imitated and cloned a Fender Bassman!). But not very many (if at all) clones an AC30. It is unique!

THE PURCHASE

ebay- it’s a dirty jungle! It’s a bane for sellers, but a boon for buyers. Being careful and knowing what you’re looking for helps in making an educated purchase.
I saw this AC30 with a Buy It Now price and a Best Offer, with free shipping. The description read that the seller “does not have the power cord to test it”. Which means IT DOES NOT WORK! I saw the blue AlNiCo speakers and thought, well, maybe i can use those speakers instead if the amp is completely dead- they do command a good price. Plus it looks clean, the grill cloth looks straight (most grill cloths are either sagging or out of place already with amps this old and used). I can also part away with the cab easily.

So unknown if it works or not, if its fixable or not, i went and got it. Took over a week to arrive, and man! did it look like it was going to be a disaster!
The box it came in was super beatup, tape all over, flaps not even p[roperly closed, some holes thats bleeding with (gasp!) peanut packaging!!! Damn you people who use peanuts for packaging. You can never take anything out of the box without spilling any peanut on the floor. So DAMN YOU TO HELL!!!
I didnt even bring it inside- i took pictures all around without touching the beatup package. I even took a video of me taking the amp out of the box.
Aside from the awful peanuts, the amp was also wrapped tightly with almost 2 inches of bubblewrap. Good! it wasnt all just peanuts. slowly i took the buble-wrapped amp out, but to no avail- i STILL had peanuts spilling out. DAMN YOU TO HELL, PEANUT PACKAGERS!!!
As soon as i got the amp out, i started to SLOWLY cut through the bubble wrap. It was tightly wrapped that it took sometime to completely remove all of it. and then i saw the amp in its entirety.

CLEAN AS CLEAN CAN BE!!! WOW! that 2-inch thick bubblewrap made up for the crummy packaging! HALLELUJAH!!!
I can’t see any breaks anywhere, no dents, no huge scratches, no tears, no structural damage at all. PRISTINE!!!
After cleaning up all the peanuts and packaging and bubblewrap in the trash (this is still outside, mind you), i hauled the monster up a flight of 17-step stairs to my apartment. backbreaking motherfucker!

THE INSPECTION
a much thorough inspection indeed confirmed that there were no breaks or dents or structural damage to the amp. It WAS missing the power cord (which is the same as what you have on a PC, which is easily acquirable. It also just means that the seller must’ve tested it,. saw it didnt work, and made a poor excuse of “unable to find a cord” for it. Whatever, it’s now in my possession).
The bell covers of the Blue AlNiCo celestions had dust on it, indicating its been around a while. It did have the footswitch to go with it, but also was missing the 1/4 stereo cable that connects it to the amp. no big deal, i have both missing cable and power cord.
Plugging in the amp, it did NOT light up at all when it was turned on. I opened the fuse holder, and it didnt look like it blew. So my first thoufght was the Power Transformer. Damn. That’s not cheap. Oh well- im handy with the solder, we may be able to fix this.

THE FIX
So i got the drill/driver and went to town opening up the back. Not the easiest chassis to take out- there were 6 MORE screws that help the chassis to the amp after the back was removed. and then it revealed the tubes (its like a peek of heaven! he he).
JJ EL84s, a Sovtek GZ34, and 2 JJ 12ax7 and 1 chinese-made 12ax7 preamp tubes. Not bad, but it can be better- if i do fix these there’s going to be some tube-rolling happening. Maybe. IF i even fix it.
After tooling around, it seems nothing was done, no noticeable changes, upgrades or mods done at all. GOOD! that means no one really did anything to it. It’s worse if someone did- you don’t know what they’ve done, which could have caused or even contributed to it not working.
I also found ANOTHER fuse that’s blown- right under the IEC receptacle (where you plug in the power cord to). I had to wait a few days to get the replacement fuse, as i didnt have the right value AND size for it. Online technical manual and schematics revealed what it needed.
Once the fuse came in, i replaced the blown one, and once i turned the power switch on, there was light!!! The tubes were glowing! HELLOOOO, NURSE! it was a fuse issue! Or so I thought.
Then as quick as my elation, the amp blew the new fuse. huh. Something is making it blow. I over-ordered a lot of fuse, with different ratisgs. Probably not the correct rating. Putting a higher rated fuse STILL blew it though. But with the chassis out and the tubes glowing, i saw the tube rectifier (GZ34) flash briefly just before it blew the fuse. AHA!
I quickly turned the amp off, waited for the tube to cool down, and pulled the Sovtek rectifier out. Let’s see if it is indeed just the tube or something else making it flash bright briefly and also blowing the fuse. I have another amp (the Epiphone Blues Custom 30) that uses an old stock U.S.-made GZ34 rectifier, so i pulled that off of it, and plugged it in the AC30. Put a new fuse, plugged it in, and turned it on.

This time it stayed on. YES!!! It was a bad tube rectifier. After a few minutes of warming up, i slowly turned the volume all the way up (to 11!) and can hear a slight unmistakeable overdriven amp hum. I plugged in my 12-string Gretsch, and KERRANG!!!!
WE HAVE SOUND! AND MAN, HOW SWEET IT IS!!!

I spent no time putting the chassis back in and the back panel on. I’m playing this now!

KNEE-BUCKLING AND ALL THAT GOOD STUFF
Wow. this beast is jaw dropping! I knew what i was gonna get, but i never knew how good it really sounds. I spent a good hour just turning knobs, fiddling with tones, finding out what does what, getting myself familiar with the amp. So much tonal variety, but so unique-sounding! Everything works! And the Vox chime – heaps and heaps of it! So much so that there is even a tone cut knob that tames it back, if you so desire. Overdriven, the Top Boost is so sweet you can almost taste it. mix both that and the Normal channel, and the grit just gets grittier. The reverb has a switch that makes it sound hauntingly cavernous, perfect for deep retro-surf sounds. And the tremolo is not just useable and passable- it adds a flavorful dimension to the already magnificent sound.

and all it took to fix it was a tube rectifier and a fuse change. This was one chance i took that could’ve turned into a nightmare waste of a purchase, instead became one of the best fixes ive ever done.

So now we have The Keepers: The Fender Prosonic, this Vox AC30 CC2X, the Fender Bassman Ten, the Plush P1000-s, The Kalamazoo line (Reverb 12, Model One, Model Two, Bass 50), The Peavey Windsor 100W head, the projector amps (RCA and Bell & Howell), and… that’s it. Maybe the cheap solidstate no-name 100W bass amp just so i have something for my bass guitar.

This is IT. now let’s JAM!!!

1996 FENDER PROSONIC

You know when a dream becomes real? This is one of them.
Ive been lusting after a Fender Prosonic ever since i learned about it a couple of years ago. At that time ive been searching for a “modern” tube amp that employs tube rectifiers. A good 95%+ of all tube amplifiers made today have solid state rectifiers. Why tube rectifiers? “SAG”. To put it simply, when a signal goes thru an overdriven amp with a tube rectifier, the signal undergoes something similar to compression. Instead of dying quickly, it dies slowly, and the sound sustains for a little longer. The impression is that it “blooms”, the sound seem like its getting louder instead of softer as it decays. This is know as ‘sag’. Awful for jazz and metal or anything you need fast articulated notes, but perfect for blues and anything that needs sustain. Which i LOVE.

THE PICKUP
Ive been on the lookout for a cheap one for months now, scouring eBay, always finding them priced beyond what i’d like to pay (and afford!). Until one day on Craigslist, an ad was up for one. It normally goes for an average of $700 to as much as $1600 for the “custom shop” (which after much research really has no circuit difference with the production model – only the kitsch of having an early model makes it appealing to collectors…). $600 is relatively cheap for it. This was LESS!
I immediately emailed the guy with an offer, and he replied back saying he agreed with it as long as i pick it up that same day. No problem! As soon as work was done, i withdrew money, and headed there.
This is the most ive spent on ANY one guitar-related purchase (im poor! and im poorer now!). My heart was racing – i will have to cut back on lunches and dinners in the next 2 weeks! 😀 But as soon as i took it home and plugged in, it was well worth above what i paid for.

HOME
The first was to test how it sounds with my guitars. I test-drove it when i was at the guys’ place, and it sounded great, but its always different when you have your own comfortable axe to use, and you have all the time to futz around.

On the front has the unusual set of controls ive seen on a 2-channel amp. There is one Volume knob for channel 1, and a Master Volume, not for over the 2 channels, but Master for channel 2 only. Both channels share the same 3-band EQ and Reverb. In addition to a Gain knob for the 2nd channel, there is an ADDITIONAL Gain 2 knob, also for channel 2 only.  The front also displays the green jewel light and the 2 inputs (1 is high, 2 is low, about -3dB lower per the manual).
On the back are the Power switch, as well as the standby switch, and the WONDERFUL 3way selector switch which sets it to Class A tube rectifier (30W), Class A/B tube rectifier (50W), and Class A/B Solid state (60W). Most of my testing was done on Class A. The back also sports the Effects Loop Send and Return jacks, the occupied jack for the combo speakers, an additional output speaker jack to wire an external cab, and the footswitch jack.

Inside are the two Celestion speakers (supposedly the 10″ version of Celestion Vintage 30’s). Hello!

I used my ’88 Gibson SG Special to test for humbucker sounds, a Korean-made 90’s Squier Strat, and a Danelectro Mod 6  (modded with a Baritone neck) for single coils.

Channel 1 sounds clean only up to 2, but man its LOUD! Past 2 and it starts to break up. NICELY! mmmm…. tube breakup…..
Channel 2 is just a firebreathing monster eating jalapenos and drinking capsacin in molten lava. HOT!!!

Not going to elaborate further, all i know is this beast needs a bit fo taming. Its over-the-top cascading dual Gain settings is WAY more than what even the most aggressive metalhead needs. After several hours over a couple of days, it was time.

THE SWITCH
Stock tubes sounded ok, but i never did like new production tubes very much (only if necessary). So i went about changing them, and dug up what NOS and used vintage tubes i got, but not before looking at the tube amp chart, the schematics, and forum-trolling. There isnt one definitive place i found that outlines exactly which preamp tubes do what, so i traced the harder ones using the schematic i found online.
left to right facing the back:
V9 = rectifier
V8, V7 = power tubes
V6 = Phase Inverter (PI)
V5 = 1/2 pushing the PI, 1/2 Reverb recovery
V4 = Reverb driver
V3 = 1/2 effects loop, 1/2 unused
V2 = 1/2 Gain 2, 1/2 channel 2 master volume (and looks like the EQ section)
V1 = 1/2 Gain 1, 1/2 Channel 1 volume

Now this is interesting. I double-checked to make sure i read the schematics right, but it seems V1 which controls Channel 1 Volume, ALSO controls Gain1 of Channel 2. V2 seems to control Channel 2 Master Volume and the EQ section, AND Gain 2. One would think that V2 should have both GAIN levels hooked to it, and maybe have V1 used for Channel 1 and the EQ section. The Master Volume for channel 2 could have dedicated itself on the unused section of the FX Loop.

At any rate this is what i ended up putting in at first try:
V9 = stock chinese 5AR4 (had to order something else for it)
V8, V7 = GE 6L6GC
V6 thru V3, V1 = GE 12AT7
V2 = RCA 12AX7A cleartop
Though it gave the amp a better sweep and a it more headroom, this also gave the amp a better sounding drive, on both Channel 1 (when turned up), and on Channel 2, which has less distortion, but still fizzy when dialed up. I usually turn this up when i want something more thrash chunky riffing.

One thing it certainly did was clean up the noisy reverb. Amazing what bad tubes can do, and how easy it is to fix them. No more AM radio static. The 12AT7 in the reverb recovery also made the reverb less swampy. Though truth  be told, i LIKE swampy! but putting a 12AX7A in the reverb recovery also added back a bit more drive and less headroom.

Now i mentioned ive always wanted to have a Prosonic since ive learned of it, but i didnt go into too much research on it -i didnt want to get an impression until i got one, so i can judge for my own. I do know that tube rectifiers give a tube amp a sweet sound when the tubes are cookin’, as evident in the old Bell & Howell tube amps ive owned that used tube rectifiers. The chinese rectifier was doing its job, but it sounded stiff and thin to me. I have an Epiphone Blues Custom 30 that uses a 5AR4 tube rectifier also, and the sound improved when i replaced the chinese tube rectifier in it with a Richardson-branded US tube that was made in the 80s. It looked like a Bendix with the off-color base, but i wasnt sure. All i know is that the sound of the Epi BC30 was rounder and had more depth with the change.
So i went about looking for used old stock 5AR4 tubes, and was prepared to get even japanese made if i cant find relatively cheaper 5AR4 tubes. Everyone points to a British Mullard, but seriously what blue-collar person can afford a $100 tube? At any rate i ended up with a couple of Japanese (looked like Matsushita) 5AR4 tubes, and one GE.
Dropping one of the Japanese tubes in, the change was immediately apparent. There was more volume, or at least the bottom end was more evident, and it was tighter. Before the change, the overdriven sound was too loose-sounding, a bit too hairy. There was less fizz now, and the overdrive is more focused. YUMMY! That one stays!

Only now that ive owned A Prosonic that ive gone in and read as much as i can about it on forums and boards, just to get an idea what people are hearing, and compare notes, so to speak. So far what ive been reading is consistent with what im hearing – this amp when given a little work, becomes much more of a jewel than it already is!

One tube ive been meaning to play with are 5751 tubes – a lower gain 12AX7, but slightly more than a 12AT7. The SRV tube, which i guess lends to its expensiveness. I was able to find some old stock online that were relatively cheap. At least compared to the new production 5751’s the ones i got were just slightly cheaper, which for old stock is a GOOD price! I also got a pair of Sylvania JAN 6L6GB’s to try.

As soon as the 5751 and 6L6GB’s came in, i swapped and moved tubes around. I ended up with this configuration:
V9 = japanese old stock 5AR4
V8, V7 = JAN Sylvania 6L6GB
V6 = GE 6679/12AT7
V5 = GE 5751
V4 = GE 12AT7
V3 = GE 12AT7
V2 = RCA 12AX7A
V1 = GE 5751

This seemed the most musical-sounding combination for me.
V9 = japanese old stock 5AR4 – gave it a better bottom end ( i can hear it now! so much so i can dial back the Bass and still sound great)
V8, V7 = JAN Sylvania 6L6GB – higher headroom, better, louder cleans before CH1 starts overdriving
V6 = GE 6679/12AT7 – clean-sounding tube for the PI, less hairiness on pushing the power tubes
V5 = GE 5751 – instead of 12AX7, the 5751 doesnt make the reverb too swampy, and also pushes the PI less
V4 = GE 12AT7 – tames the reverb well, maybe TOO well. im thinking of changing this back to an RCA 12AX7A cleartop
V3 = GE 12AT7 – FX loop. easily the most useless tube in the chain to me – i dont use it. why waste a 12AX7 tube in it? just an old stock quiet 12AT7 to keep the chain going. Cant NOT put anything, since removing it makes the amp go silent.
V2 = RCA 12AX7A – keep Gain 2 real hot, and also drive the EQ section strong. Also debating on putting a 5751 (swap with V4)
V1 = GE 5751 – 12AX7A was too hot, didnt give Channel 1 much sweep, and overdrives too soon when turned up. 12AT7 seemed like a blanket was on channel1, 5751 opens it up and brings back that chime without losing clarity at higher volumes.

One thing ive also wanted to use are 7581 tubes, an industrial version of the 6L6GC. Known for its higher plate voltage, it can be subbed without worries. One strong tube. Thats the thing though- i have only ONE. it came out of a mismatched pair (with a 5881!) in a Bassman 10 i got a couple of years ago. Tested in my Hickok 600, it tested very strong. But without a pair, its been sitting on the shelf. So i went on ebay to find one. Its a dicey move, since who knows what condition tube i was going to get. But i went ahead and placed a bid.

While waiting for the auction to end, i decided to swap out speakers. Granted the stock celestions are loud and had character, but it seemed too midrange-y to me. From what ive been reading, Brice Zinky had intended for the Prosonic to contain 1 12″ Vintage 30. I almost bought one, until dug up my speaker overstocks, and found a WGS Veteran 30. DAMN! i forgot about this baby. Similar to a Vintage 30, but supposedly a fuller sound. I did a quick and dirty hookup, and it sounded promising. This led me to measuring the baffle and looking at options. Either i can get a custom-made 1×12 baffle for $100, or i can just order a half-yard grill cloth for $16 and make one myself. Im pretty handy with a jigsaw and i have 1/2″ birch ply materials in my basement, this shouldnt be too hard. As im cheap, i ordered the grill cloth.

There was also a THIRD option: get another pair of 10″ speakers. AS i dug thru my speaker overstocks, i also spotted a pair of CTS speakers dating to the 18th week of 1973. I remembered this being an early replacement for the  Kalamazoo Bass 50 projects i had some years back. Theyre essentially derivatives of  ceramic Jensen C10N speakers. Oddly enough Weber doesnt make this specific model (they do have C12N copies), so i was forced to scour ebay for something old stock thats similar.
Wouldnt you know it – the midrange-heavy sound was definitely less, the bottom end was MUCH tighter, and the clean sound has more shimmer to it! I like!

One mod i did do was to cut the cap found on the preamp tube socket of V1. Supposedly for more headroom. Man, the clean sparkle got MUCH better!

For a couple of days ive played with this configuration, and im finding it sweeter each day. Until i got the 7581 tube i won. Crossing my fingers i tested it, and wow – this one was as strong as the one i got. A GOOD PAIR! I immediately dropped it in the amp and fired the amp up.

JAW DROPPING.

What i started out for this amp – trying to get that overdriven tube-rectifier laced sound, i ended up loving the clean sound of it. With the 7581 tubes in place and on CLASS A, the sparkle that i thought was better got so much more clearer. If they were talking about a real Fender clean, these tubes in this amp certainly got it there. And it made the sweep wider, there is much more clarity in the notes in higher volumes before overdriving in Channel 1. And When it DOES overdrive, the sound is much more defined and less scattered.
Switching to Channel 2, its even more eye-popping. The distortion isnt fizzy with just playing on Gain 1, and only gets to that point when turning up gain 2.
UN. REAL.

its turned from a high-gain beast to a multi-faceted, multi-horned monster! Cleans are sparking (and louder!) on the Danelectro single coil lipsticks, brittle overdrive gone with the humbuckers, fizziness virtually gone in lower gain settings, and the squawk is back on the lowly strat!

SPEAKERS
Ive come to find that this versatile amp can work wonders on different speakers. I still like the inherent sound of the stock Celestions, and id like to still use them. For that i have a 2×12 closed back speaker cab on the way. I plan on putting 10″ conversion baffles for it too.
whether id put the stock celestions in it, or put the celestions back in the combo and move the CTS speakers in the cab, im still not sure. There is still the 12″ option.
What i truly want is to have the prosonic in a 2×12 combo, but from what ive canvassed online, it will push me back close to $300 to have a custom-made 2×12 cab for it. I can simple get a Twin cab, and adjust the chassis to it (Twin cab is about 2 to 3 inches longer). Then i thought, thats can be a lot of work!Especially when im already getting AMAZING tone now.
Ill wait til the cab comes in. For now, ill enjoy and keep playing this wonderful amp!

DATING THE AMP
One thing that intrigued me was WHEN the amp was made. Its certain that this was a production model from Corona, not from Lake Oswego. Though some people put a lot of weight into where their prosonic was made, i could care less in this matter. There were NO circuit difference from the ones made in the custom shop in lake Oswego with the ones made in Corona. The only real difference is that (supposedly) the first few Prosonics were made with “better” PCBs, and higher grade components. Be that as it may, These “higher grade” components arent that much different with what was in the production models. Now if it was the use of different TYPE of capacitors (say, orange drops Vs. Paper in oil), or resistors (say carbon comp Vs metal film), id allow a good leeway that the tone would be vastly different. Its not (ive seen the innards of a lake Oswego made Prosonic and a production Corona Prosonic), and to me, i couldnt tell major differences. Not even any differences in the values used. To me, its just a matter of corksniffing now, and at worst, a peeing contest. But thats just my humble opinion. Some people hold in high regard the Kitsch of something Collectible. Im not a collector, im a guitar player.

That said, the production code should have been stamped on the tube chart, but its blank. The QA Inspection sticker does have two handwritten initials, one scribble in black next to Electronic Test, and what looks to be 6K or GK next to Sound Test. Looking at a couple of pictured samples online, GK seems to be a better decipher. According to fender’s own reference, this dates it to 1996, November.I must say, this amp has been pretty well-kept, considering its  made during the first year of production.

SOME THOUGHTS (for now)
Save for a couple of tweaks, i must say, im DONE! Ive arrived to where i want to go with this amp. I didnt expect to get Fender-clean sounds, i was content to get something decent, but that just blows me away each time i strum my lowly squier and my baritone Mod 6. The blazing hot gain i first experienced was acceptable, but now its more easily controlled. If i want to let it loose, i just turn up both GAIN knobs!

In the process of doing the cap cut on V1, i was able to inspect the innards of the amp. I must say, its relatively simple layout on the PCB. Ive opened the Epiphone Blues Custom 30, and just getting to the PCB was a pain. Once open, the Prosonic chassis reveals a clean layout where you can follow the schematics easily. Another plus for this amp is that the pots arent directly soldered onto the PCB, like some modern amps that ive seen. This makes for easier pots replacement. This is significant because apparently the pots are linear, not audio taper. This is one reason why the sound JUMPS from full silence to a loud volume when going from zero to 1 on the dials.  An audio taper allows you to open the amp SLOWLY from silence to a more gradual increase in perceived volume. This looks to be a good mod for Channel 1, to make the clean sweep a little better. Maybe for Gain 1 also, but i think its control is ok.

The amp didnt come with a footswitch, just the cover. It wasnt hard to get a footswitch, its just a matter of expense. The stereo 1/4″ type it uses isnt available from Fender anymore, and no one sells it. But apparently the REV/VIB version works well. I got the REV/VIB footswitch i used for the other Plush amp i got (P-1000S), used a 1/4″ stereo plug to dual RCA adapter, and i was in business. I did order a footswitch with had the direct streo 1/4″ plug to dedicate to this amp.

Playing mostly in Class A, this has been truly amazing. But switching to Class A/B, ad even solid-state rectifier, it even makes for a more versatile amp. The dough i dropped for this doesnt hurt anymore – im glad for the little sacrifices i have to make, now that i have this tone machine to play with.
🙂

UPDATE: MORE SPEAKER OPTIONS!
This week came the 2 used 2×12 cabs i got online.
1) Black Tuck n roll – no brand, one handle on the longer side top. looks like a cab made by Kustom with their tuck n roll line of amps (or the plush/ Earth amps of the early 70s), though the silver (turning brown from dust and age) grill cloth gives it away – all the grill cloth ive seen Kustom and Plush use is black. Inside the cab the construction seems home-made (gobs of glue  lining the edges), but exhibit no wiggle – this cabinet feels, looks, and IS solid. Outside the cushioned soft vinyl  sets it apart from  the usual rectangular 2×12 cabs, while the inside reveals a unique speaker positioning – the speaker panel is ANGLED. the speaker baffle/panel is separate from the grill cloth panel, and the left side is set about 1/3 of the way back into the cab. In effect the speaker on this side is further back when facing it, while the other is closer to you. This didnt make any sense until i inspected the outside of the cab further. One short side is FLAT – it didnt have the cushioned tuck n roll cover on it, just a soft tolex. It seems it can convert to a VERTICAL cab! When in this position, the ANGLED speaker panel inside projects it at an upwards angle, much in the same way a fender combo front is a bit angled back upwards, or how the tilt-back legs of some fender combos allow it to lean back a little further. THAT’S WHY!
I ended up installing the WGS Veteran 30 12″ speaker on the more angled side, and a fender branded speaker on the other side (it looks like a jensen C12N, but it has a slightly smaller magnet. Im still researching its make and model. Both Are 16ohm speakers, and i decided to just wire them parallel for 8ohms. The cab has a panel to fully close it up, but has a 4″ diameter hole in the center.

2) Empty 70’s Fender Twin Combo Cabinet – came with caster, fender logo on the silver grill, back panels, and even the tube layout diagram. This is EXACTLY what i was planning to transplant the Prosonic chassis into, even if it was about 2 inches wider. It just cost too much money to get a NEW custom made combo cabinet. This too was structurally SOLID.
I put in vintage 12″ Pyle Speakers from 1968 and 1969 (what looks like Jensen C12N’s). Though not from the same batch and year, it is exactly the same model. Both are 8ohms wired in series for 16ohms total.

UPDATE #2:
After an exhaustive testing of the different speaker options, it’s been decided: the 10″ Celestion speakers in the Prosonic are getting changed out. Its too mid-range-y for my tastes, and it lacks the sparkle and top end cleans that the jensen ceramics (C10N or C12N) have, and lacks the overdriven smoothness and depth of the 12″ WGS Veteran 30 (which is supposed to be an improved Celestion Vintage 30, with a better clarity on its higher midrange).  The Celestions just did not have enough chime in the clean sound, and was too brittle-sounding when switching to channel 2’s overdriven sections. Hell, even the WGS had that elusive Fender chime when played clean! A very versatile speaker.

And that’s whats going in as a replacement in the Prosonic combo. Ive already started cutting up a 1/2″ thick birch ply to its baffle dimensions, and im about to cut the hole for the single 12″ WGS speaker (from the tuck and roll 2×12 cab). If i was able to find the jigsaw tonight id have been done with the rough cut.
The silver grill cloth is ready, and i finally got a matching fender logo with the sweeping tail taken out of an empty ’69 Fender Bandmaster Reverb head (which will then house a Bassman 10’s chassis – yes, another project).
$8 – 2’x2′ 1/2″ thick birch ply
$16 – 1/2 yd Fender Grill cloth
$15 – used fender logo
$1 – set of screws for speakers
[u]  $0 -[/u] some elbow grease and existing tools (jigsaw, circular saw, sander, drill/driver, staples, glue, black paint)
$40 – Total
compared to $100 if i had it done professionally, with the same results. And it doesnt even have a Fender Logo!

Im excited about this mod, as it makes the combo my favorite among all my amps. Its versatility, its tone, and its unassuming look that betrays its monstrous capability… and now its lighter! My back is thanking me already. 🙂

Update #3: Just changed out the Japanese 5AR4 rectifier with a used old stock Gibson-branded Holland-made Amperex 5AR4 that tested well above new. It retained the clarity and depth of the Japanese 5AR4, with a better bass response. And still provides that “bloom” on overdriven settings. Mmmmmm…. delicious!

UPDATE #4: 1×12 conversion DONE!
didnt really take that long – its the planning and measuring that took some time. Preparation is necessary to make sure errors and mistakes are avoided or minimized. Its a professional tube amp -it should NOT look sloppy!
What exactly is involved in the prep?
1) Direct tracing the original baffle onto the 2’x2′ birch ply.
2) Deciding WHERE the 12″ speaker will be located. Centered seem ideal, but the heavy transformers are off to the right side more (facing the amp). Centered will make the amp heavier on one side and awkward to carry.  I set the speaker hole about an inch and a half from the left, top, and bottom edge.
4) Tracing the speaker hole. I used a twin reverb’s speaker hole as the guide, traced it onto a stiff cardboard, and cut the cardboard out. Or if you have a compass you can use that instead. Also traced the screw holes. I just used 4.
5) Once all the measurements are traced, i double checked. Measure twice…

After the prep starts the cut. First is the dimensions of he baffle itself. Once thats done, the speaker hole is next, using a jigsaw with the smallest width blades i can find. The circle cut wont be perfect, but at least the smaller width makes the curving cuts easier. Slowly following the trace makes it look less sloppy. It wont really be seen once the grill and speakers are on, but as a perfectionist on denial, i will know, and it will nag at me if i didnt do the best job i can. Gotta hold yourself accountable.

after the cut is a spray of flat black enamel all over. Not to really color it but just enough to cover it. Its subtle  but it makes the silver grill cloth sparkle more against the dark background. Think about it – stars are visible more in the dark night sky. Same principle.

Once its dry, its on with the grill cloth. Ive only done this once before, and what i can remember, the grill cloth need to be pulled taut and as tight as possible, so it wont bulge or sage, and must be done square, or it would look uneven, with random diagonal patterns. By following the straight rows and columns of black and silver threads and folding them according to the baffle’s dimensions, it allowed me to align the straight pattern squarely with the baffle frame itself. Doing about 2 inches at a time, i pulled and aligned the grill cloth square, and then stapled it.

Once thats done, the next thing i put on was the Fender logo. I got the fender logo from a 69 Bandmaster head, with the tail. But before i did this, i decided to follow the exact placement of the Prosonic logo. I took a letter size paper, and aligned it at the top right angle of the original baffle. i then took a pencil and traced the logo from the top, more so where the 3 screws were. I then transferred this trace over to the new baffle, and punched a guide hole where the screws would be. I then screwed in the fender logo.

Last was the 12″ speaker. Using the screw guide from the template i also bore some guide holes fot the speaker screws. Once thats done, its a simple matter of aligning the speaker right and secure.

I took the wires with the right angled 1/4″ plug from the Bassman 10 cabinet and used those for the connection between the speaker and the amp. Looked more original with the round Fender “F” plug cover.

Some thoughts: It came out well! In hindsight i should have used a 3/4″ thick birch ply instead of the 1/2″ i got now, but i dont hear anything rattling or buckling from a loud setting. Its holding up well! I may make a second one, this time using laminated pine. I know exactly what to do now.

Im definitely MUCH MORE satisfied now with this 1×12″ configuration using the WGS Veteran 30 speaker. It;s 3:30AM and im more inspired o keep riffing away! Sleep beckons though, but its gonna be a nice slumber, knowing when i wake up i can play again, and mre time to play thru this amp. 🙂

UPDATE! July 03, 2013
CLOSE TO 3 YEARS LATER… WE FINALLY HAVE THE PERFECT COMBO!

it took til the early months of 2013 to finally nail down the perfect speakers for the Prosonic. and wouldn’t you know it, the winners are CELESTION GREENBACKS!

what made the Greenbacks THE speakers for the Prosonic is the emergence of the TRUE FENDER CLEAN. The overdrive channel will always be THE hallmark of this gem of an amp, but not until i attached the Greenbacks to it did it finally show the TRUE Fender clean sound, the unmistakeable chime a Fender is known for, and only a Fender can provide. You know, when you play a strat clean thru a Fender amp you get that trademark Fender Strat ‘squawk’ in the 2 and 4 positions – this amp finally got that going for it with the Greenbacks. I thought the 1×12 WGS Veteran 30 speaker had it, man! i was so wrong.

Going from 1×12 WGS:
Instead of dedicating a separate 2×12 cab for the prosonic combo, i ended up using the empty 2×12 twin combo i mentioned. I had already drilled the holes on the twin cab for the Prosonic chassis before, so it was (relatively) easy to transfer it. It does have about an inch of space on either side of the front panel, though at first glance it doesnt look as funny as it first did before. a closer look does show the gaps. Im still in the process of thinking a way to cover them.

1972 PLUSH CONGRESS IV

Another great craigslist find, this vintage all tube amp hails from the early 70s (01/05/1972 to be exact, as penciled in under the chassis itself).
As a background, Plush amplifiers were made from 1969 thru 1973, apparently direct copies of either a fender showman or a twin reverb. Its main striking feature is the now identifiable “tuck and roll” covering not unlike that of Kustom amps, instead of the regular tolex coverings. Not surprisingly they were sued, either by Kustom or Fender (not sure), and its been said they went out of business due to bankruptcy from these litigations (but the story goes that they went on to become Earth Sound Amplifiers, who in their early life also had tuck and roll covering on their early amps, though nothing real conclusive to support this Plush-Earth Sound connection other than whats been generated thru forums and posts). Earth Sound also went the way of the dinosaurs eventually.

THE FIND:

Trolling craigslist one night, the ad “PA tube head – $100” immediately caught my eye. “Tube” and “$100” together are usually a good indicator of a possible good deal, so i checked it out. Two things stood out – the tuck and roll covering, and a bunch of knobs to match several inputs. Whoa! Mixer? it said PA, so i googled the name, and sure enough most results were about asking the same question i have: what is a Plush Amplifier?
A website about plush amps did answer most of that question, so i called the number on the ad right away, and left a message. As a follow up i also emailed. I got a response the next morning, and proceeded to arrange for pickup. A little drive out to the country on a sunny early sunday afternoon is a nice change – one of the cool things i enjoy about out-of-the-way weekend drives.

The guy was nice, and spoke about how being a drummer, he had not much use for the amp anymore. It did power on, but didnt hook it up to a cab or even try it out – we spoke at length about the virtues of vintage and analog instruments. In the end i almost got an unloaded 2×18 ampeg cab, but having a small car, it wasnt going to be an easy transport (and i have no  more space in my house!). i DID get a project solidbody guitar also for cheap – a double cutaway setneck Aspen brand, no pickups and with a bad paintjob, all other hardware included. for $5 with a harshell case also included, i couldnt turn it down. But that’s another story….

INITIAL START-UP AND RESTORATION:

As soon as i got home, i plugged the head into a 2×12 cab loaded with Carvin British Series US-made speakers. The tubes glowed and warmed up, and sound came thru once i plugged a guitar in.
It sounded clean, though several knobs wont turn, and some are stiff. The reverb didnt work, and channel 4 sounded weaker than the other 3. No big hum or extraneous noise were heard (other than what i was making thru the guitar), so the guitar is functional as is, just needs work.

A couple of contact cleaner sprays allowed the stiff pots to twist easier, but there were 6 that were completely stuck and frozen – no amount of clamping and pliers twisting freed it loose. I ended up replacing them temporarily with my own stock of Alpha pots, eventually ordering the correct size (i had the correct values, but they were mini pots – it works great, but it just looked odd to keep it that way). Reading the stamped code on the back showed they were Centralab pots, and man – NOS are expensive! So i got the next best thing – CTS pots. as soon as they arrive, ill be opening up the amp again.
I ended up changing two pots, and moving the current pots around to get all of the untwistable ones on one channel – at least i can have 3 usable channels til i get the orders in.
Workng without schematics (there were NONE found anywhere online), i made sure the values are correct for the gain and master volume pots (1 Meg), but i didnt inspect the tone pots – i thought they were all 250k. I realized it later when i saw the midrange control pots were 100k. oops! oh well, thats a lesson for me, plus its not that big of a deal – it will be corrected when i get the order in.
but for now, the goal is to get it to full working condition. After the pots were properly cleaned with contact cleaner, i proceeded to check the preamp tubes by removing one at a time and playing each channel- with a pencil, i marked each preamp tube that corresponded to what channel, and to what sockets they were in. Doing this i found out each channel had its own preamp tube. More on that later…

It had a reverb in and out RCA connectors in the back, and seeng no reverb tank inside the cab, i hooked up a working spare reverb tank i had lying around, and didnt have any reverb response either. Bummer.

So on to the next step – I removed the preamp tubes (6 12AX7’s, one 12AT7, and one 12AU7), and replaced them for now with new production EH 12AX7’s, 1 NOS RCA 12AT7 and 1 RCA 12AU7.
THE SPRING REVERB CAME TO LIFE!!! I dont know where its coming from, but its there alright. The old preamp tubes must be in their last legs. The dry sound is now much stronger, and the reverb sounds quite lush.
The 4 power tubes were  Japan 6L6GC’s, and were all branded the same (Realistic), looking like they were all from the same batch, a good chance they are matched. Ive seen several plush amps online pictured with non-matching pairs. I havent tested these yet, but they sounded strong. I may tube-roll with my own old stock 6L6’s, but i I see no need to replace them at this time.

After a few more sound checks, i decided to replace the bigger electrolytic caps. After almost 40 years, even though it sounds strong and there were no tell-tale signs of impending burnup or blowout or simple failure, its best to be on the safe side.

Unscrewing what looked to be non-original non-matching screws, the bottom easily fell off, and revealed not just point to point wiring with most of the smaller components on a slim turret board, but the reverb tank as well – THERE IT IS! It looked exactly like my other non-working accutronics tank, only the Plush’s tank had pitting and some rust on it.

There were 3 big caps in the under side, and one cap can on top of the chassis. I had to snap a picture of the cap can’s side where the cap values are stamped on since it was in an awkward unreadable spot. 3 20uF/500V were needed for this cap can, and 2 80uF/350V and 1 40uF/450V (in series with one of the 20uF caps) under the chassis.
I kept close to the same values, only substituting a 100uF cap on the 80uF ones, and a 47uF for the 40uF. I figured i can replace with a bigger value, but i wanted to hear as close as possible to how the original amp sounded. I can always up the values later on if i see fit (this can also raise the headroom, which for a PA system is paramount, then again im mostly going to plug in a guitar here anyway).

THE JAM TEST

So now that most of the amp is in working, its time to put it thru the Jam Test.  😉 I must say – this is one LOUD MONSTER! Cleans are excellent, and not ice-picky at all. It barely overdrives! To most modern-day guitarist looking for “breakup” or high gain sound, this is a drawback, and not for them at all. This is after all a P.A. amplifier, so its setup NOT to distort. I guess thats one reason the Plush engineers thought best to copy a dual showman or a twin reverb, all for their clean headroom. This one has it! and i thought thats all it had.

My buddy came over for our regular jam, and we put it thru the paces, and lo and behold, he dialed it enough to get real overdrive. WHERE THE HELL IS THAT COMING FROM?!? we took a look at the settings. He was plugged in to Channel 1, gain at 8, bass at 2, mids at 9, and treble at 3. Main volume is at 8. DAMN! turning up the mids and lowering the bass and highs, made this a promising dirty rockin machine!

Further tests revealed the amp is picky about what to get it to overdrive – it wanted bright pickups. The guitar he played was a Luna brand, while i tested a dean Evo (both humbucker equipped). The Evo sounded dark on the Plush (detuned to D). I plugged in a Squier ’51 that had a GFS Dream 180 bridge Humbucker (described on their site as having “…the sparkle and chime of our Retrotron Nashville, along with the fat warm bottom of our Alnico PAF pickups”,  “…made to combine the chime and jangle of vintage Filtertrons with the warmth and body of a great pair of vintage PAF’s). It worked well – the biting overdrive pushed the amp to sound grittier at higher volumes.

For about 3 hours of jamming we used it, plugging different guitars and in different settings. Everything worked, even the 4 frequency cut/bost switches. All 3 hours were earcandy!

On  a separate note: You can tell we used “cheap” guitars. It only goes to show that even low-end guitars can sound amazing if you use a great-sounding tube amp. You can have a super high end guitar that costs a couple thousand dollars, but it will still sound like crap if you pair it with a bad-sounding amp. This is one reason why i shifted to tube amps from being a simple guitar-tech. Knowing tube amps completes the circle of a great electric guitar sound.
And it DOES helps to play decent too. 😉

ONE BIG SAFETY ISSUE

I’m putting a 3-prong cord in.
The amp came with the original power cord, but with a replacement 2 prong plug (must have worn off, as ive seen pictures with the original plug wearing out right at the plug base). This is then hooked up inside the chassis to a standby switch and a separate 3-WAY toggle switch that has the ON setting up, the OFF setting in the middle, and a Reverse Polarity in the down position.

Back in the day, there used to be NO standard for grounding electrical appliances and equipment. When you have several amps plugged in to the same mains, hum can occur, which can be mitigated by turning the plug around. In this case, you just flip the switch down to set it to the correct polarity.

The problem here is that one you flip down the switch, the chassis or any metal part of it becomes HOT instead of the Ground, and electricity will pass through it. and once you plug your guitar in, it will pass through YOU as well. touch a metal part of another amp that isnt in the same polarity, and you complete a circuit – YOU GET ELECTROCUTED! happened to me. On THIS amp. I flipped the switch down too hard, and kept it ON (reversed polarity), and touched a different properly grounded amp. YIKES!

Im tempted to make the all-in-one bypass/off/on mod with the 3way toggle switch, but i much prefer a separate bypass switch. Seems more traditional, plus might as well use whats on it.

DEDICATED CHANNELS

During the jam test, after we figured out how to coax a crunchy tone out of the 1st channel, we copied the same settings over to channel 2, but it didnt sound as good, and didnt crunch as hard. At first i thought it was the overstated midrange value (250K instead fo 100K), but later on i figured it out with an experiment.

A clue was in one of the post of another Plush owner about modding it, who mentioned that “…each of those channels have a separate section”. Preamp? Hmmm…. so i went to town and one by one took out a preamp tube and turned the amp on. Lo and behold, it revealed the tubes that were responsible for getting the sound thru each channel (duh), and even for the reverb! Looking at what i thought was the correct layout, the tubes did NOT line up. I had a high-gain 12AX7 tube in the reverb section, and a 12AU7 in for channel 2. No wonder it sounded weak! a quick change of tubes effectively brought up the volume AND gain, now at level with the rest of the other channels. This also made for a much tamer reverb with a 12AU7 in it.

Now that i know which tube sockets control which channel correctly, there will be preamp tube rolling! I do have a stash of used old stock Mullards, RCAs, Sylvanias, and GEs i can try – heck, i may just simply put a different preamp tube for each channels, and then do an ABCD test.

FURTHER THOUGHTS

Pots will be set back to stock values as soon as the order comes in. Later on one channel will possibly be converted to a twin reverb tone stack. Im thinking another can be set to blackface bassman tone values. along with mods as well (maybe). Im keeping at least ONE channel stock.

A review of the schematics for a dual showman and twin reverb shows that there IS a difference with the pot values, at least the mids. Dual Showman and Twin reverb mid pots show a value of 10K, while the mids on the Plush are set to 100K. The Bass and Treble are all the same at 250K. Bassmans have theirs at 25K.

This was one of my oversights – i assumed the gain and master volume were all 1Meg, and all the tone controls were all 250K, so for the tone, i just simply transposed some over to the other channels without checking. So for channel 2, the mids are set at a higher range (250K), though the difference is noticeable only at much higher gain – the overdriven sound has that thick discernible midrange tone. at lower gain settings the clean tone sounds the same as the other channels, though this is only a preliminary test. Most of the Jam Test were in full overdrive on channel 1, and clean tones across the other channels.

SOME FINAL THOUGHTS (FOR NOW)

This is THE most interesting amp ive acquired, hands down. 100W via 6L6GCs, FOUR DEDICATED CHANNELS (!!!), and an excellent-sounding reverb. ON ALL 4 CHANNELS, SWITCHABLE! It even has 4 frequency boosts/cuts (though i leave them all ON, and just futz with the tone controls). Its simple when you boil it down to what it does, but it does it 4 times! This has the potential to be a versatile guitar amp. And who can deny its quaint soft tuck and roll cover? Its almost perfect – there arent any noticeable tears, just a little nick here and there – pretty clean for something thats been around over the 38 years.
Almost looks like a foot stool. i beter make sure this is NEVER put on the floor – people may think its furniture to sit on!

Great amp, even better price! 😉

Some more pics:

PEAVEY VALVEKING 112: Say Hello, Wave Goodbye

This is my very first tube amp, in my awakening from my guitar coma. Im kinda sad about letting this amp go, as it held a sentimental value, but i dont play it as much, and only used when my buddy comes over and uses it with his modeling guitar unit.
It wasnt a bad tube amp at all. It was very transparent- it allows modeling units and stompboxes come through and flavor its sound nicely. It was quiet at its maximum levels, and it was LOUD.

But one reason that i gave it up was that it was TOO transparent. It had no specific tone to me, especially when i run its Lead Channel. Its Clean Channel was as classic a clean tone as you can get, but once the Lead Channel is kicked in, i had to futz around with its settings a bit much jut to dial in a useful tone. I guess i got spoiled from the other tube amps ive gotten, where getting a great overdriven tone was easy – all i did was turn up the volume and start getting a great tube growl. The kind of distortion and overdriven sound you look for in a tube amp. Dont get me wrong – the great tones are there, just too much dialing takes place when i play it, and i prefer to simply plug and play. Or plug, turn it up, and play. 🙂

One thing i did read about it was that it came from the factory cold-biased, meaning its power tubes were running underpowered, or cold. Not enough current is passing thru it to overdrive it naturally without turning up its gain knob. It didnt help that it did not have a way to adjust the bias – it had no bias control pot like most amps (not all amps have bias controls, but those that have it can have more or less current flow thru the power tubes, and this alters its sound or how soon it breaks up and gets that overdriven tone). There are actual bias pot mods that can be found online, but i stopped short of doing that on the Valveking. I already have other amps that can produce great tones by simply turning up the volume control, and also i have way too many projects already. Plus i feel a mod will decrease th amp’s value. I know id rather get an amp thats stock and unmodded, so i know  I can mod it myself, and not worry on NOT knowing what was changed before i got it.

So last week, after 20 months of ownership, i got a buyer to finally come whisk it away for cheap. At least now someone can enjoy it more than i did. So long Valveking, i dont think ill miss you much, but you’ll always be my first! Hope you have a more productive life! 🙂

EPIPHONE BLUES CUSTOM 30

Scouring Musician’s Friend’s site, i stumbled on a bunch of neat amps that are USED. Among them was the Epiphone Blues Custom 30. One thing that intrigued me was the use of a tube rectifier – not always seen on modern tube amps, its mostly considered by engineers to make amps unreliable, one reason why most tube amps are built with solidstate rectifiers. Also another consideration for NOT using them is the increase in power that an amp can generate, which in these days, something that majority of users seem to prefer. Or something that amplifier manufacturers seem to perceive is where the market is. People want more power, lets make one that can do give them that. Tube rectifiers eliminated, amps can push farther out in its power handling.

Tube rectifiers do change the way the amp handles its power, and consequently affect the tone it produces. Theres always what is called a “sag” when teh amp is pushed. In a nutshell, when an amp is turned up as in guitar amps, playing notes seem to produce a compression-like effect, sustain on a note a lot longer. The other effect is that picking isnt as respinsive as with a solidstate rectifier. Jazz and metal players, or anymusician who needs to be able to articulate 32nd or even 64th notes will find this unuseful. However, if you play blues, or use less notes in phrasing, this can manifest a unique, and in my opinion, a more soulful tone.

That being said, ive personally heard the difference, using an old modified Bell & howell 15W 6V6 amp turned up. Sweet, unmistakable bluesy tone. Initially, ive always been after a Fender Prosonic, a discontinued tube amp made in the mid 90s. Its unique feature – a switch to use solidstate rectifier OR a tube rectifier. Something that Mesa amps have used in a while now. Though it doesnt have a solidstate rectifier switch, when i saw the blues custom 30 being sold used in working condition at a ridiculously low price (after a coupon added), i decided to just go for it. I wasn’t disappointed.

I decide to keep the speakers stock (2 12″ 70W Lady Luck speakers made by eminence). The tubes though, i pulled and replaced with old stock i keep in my collection. Sylvania and Raytheon 6L6GC tubes, mullards for the V1 and V2 preamp section, a US RCA in the phase inverter, and telefunken tubes in the reverb section. The footswitch i ordered separate, but came in quick (though not a sponsor, Sweetwater gets a big thumbs up!)

After about 2 weeks i decided to dive into the mods, only after i got some problems with the volume control of CHannel1. It started to go in and out. After some diagnostics and poking around (methodically done with safety precautions of course), i found the issue to be a loose lug, most likely having a bad solder. After fixing it and the chassi still out of the cabinet, i decided to cut one leg off of a couple of capacitors as suggested in the Epiphone forums. Sure enough the noticeably treble-y stock sound of the BC30 tamed a bit, but the most noticeable imrpovement in tone was paralleling a 5.6K/2W resistor on R15. The overdrive on Channel 2 is more distinct, clearer, brighter, less muddy at lower drive settings. It has a perceived increase in loudness as well.

Though the stock sound of the BC30 is actually good on its own, these mods improved its tone. To me, options are always a good thing. To offer that in this unit, im installing small switches. Im more likely to put the switches on the back to keep the front panel clean.

Bottmline, i enjoying this amp a lot! right now im tube rolling with the tube rectifiers. From what the circuit calls for (5ar4), to 5U4, 5V4, and a solidstate rectifier.  During research and comparing their datat sheets, im very well aware that this can wreak havoc, even damage the amp. This definitely is NOT for the faint of heart, or those who prefer not to have that scenario even taking place. SO i do not suggest what im doing for anyone to do. This is my own experiment, and however foolish or misguided it may seem, its not without proper and more than sufficient knowledge and research. I am prepared to suffer consequences in pursuit of sonic research.
Plus without being brave or foolish, no one would discover other lands, no one would have rock n roll. If Jimmy Page or Eric Clapton or Jeff Beck or Jimi Hendrix did not turn their plexis and AC30s all the way up, they wouldnt have made those sounds we now consider legendary, and wouldnt have spawned what we know of now as ROCK MUSIC. Of course, it made their ears ring permanently, and amps blew and had to keep going back to repairs, but such little sacrifices are worth it!

For the BC30, its a gem of an amp, that im amazed at how good the sound is with very little futzing around. something i cant say about my valveking 112. Whatever the case may be, this amp is a keeper!

The way i see it: play safe, or play loud, just play!!!

1960s SEARS SILVERTONE 1470

Yet another gem from CL: a mid to late 60’s Sears Silvertone tube amp, model 1470.

From some online research this puts out a whopping 2W, from a very non-traditional set of tubes: 12AU6, 35E4, 50C5. Nontraditional for guitar tube amps, that is. The trio is usually found on tube radios. Plugged in it pushes the sound thru an 8″ speaker (unknown brand), with only ONE volume knob. Its as simple as it gets.

What i wonder about is why most of these low-wattage amps have TWO inputs. I guess back in tube heyday, theyre enough to power an instrument and a mic. This IS afterall a small amp, and light enough to travel with.

Plugging in another 1/4 plug thru the other 1/4 input jack doesnt increase the volume level, as is the case with most older tube amps. if you have an older tube amp with at least 2 inputs, try it. Plug your guitar in input 1, then find another 1/4 plug and plug it in the other input. Dont plug any instrument on the other end of the second 1/4 plug/cord. Most cases, the bigger amps get a noticeable volume boost.

This one i got for ultracheap – so cheap im not even going to mention it here. Lets just say it wasnt free, but the price i got it for was insane.

The cabinet itself is in pretty good shape, considering its over 40 years old. There are some small tears on the silver pepper cloth tolex, but nothing big. One corner has scuffed and noticeable ding, exposing the plywood construction, but otherwise the cabinet is sturdy.

The one volume knob is a bit loose on the pot, and the pot itself is a bit scratchy. There IS some hum, albeit normal for all old tube amps, nothing loud and uncontrollable. In fact, plugging in, its fairly quiet and inaudible once you start playing. The inputs also tended to lose the signal intermittently, but the sound itself? clean til about 1oclock, then starts distorting past that. Fully dimed it has a nice singing sustain and respectable growl. All this at decent loudness. Great for bedroom or even backstage practices!

WIth the fix on the knob, the pot, and the input jacks, I ended up doing only one major mod (for now!). I noticed the sound was clearer and louder when i turned the amp around. Something was muffling the sound coming from the front. Disassembling the amp (as i always do with ANY amp i get), i found the front speaker holder had one 1-inch wide piece of wood (its actually fiberboard) that goes across the speaker front. Most likely it was thought of to prevent anything from poking thru the front. The problem is that it muffles the tone too much. For a small speaker, anything in front of it thick and wide enough will block the sound. So off it went. This resulted in a much stronger, fuller sound. This DOES alter the cabinet drastically, so think about it if youre into collecting pristine, untouched amps. But to me, i play ALL amps i come across with (every amp i own is a working person’s amp – it will get played and NOT sit in a cabinet or on a mantlepiece behind some glass just for show like a museum. i fulfill its purpose!). But the mod is invisible, and the better sound you get is more than enough to justify the mod.

The next mod is most likely a 3 prong cord, for safety reason. The cord on it now is clean and supple, and even shiny – proof that this amp hasnt been played much. This one is a keeper, unlike the Harmony amp i had a year ago. Although small and dinky, this silvertone has a power transformer as well as an output transformer. The Harmony amp had NO power transformer, which means it gets its tube fed with electricity STRAIGHT FROM THE MAINS! SHOCK HAZARD! Even the old man who sold me the harmony amp told me about it (used to repair tube equipments for 40 years) – there is simply NO WAY to make it safe, and admonished strongly to use an islation transformer. Even then its not a guarantee to make it fully safe. At least THIS 1470 has one, even if its a small transformer. For the output power, its sufficient enough.

Another mod i may end up doing is an external speaker jack. Not sure how a 10″ or 12″ speaker will sound with it, but a simple mod should help me find out!

Other than that, i dont see any other mod i would WANT to do on this amp. Maybe a better output tranny? It’s a great little amp  – the tone it puts out is at best sweet and singing, and privides little to fuss around with. This allows you to play more, instead of futzing around looking for that great tone. This amp ALREADY has that. just plug, and play!

1960s CORDOVOX CG TUBE AMPLIFIER

Jumping over several tube amp projects is this gem of a CL find: the CORDOVOX CG TUBE AMPLIFIER. or more precisely, tube amp AND tone generator. Research shows the amp is rated at 30W, using TWO sylvania and RCA 7591 power tubes, and TWO 12AX7 preamp tubes (that look to be telefunkens!). The tone generator uses 60 (yes – SIXTY!) tubes (6X8 and some other kind that arent useful for guitar amps), not including six other preamp tubes (a mix of 12AU7 and 12AX7 tubes, some branded Mullard, Lafayette, RCA, and a couple im unable to discern the name off). The 2 speakers are TRUE vintage C12R jensens, dated in 1964.

back up: what the hell is a tone generator?

Let me first preface that in the 60s, CMI (Chicago Musical Instruments) had several brands under its name, such as Gibson (for high-end guitars), Maestro and Kalamazoo (for their economy line), and Cordovox, for accordions.

Accordion manufacturers at the time (1960s) are scrambling to compete against the skyrocketing popularity of electric guitars. They tried to follow the concept of plugging in an electric guitar to an amplifier, but it was not easy to amplify an accordion back then (other than to simply mic it). Unlike electric guitars that simply had at least one pickup for all 6 strings, each chord on an accordion had to have its own tone bank that had to be amplified. In this particular Cordovox model (CG), this resulted in using at least 4 up to 6 vacuum tubes EACH chord. For a full octave (12 notes or chords), 60 tubes had to be used. These tubes had to be housed in a cabinet called a TONE GENERATOR, same size as the SEPARATE cab that housed the amp section and the two 12″ speakers.

The tone generator also housed the preamp section. This cab then connected to the main amp cab via a thick cable that contained all the wiring for powering and feeding the current to the main preamp tubes AND the 60 tone generator tubes (not including the SIX other preamp tubes for those “chord” tubes). I was able to count 71 tubes TOTAL (including the main amp’s tubes: one 12AX7, and two 7591 power tubes). OF those, NONE of the tone generator “chord” tubes are useful for guitar tube amps.

Curiously enough the controls are connected and housed in the tone generator cab, NOT with the power amp and speakers. To use this cordovox amp, you HAVE to have the tone generator cab connected via the thick cord from the main amp section. Both cabs are HEAVY (made of what looks to be thick birch ply), and each even had STOCK removable casters to roll it around. There is simply no convenient way to haul them around for a gig or a show. For accordionists this does NOT even include the accordion itself, so you really have THREE BIG pieces of heavy equipment to amplify ONE instrument.

This inconvenience may have led to poor sales, not counting the fact that in the 60s, demand for polka (or for accordionists for that matter) is seriously on the decline, now that rock n’ roll has taken over pop music. Of course that did not stop Lawrence Welk, but thats another story.

What’s more interesting about the tone generator is that it has another LONG thick cord that connects to the accordion. The end looks like a connector used for parallel cables for printers (if you can remember those). If you can decode the wiring, you basically have a TRUE analog synthesizer in the tone generator. I dont have that much patience, but apparently some guy did, and is selling the adapter for $585 on ebay. JUST THE FRICKIN ADAPTER! well, i guess if youre that dedicated to analog sound….

I’m not, but i do have SOME patience. I really did not have any idea what cordovoxes were when i responded to the CL ad, and was surprised how big these cabinets were when i saw them. But as i tried it, it sounded clean and LOUD. PROMISING! After bringing it home and examining the two cabinets i set out for a goal: make ONE whole amp out of the two. I called this Project: INTEGRATE.

first off, a WARNING:
HIGH VOLTAGES AND HIGH CURRENTS KILL! TO be specific, its not the voltages, its the amp (current). 0.5 Amp is all it takes for a human heart to STOP beating – the electrical mains in a regular household generates up to SEVERAL AMPS.
DO NOT ATTEMPT THE FOLLOWING PROCEDURES UNLESS YOU HAVE A WORKING KNOWLEDGE OF ELECTRICS AND ELECTRONICS. SAFETY FIRST!

At first, it looked easier to move the preamp section of the tone generator over to the main amp cabinet with the speakers. Everything is set in the main amp cab anyway. But that would mean i’d have to cut out a section on top where the controls would be. That means MORE work cutting, and hiding the cuts, etc., not including the preamp and amp integration (the meat and potatoes of the project).

The other BETTER alternative: move the speakers and amp section over to the tone generator cab. I have no need of the tone generator itself, and with closer inspection of the wiring, it seems that the preamp section can be removed safely from the tone generator.

at this point in time, id like to mention that i have NO wiring schematics to work with. An exhaustive and long search online turned up NOTHING free (they should be!), and the ones being sold did not mention the model CG at all. And true to its non-popularity, not much is known about this specific amp.

First thing to do: find out which of the 8 preamp tubes found in the tone generator affects the MAIN amp’s output. To do this (and this is potentially dangerous!), i pulled one preamp tube at a time while the amp is plugged in and turned on, and one of my guitars is pluggedin to make an audible sound (i set it on a guitar stand). If the sound cuts off, the preamp tube is part of the main amp’s audio path, and so it stays. 6 were taken out without affecting the sound, and the remaining 2 were on a separate protruded section. Wwhich now made sense, as right on the diagram on the piece of paper glued on the inner side of thr cabinet, it did indicate that they were “microphone” preamps. Without a true schematic, it was not possible to be fully certain that these were the only preamp tubes that affected the main amp, unless this somewhat dangerous procedure is done.

*ANOTHER DANGEROUS PROCEDURE!* The next is to find out what feeds the current to the guitar-useless 60 “chord” tubes (I decided to leave them in on the tone generator panel. At this time as the amp is turned on, you can see ALL SIXTY TUBES GLOWING!). Locate the wires coming from the connector that links the amp cabinet to the tone generator cab, and trace where they lead to. Once ive determined which of the wires carried the electricity, i turn the amp off and cut out the wires. I then turned the amp back on and see if the sound again is affected. After a couple of successful cuts were made, i saw that none of the 60 chord tubes lit up, while the amp is still on and the guitar is still making the same level of sound.

At that point its simply a matter of finding the rest of the wires that do NOT belong to the tone generator-main amp connection, and cut those out as well. I did not keep count of the different wires needed to extract the preamp section, but (relatively) safe to say that i was able to successfully isolate it. With the tone generator’s wiring isolated, it was easy to lift it out of the cabinet (held in place to ingeniously pivot and swivel out via 2 blocks of wood and a retaing metal lock). Once the piece of metal is out of the way, its an easy lift out of the cab. NOW i have the tone generator out of the cab (with the panel of 60 tubes), and i can easily move the main amp section and the speakers over to it, where the preamp and the controls were.

easier said than done.

The idea is to move the speakers also, and that meant moving the front panel that they were installed on with them. Both cabinets have the same dimensions and size – the speaker panel is an exact fit inside the tone generator cab. Better than cutting out an imperfect circle out of the tone generator cab front panel (more unnecessary cutting). But before that, the front panel of the tone generator HAD to be removed. A swap was needed, but removel isnt the easiest. There were screws and nuts in the way among other things. All these had to be cleared before any easy removal and swap can be done.

Once they were removed, the front panel/grill cloth swap was done. One issue is that the speaker panel was thicker than that of the tone generator cab, and a small section on the top had to be sawed out to thin it enough so the control panel can fit right back in. In my zealousness and (running out of) patience, i cut out a wider section than is necessary. Not that big, but if i had seen it only needed less than an inch, i wouldnt have taken off a good 12×3 inches worth of ply. It didnt really matter in the end- its in the inside, and totally invisible, so it wasnt a big deal. This wasnt a collectible either. Rare maybe, but not much worth in the musical circles. Bottom line: the controls are back on and fit the way it did.

After that, it was smoother sailing – 4 holes drilled from the bottom (like the main cab) to securely screw the amp in place, and everything is now integrated in one cab.

A note about the control panel: aside from the usual toggle switch, lamp, and TWO mic inputs, there are 3 pots: one for accordion volume, one for mic volume, and one MASTER volume. Yep – no tone pots. which i find ironic to have NO tone controls for a TONE generator. But this lack of tone controls made for a much simpler amp to play. Plug in, dial your sound, and GO!

The tone? USEFUL and BEAUTIFUL! With a MASTER volume dimed, you can dial in the loudest CLEAN this amp can push at 30W, just back off the mic volume to about 10 oclock. Dial past that and you start overdriving the sound nicely, and diming it produces a gritty distortion that has bluesy sustain. Another cool thing about it? keep the mic volume dimed but back off the master volume, and you have bedroom levels that retains that overdriven tone!

I decided to unhook the connections to the accordion pot, since there was NO accordion connection anymore (via the tone generator), and it also contributed to extraneous noise. This pot can be used for a tone control later on.

It does hum at high MASTER volume levels, but backed off the hum diminishes and even disappears, while still having decently playable output levels.

It has to be noted that at this time, only the extraction of the preamp section and its integration with the main amp section was done. Cap changes and rewiring has yet to be completed. But it IS playable now.

What else needs to be done? there is at least 6 feet of thick cabling that connects from the main amp to the preamp, a very good recipe for hum. This has to be shortened as small as possible. Maybe even completely integrate the 2 preamp tubes with the main amp. Looking under the chassis, there is a LOT of space that can be used. Simply a matter of tracing the wires from the amp to the preamp. I shouldnt say simply – none of these are!

The electrolytic caps are still original (albeit working well). For safety’s sake, these need to be replaced. This could be another source of extraneous hum.

At any rate, the payback is sweet. For about a couple of hours there i tested the amp using my Gibson SG (with high output dimarzios), the 335 clone Stagg equipped with GFS retrotron Memphis and Liverpool pickups, the dano-corrected Hearsay with the chimey single coil lipstick tubes… i must say, it was fun as hell! both volume controls dimed, it was great to hear the overdriven sound, while still hearing the clarity of the treble notes without the piercing sound accompanied with loud amps, the growl of the distortion without sounding buzzy or fizzy. Just right!

One thing i DID notice was how the speakers were connected – PARALLEL! Usually the speakers are connected in series. Again, without schematics, im not certain if this is exactly how the speakers were supposed to be connected in cordovox amps. I have no knowledge what impedance the output transformer puts out, and i havent yet measured how many ohms the speakers are (i will soon). But i do know that the original Jensen C12R’s are rated at 25W each with an impedance of 8 or 16ohms. If the speakers were each 16 Ohms, we have a total of 8ohms for both in parallel (32 ohms in series). If the speakers were 8 Ohms each, we have a total of 4ohms in parallel for both (16ohms in series). This would mean the amp is putting out between 4 to 8 ohms. Regardless, its much safer (in my opinion) to hook up the two speakers in series instead. This gives the amp a little more leeway for the output transformer, and if one of the speakers blows, you know immediately (there wont be any sound at all). I guess thats one reason why they were hooked up in parallel – if one blows, the other continues to work.

So i ended up rewiring the two speakers in series, and BOTH still worked! They both still sounded warm, has that singing midrange-y distortion in the lower registers, giving the amp that thick growl when doing power chords.  playing licks provides a smooth even tone across the frequencies. This is no metal machine, or a super-clean jazz amp. Think late-60s heavy rock sound, without being over-the-top. Bottomline is that its just a fun simple amp to play with!

Another note on the main amp – it had an extra output 1/4 jack for another set of speakers! This allowed me to test the amp with other speakers. One was a 1×12 epiphone cab (a mate to the valve jr. head). It sounded more full, most likely due to the closed back, deeper cabinet it had, and the higher wattage (70W). I have yet to test it with my favorite set of speakers (jensen C12P series froming from Bell & Howell cabinets , rated at 25W). Later on i aim to test the amp on C10N jensens as well from my Kalamazoo Bass 50 amp (rated at 50W). Should be MORE fun!

Some to do list:
1) shorten that long cable cord to the preamp
2) integrate the preamp to the main chassis
4) electrolytic cap change
5) play the living daylights out of it deep into the night!  😀

UPDATE:  January 10, 2010

7591 tubes are relatively expensive (NOS, used, or new). there is however, a way to convert 7591 amps to use 6L6 tubes. Thanks to this link:
http://www.triodeel.com/7591.htm
in case that link doesnt work:

Quick fixes for 7591A amps

1. Quick change from 7591 to 6L6-GC or Russian 5881 (Scott and Fisher amplifiers)

On these amps the cathodes of the 7591A’s are usually tied together. Find the wire connecting between the two cathodes and remove it. Some amplifiers have a balance control between the cathodes, this must be disconnected.

NOTE: If a bias rectifier is used and has not been changed we recommend doing so as the original part is selenium and will deteriorate and eventually fail. Also, the replacement silicon rectifier will give slightly more negative voltage. If the amp has tubes (usually 12AX7’s) with filament voltage sourced from this rectifier, a series resistor (usually between .5 to 2 ohms) should be installed in series with the filaments to produce a DC voltage across the filament about 10% or so less than the normal filament voltage. (e.g. about 5.2 to 5.7 volts for a 6.3 volt filament or 10.5 to 11.5V for 12.6 volt filament).

Assuming that amp uses fixed bias (e.g. negative voltage on grids, not a big cathode resistor), do following:

A. Find the base diagrams for 6L6 and 7591 (remember they are the BOTTOM view of the socket.) Rewire socket as follows:

7591/7591A    6L6/5881
Control grid (g1)    Pin 6    Pin 5
Cathode (k)    Pin 5    Pin 8
Other connections are the same.

7591A has two screen leads pin 4 and pin 8. If there connections on pin 8, move them to pin 4.

B. Between the cathode of each tube and ground, install a 390 ohm 5 watt resistor. Parallel to that a 220 uF 50 volt capacitor. If the tube is drawing the correct amount of cathode current the voltage across the resistor will be between 19 and 20 volts.

If the voltage is too high, make the resistor higher in value, or vice versa. A small value pot could be used for this adjustment. The voltage drop should be equal to 50 ma no-signal current.

Optionally you can install parallel to the resistor a 1 watt zener diode (in place of the capacitor) equivalent to the correct voltage drop or a little higher, this gives 100% fixed bias operation. If amp is cathode biased, changing to 6L6 tubes is a simple matter of changing the connections as noted above, then increasing the size of the cathode resistor by a factor of about 2.1 or 2.2 times the original. For example, if the original was 170 ohms, increase to 390 ohms. The replacement should be twice the wattage of the original, and the bypass capacitor parallel to it should have twice the voltage rating.

C:\>The other, more difficult way to change bias is to change the bias supply either to (A) (if bias supply is 26VAC, check it first!)to a voltage-doubler (if you need explanation, probably better to skip this idea), this requires upping voltage rating of the bias-filter caps, most of these amps derive DC for 12AX7’s from bias supply, and will require rewiring filaments of 12AX7 from series-parallel to straight series connection, possibly with a series resistor to bring filament voltage back to correct level (between 10.5 to 11.5VDC across each tube). Then rechange the bias resistors ie voltage divider to produce approx -37 volts. Alternately, you could install pots, and copy bias circuit (except for rectifier arrangement) out of ST-70 Dynaco, installing 10 ohm 1% 1/4w resistors for metering cathode current (set at 50ma per tube)

Some amps have a 50 or 52VAC bias winding, this simplifies getting the higher bias voltage need and only requires making changes in the voltage divider as noted above.

You may wish to fiddle with the feedback resistor. Since the gain of the amp is cut more or less by half, try cutting the value by half.

Sometimes the preamp section will not put out enough voltage to cover this change in sensitivity, most of them will. You will notice you must increase the volume control a bit more for the same output.

However, not adjusting feedback levels with the tetrode-connected output arrangements usually employed will often result in pretty flaccid bass response.

FENDER SUPERCHAMP XD

One of only two amps i own that are made in the “modern era” – built in the past 2 years. This is an updated version of the venerable classic Champ that fender made in the 60s, well throughout the early 80s. The difference being it has a 10″ speaker (original Champ has 8″), the added DSP effects section, and no tube rectification (this has been a debate i some circles, whether it was good or not to have no 5Y3GT tube rectifier in the new amp). But it still retained the 1x 12AX7 preamp tube, and 2x 6V6 power tubes, effectively giving a 12-15W of power output. The rest of its description can be found in Fender’s Own Website.

Im not going to elaborate much with it’s sound – there are tons of material written about it, experiences that i must agree with. Soundwise, it is indeed a champ. Plugged in, it provides true tube tone, and remains relatively clean even as the volume is dimed. Its versatility (for better or for worse) comes in the DSP effects that engages either through the switch on its front panel, or via the optional 2-button footswitch. It can stand alone and produce plenty of sounds without any additional pedals linked to it. Simple, which is why i think it shines.

That being said, this article is for the cabinet repair done on a new champ XD (or as its known around the net as SCXD). Bought off of ebay for half its MSRP, it came in its original box, pristine and completely stock. The big issue as described in its auction listing was that it fell on its top right corner (probably during its original shipping to the store or on its way to it), cracking that cabinet corner, separating itself clean through. Ugly to say the least. Plugging a guitar through it has not affected its tone or any of its other DSP features. It was in excellent working condition, like new off the shelf (which i think it is – still has all the decals and tags still attached to it), only with the big crack.

This was easily fixed with wood glue and corner braces screwed in that run along the crack. After a day of curing, the amp is now probably sturdier than it it was new and unbroken – i grabbed it on its handle and yanked as hard as i can picking it up, and even swung it around literally, and it held well (its not that heavy with its stock speaker). The only thing that gives the repair away is the tolex tear, and only thru very close inspection. It looks like a production flaw, where the ends of the tolex meet unhidden (usually found under or away from sight).

One of the other things i did was to change out the tubes with NOS Magnavox 6V6s and RCA 12AX7. The Electro-Harmonix tubes are probably good (and ARE strong as measured with my tube tester), but i prefer the sound on the old stock tubes i put in. Its relatively weaker compared to the stock tubes that came with the SCXD, but that gave it a more mellower tone for me. If you search the internet, you can find instruction on how to properly adjust the bias with the handy trim pot that was built in to the PCB.

One other thing i also did was switch out the stock speaker. It was not bad at all, in fact i could have left it in there, and still sounded good. But i ended up changing it to  Jensen C10Q. There are other suggestions that are believed to be better, and makes the amp sound louder (look online), but the jensen speaker gave the amp a much better clarity (compared to the stock), and since its got slightly higher wattage, it made it break up less (if it even broke up the sound at all). My goal was to get that fender clean sound. With the speaker, i have achieved this. In fact it gave it a better clarity at the low end. The pic shows the speaker in the background, with the chassis pulled out to measure the current flowing thru the NOS tubes just put in (before the bias adjustment with the blue round trimpot. You can see the older tubes running cold, drawing not much current).

The last thing i did was to put the rocker power switch on the front panel, right next to the pilot light. I never understood why a power switch would be on the back. I find it very cumbersome and annoying to have to reach around the back and fumble around to find it (its not like its the only thing on the back panel). So i cut away a rectangular hole to the right of the pilot light, stuck the rocker switch in, and simply moved the leads along with it. They were long enough to reach to the front panel, but had extended out a bit on the back that it pressed against the PCB that the electronics sat on. Its covered with insulation, so it wasnt cutting into anything (and there weren’t any signal path to short out). If you’re going to do this, make sure the power is unplugged. If not, it serves you right whatever happens to you.

I was tempted to do the famous mod that people had already done, which is to add a line IN (the SCXD already has a line out). It basically cuts out the DSP and allows you to plug in your own. I like the amp as it is, and personally i don’t use sophisticated DSPs (other than my chain of pedals which i use sparingly). I may do it next time, just to add more tonal options (hey, thats why i do these!).

1967 KALAMAZOO REVERB 12

This is one of those clunkers-turned-golden stories. After the mods ive done, this is NOW my favorite amp (so far!).

This is the 2nd hardest to come across to Kalamazoo amps on ebay, next to the Bass 50. Models One and Two, and even the Bass 30 are easy to find, or come up in listings often enough.  Its tube complement can be compared to that of a Vox AC15 (12AX7 preamp tubes, and two EL84 power tubes).  Its been described to sound close to a Fender Princeton. This had a tremolo and reverb (obviously), thats footswitchable. The only other Kalamazoo amp that had an extra feature is the Model Two (tremolo). The rest have the rudimentary and simple Volume (or LOUDNESS) and Tone, or Volume and Treble and Bass.

Most of the mods that follow are based on the recommendation by Miles O’Neal at his site Kalamazoo Amp Field Guide. A Highly recommended site for all things Kalamazoo, to which we are grateful for.

The purchase:
Ebay is a great source for used, second-hand and vintage equipment that may otherwise not have seen the light of day, or the world’s exposure. Suddenly sellers have the whole world to sell to, and buyers now have access to remote far flung areas they cant reach or get to. I wont talk about how it has spawned unrealistic prices for what is usually less than a tenth of its value on most items. Instead i will talk about how there are still a lot of GREAT values to be had, if you know what to look for, and what to do with them, coupled with a certain amount of bravery (or in our case, foolishness!).

The seller described the amp as:

This Kalamazoo Reverb 12 amplifier is in good cosmetic condition but needs work. When turned on it either makes no sound or a very weak sputtering sound. I have changed the tubes so they are not the problem. I can see a number of new orange drop caps on the chassis so it looks like someone has tried to work on the amp. The amplifier has reverb,tremolo ( the pedal is included) and a 12″ speaker. I am sure this is a cool sounding little amp when working properly and will be worth the time it takes to fix. Shipping will be by FedEx ground. Figure 30 pounds from 23505.Use the FedEx website to figure shipping. The eBay calculator always show a shipping cost that is higher than it should be. I will not ship overseas … OVERSEAS BIDDERS PLEASE DO NOT BID.

took a chance, and I won it. Cheap! Within a week, it arrived.

FIRST LOOK: Outside
The seller packed it well, with NO annoying and messy peanuts (sellers, take note!). It arrived with no noticeable shipping damages. The tubes were in a separate box (good job!). What i saw inside was a surprise.
The amp was very clean! Usually amps this old have accumulated dirt, grime, dust, dried spills, and they usually are not easy to clean on those rough hide tolex covers. Colored black doesnt help, as it exposes those hardened dust between the light ridged texture of the tolex. This amp only had slight signs of this on top – the front sides and back look darker (less dust) than the top. Its like this amp had been stored with a cover for a while not long after it was bought.
It does show rust on the metal corners and on the handle ends. The front chassis was a bit dusty, and the knobs had the typical dirt on the sides from dirty hands. Its gonna get soaked and brushed clean.
The handle itself hasn’t had much wear on it, proof it hasnt been lugged and taken around much.
The grill cloth didnt have any stains, or has not been changed . Its the original cloth – its the same as the ones in my 2 Kalamazoo Bass 50s, and Models One and Two. The cloth had been slightly pushed in, as on an angle you see the round outline of the 10″ speaker circumference.
There were maybe 2 visible edge tears the size of a quick release tylenol pill, but no big rips. The corners are sharp, and the bottom shows no scuffs. Even the 4 rubber feet are still intact.

Second Look: Inside
To get to the chassis, The top back panel had to be removed first by removing 5 black screws. There were considerable rust on 3 of them, and one corner had already been stripped. Not unusual for an older amp, but the other 5 Kalamazoo amps ive worked on didnt have this much rust on them (or at all).
4 screws from the underside of the chassis needed to be taken out to slide out the chassis itself. It wasnt an easy slide out. The amp handle had screws sticking out from the underside that prevented a smooth pull – the chassis had to be lifted up to get it past the long screws.

Inside there were 3 electrolytic capacitors (1x47uF, and 2x22uF rated 450V) that stood out from the rest. A good start, though sloppy in execution. They were floating, held up by its long leads. Some cobwebs, usual dust, and what looks liks some odd dried out spills of some sort (not sure where it came from). The rest of the capacitors were sprague orange drops. The seller described it as having some orange drops replaced. I couldnt tell which ones, as they ALL were orange drops, and were all equally a bit dusty. There were still some non-electrolytic polarized caps that had not been replaced.
It still had the 2 prong cord (about 12 feet long!), and it was supple and firm, not dried out or brittle. The Footswitch was present! There was sadly NO schematic found. Important since this will easily tell me what version it was (theres at least 2). Now i have to compare the component placements with whats online.
The tubes that came with it are not original (theyre usually CMI branded), though they seem old stock tubes. 1 Fender branded 12AX7 made in the USA (most likely a westinghouse), a raytheon (made in japan), and an RCA. The EL84 tubes were magnavox brands, 1 made in italy, the other made in austria.
Though described as 12″, the 10″ speaker is not original and of unknown brand. It had an unusual square ceramic magnet (signifiying it may have been from the early 60s). There were some numbers on it, but it didn’t match any of the speaker codes found online. When it was taken off, it weighed very lightly, compared to, say a Jensen C10Q. The cone was intact and clean.
The reverb tank is about the size of the amp’s inner width, and had 2 spring in it.

PLUGGING IN!
True to its description, it sounded weak and sputtering! It’s hum and hiss was louder that the actual sound. Turning up the reverb only made it squeal. The Treble and Bass knobs had little effect. Connecting the footswitch DID turn on the tremolo, but it was spotty and intermittent.

THE MODS

SPEAKER CHANGE (and correcting the weak sound)
At first i thought the speaker was the problem. The description said the tubes are good (they were), but did not mention if they did anything to the speaker. First reaction (as described earlier), was that the speaker was very light. Even the CTS speakers from Model One Or Model Two are heftier. I swapped with it a MUCH heavier one, an oxford speaker (circa early 80s), thats similar in construction as a Jensen C10N, about 50 to 60 watts rated. The reverb 12 puts out about 10 to 12 watts. My aim was a better speaker than what was in it, not necessarily geared towards , say, an earlier breakup, or a certain sound. The higher wattage of the oxford speaker will tend to lean it towards a cleaner sound with a later breakup. At least, thats what i expected.
After a temporary connection, the sound was much fuller! And it sounded good too. I decided to leave the oxford speaker on, and reconnected the wires. Trying it again, the sound went back to being weak. It wasnt the speaker (but i kept it). I remembered NOT hooking up the reverb tank connections during the temporary connection. I took it off, and the full sound was back.

It was the reverb tank.

At that point i though of replacing it -i had 2 extra reverb tanks, one was the same size. 4 screws unhooked it off the cabinet floor. Once out, i noticed something wrong. The connection coming from the chassis was going to the OUT RCA jack of the tank, and the speaker connection was going to the IN RCA jack of the tank. I swapped those around, turned it on and got a full sound, with reverb!

So it wasnt any of the bad cap job, the speaker, or the reverb tank, it was just simple wrong connections. On to the rest of the mods!

TUBE CHANGES
The reverb is out of control (past 10 o’clock it feedbacks loudly). I decided to swap out the existing Japan Raytheon 12AX7 that feeds the reverb with a much lower gain preamp tube (a USA Tungsol 12AV7). Maxed out, hum feedback hum is gone. Later, after some cap job improvements, i was able to put in a slightly higher gain USA Ultron-brand 12AT7 with no feedback at max. I may try a 12AX7 again later.

MORE CAP CHANGES
There was still considerable hum and hiss and static going on. I had a new multi-cap can (4 values of 40/20/20/20 at 500V) ive been meaning to use on either Bass 50s, but this should make it a neater layout inside the chasses with just 1 ground to deal with. It can also free up the 3 floating caps for use on other areas.
I followed some of the recommendation from the Kalamazoo Amp Field Guide on where to place what caps – you can read more there. I didnt have some of the values recommended, but radioshack (gasp!) had some for relatively cheap. I coudnt wait for any online orders with this cap job.
This mod took the most time, done in between other mods, since it was mostly a juggle of what values sounded better for me, or which ones made the hum less.

NEW AND BETTER FOOTSWITCH (and removal of the useless PHONO/TAPE RCA INPUT jack)
There were 2 schools of thought i considered. One was to keep the same footswitch and just replace the very long 4 conductor gray to eliminate the intermittent reverb and tremolo. Or (after opening up the footswitch  and seeing its simplicity) MAKE a new one.

2 footswitches, a die-cast aluminum box, and a long 4 conductor cord can easily be put together. The problem was the connection TO the amp – it was an older 4 connection plug that can usually be found in older organs. But its not usual enough to appear in online stores that carry old stocks, or in salvage parts on ebay. So replacement of that is scratched – a NEW plug is needed. This means replacing the existing jack in the chassis, but with what, i havent a clue at that time.
I examined the connections to that jack in the chassis, and looked at the schematics online. Instead of 4, it outlined 3 connections (2 were combining) as it goes into the amp. This makes it easier, as i can use a STEREO 1/4 jack and stereo plugs, the same kind that most modern replacement footswitches use. Instead of MAKING a custom footswitch, i decided to use an already-made 2 button footswitch that i got for my Peavey Valveking 112. Making one is easy, but the time and cost is about the same as getting one ready to go. My main goal was to make the footswitch easily replaceable, in any case it gets lost or damaged.

I did make a mod in the footswitch – removed the LED connection out of the circuit, and hook up the conductor wires from it direct to the switches. It didn’t work with the LED leads in the circuit. It may work, and even light up the LEDs with another battery mod, but that just makes it more complicated, and may introduce foreign voltage into the amp once engaged. Keep it simple, stupid.

Connecting to the original reverb jack was simple – nothing needed to be desoldered and unhooked. All i did was piggyback off of it, and connected to it an open circuit stereo jack (again from Radioshack). The placement was crucial – i didnt feel like drilling a hole in the chassis, but still wanted to make it an easy access. I decided to remove the PHONO/TAPE INPUT RCA jack (and its resistor and wire connections), and put the reverb stereo jack there instead. It was a working input, but it was useless. I dont even use the second instrument input in the front. It was perfect – now the NEW footswitch can easily plug in without having to turn the amp over, or using a flashlight to find AND plug the reverb footswitch plug correctly, as is the case with the original. Try that in a dark stage.

Take note that this did NOT require removing and desoldering the reverb connections, nor did it require routing or drilling extra holes in the chassis or cabinet. You get to keep it relatively stock, and you save the original footswitch for posterity. The new footswitch is far more durable and can take the abuse from heavy leadfoots (being made of stamped metal). The original was made of molded RUBBER, with a fiberboard slide-in base. How it survived l those years is a wonder. This also makes footswitching easier with the wider space between switches. There was only a full inch between the switches on the original, on a 4 x 2 1/2 frame.

ADD MORE dBs! (make it a noticeably LOUDER)
On some older amps with  2 or more input jacks, using an empty plug on the second jack makes it louder. This is true for the reverb 12 (as is for the Bass 50, but not for the Model One and Two).
This was a no brainer – of course lets make it louder! But using an empty 1/4 plug into input 2 not only made it louder, it also induced some buzzing. The jacks were closed circuit, and plugging in to them opens a connection that makes the amp louder (which makes sense if you use more than one instrument in the amp, like a mic or as they used back then, a phonograph or a tape player). The trick was to sandwich a piece of stiff cardboard between those two poles that connect to separate them. The tension betwen the poles will keep it in place. No metal touching the tip or sleeve to induce unwanted buzz (in theory the metal parts may be acting like an antenna when an empty plug is used). Plugging an actual instrument still works.

SAFETY: 3-PRONG! (and moving the Power switch  and Fuse holder around)
This came later, as i did not have any toggle switch to use (another ratshack buy).
The Kalamazoo Amp Field Guide suggested to move the Power switch away from the tone controls as this induces hum, and use a 3 way  (On-Off-On) DPDT toggle switch, to have a standby in the same switch. I decided to use just a simple On/Off toggle switch. Per the site author himself, Miles O’Neal doesn’t feel this is necessary, though old-school thought strongly suggests making the standby switch necessary to prolong tube and amp life, as direct power On causes too much of an electrical shock to the tubes. The practice is to warm up the tubes for a good minute BEFORE turning on the rest of the amp. I dont practice a lot. 🙂 (OK – i may change this to one with a standby option later).

The toggle switch is now in its rightful place, where the fuse used to be on the front (next to the pilot lamp), and that left me thinking where to put the fuse holder.  I decided to “hide” it, away from kleptomaniacs and the occasional knob twister that cant refuse twisting the fuse holder open (and consequently either losing or taking the fuse, and sometimes the fuse cap too). It needed to still be accessible easily, but away from the misguided and curious. Taking out the 2-prong cord revealed the hole it came in thru the chassis. It was another perfect fit! With the top back panel on, its hidden – you have to turn the amp over and look behind the panel to see it, and its still within reach and easy fuse replacement.

The 2 prong AC outlet on the back has been effectively removed, for safety purposes.

And where does the 16-gauge 3-prong cord go thru now? I decided against using a 3 prong receptacle, the kind you see on computer power supplies (and in most modern amps nowadays). Its a perfect fit for where the AC outlet used to be. Its a decent way to replace the cord in case it gets worn or damaged, but the cord is also easy to pull off and lose. A permanent cord will NEVER get lost or forgotten when packing. Not good when your on stage and cant plug in to any electrical outlet. The new 3-prong cord now goes thru where the AC outlet used to be. I still need to put a rubber washer around it to prevent it from moving around the metal. At 16-gauge its thick enough to withstand cuts, but still a good idea to keep it stable. A knot inside prevent its from being tugged out.

EXTEND THAT EXTENSION SPEAKER
Plugging in a second speaker cabinet in the extension speaker jack cuts out the main speaker. Ive always wondered why this was so, spoiling whatever speaker you already have. It should be named Optional Speaker Output. So i decided to jumper 2 of the leads in the jack to keep the onboard speaker on with whatever speaker plugged in.
Then i thought about the other school of thought: Maybe some gitar cabs sound better on its own, and sometimes you just want to use another speaker for testing, recording, or just A/B’ing speakers. I ended up adding a small On/Off toggle switch to facilitate this option. Again, placement was key. I can either drill a hole thru the chassis on the underside, or somewhere easier to get to, like next to the output jack itself (where i have to drill not only thru the chassis, but also thru the top back panel). The less drilling the better. I ended up widening one of the holes that original held the AC outlet in place. This was right next to the power cord. Now if i have a speaker cab, i have the option to use it only, or in conjunction with the main speaker. The way it looks (and sounds), the speakers are in series, so theres no danger to the amp working harder (parallel speaker connection creates a lower resistance, making the amp work harder than it should. For a tube amp, this is dangerous and can lead to premature amp death. Or replacement of expensive output transformers. That’s wallet death).

EASIER CHASSIS ACCESS
This was outlined in the mods that Miles O’Neal wrote for the reverb 12. I didn’t fully understand it until i did the mod on my own, and reread it again.

The handle is fastened to the body (under metal covers) with the use of 4 screws, 2 on each end. These screws extend a good 1/2 inch down. When the chassis is removed, these extended screws prevent it from a smooth pull out. There was a cleaner way that Miles O’Neal described step by step. I Opted for the easy way – snipped them with a pair of metal cutters. I could have used my dremel for a much cleaner cut, but i was out of cut off wheels. After cutting them off, i taped over them with a thick adhesive tape. Theres still a bump, but nothing to fully impede an easier slide out. This beats having to removed the tubes, then the whole chassis, and finding a way to balance it on a table, when you can simple pull it out and start working on the components.

SUMMARY
Other things i did were more cosmetic – cleaned the front face with a damp rag, wiped down the tolex, and applied car dashboard polish to it. Also pulled off the knobs and found it had thick cobwebs behind 4 out of the 6 knobs. Cleaned them out, dipped them in hot soapy water, and scrubbed the remaining gunk out.

Some other things i may do:
– Remove the rust off the metal corners, and possibly spray it with clear rustoleum. Replacement is an option, but i’d like to keep it as stock as possible, with as little cost as possible.
– Replace the rusty back panel screws, maybe fill in some of the threaded screw holes.
– Reseat the grill cloth to make it more taut.
– Figure out a way to get reverb to the extension speaker. The reverb only works when the main speaker is on, nothing when the other speaker is on its own.
– Figure out star-grounding to try and reduce any other hum. With no guitar plugged in, Its quiet up til 11oclock on the Loudness knob. Past it it starts to have a mild hum.

This amp is made to RAWK! On the Volume/Loudness knob, the sound stays clean til 10o’clock, then starts to get gritty at 12o’clock, then starts to really breaks up at 2o’clock. Its the hounds of hell dimed – PURE TUBE DISTORTION! No pedal needed, unless you want to fuzz out more, or do more metal tunes, then a pedal can push it further. But nothing beats playing on simple direct guitar-amp connection.  Ive initially used a Danelectro Transparent Overdrive (used as a preamp) to try and boost the signal. I ended up liking the amps own tube overdriven sound, i ended up using the Dano in reverse, as a “cleanup” pedal – it dials in a cleaner sound with the volume and gain rolled back when its engaged, and when its off it kicks in the amps overdriven sound.  Ive discovered since that this is  one amp ive been able to play that cleans up well using the guitar’s own volume and tone controls, and now i dont use the Dano pedal at all.

The reverb is decent, not overly cavernous, but just right. The tremolo does its effect unexpectedly well. It does a simple soft modulation, and no option for a hard stuttering tremolo effect. Fine by me, because what it does is much better – on an overdriven sound the tremolo is more apparent at the DECAY of the note. You hear the effect more towards the end, instead of throughout the note. Its soft modulation makes it sound “more natural-sounding” too, like a singing voice vibrato when a note is held. I can still use my digital reverb and tremolo pedals, but if i have to use just the onboard effects, i’ll be fine.

This amp’s resurrection from near-death is a joy to think about. It makes it even sweeter that it cost well below what it normally goes for, and its cost of repair and improvement is even less than expected. Its playable again, ready for another 40 some odd years of playing. Im thinking this probably will be played more in the next 10 years than it did the past 40!

CHEERS!

1967 KALAMAZOO BASS 50

This is what started it all.
Ive always believed and liked the way electric guitars sound thru a tube amp. Maybe its the way the older recordings sounded, but solid-state amps dont quite capture that tone. Not that solid state is bad – its different sounding. Tube amp purists deride it, since compared to tube amps it sounds too sterile. There is no “warmth” to its tone. I call them snobs.

In defense of solid state amplifiers, theyre safer to use,require very little maintenance, and far cheaper! These days there are modelers and effects processors that can emulate ANY tone you wish, and yes, even  that “tube sound”. Solid state amps can provide quiet and clean operation, and can crank out much higher wattages, and are far more versatile soundwise. Its nt easy to get really good metal riffs on tube amps – solid state amps can get meaner and nastier. If thats your cup of coffee.

But going back to my point – why bother trying to emulate a tube sound, when you can get tube sound with, uhm, TUBE AMPS? With that in mind, ive found that there simply is no substitute for the real thing.

After my very first tube amp (a 2008 Peavey Valveking 112), i decided to check out the used section in a store chain. On  the back wall stood this old Kalamazoo Bass 50 amp along with other older amps. It was relatively cheap compared to the marshalls and fenders and musicmans that are priced 3 to 4 more times higher. I did not pay any attention to it until i got home, and looked it up.

In a nutshell, these Kalamazoos were old and (or in the guitar amp vernacular) vintage. Made in the mid to late 60’s, they were in a limited run, as the parent company CMI (which also produced the more expensive Gibson amps) had been sold, which effectively ended the line immediately. They were Gibson’s economy line then.

The website created by Miles O’Neal dedicated information regarding these amps, and thats when i found out that Bass 50’s are not that easy to come by. I immediately went back the next day and bought the amp.

October 13, 2009, 11:58:54 AM
The Bass 50s are admittedly a beast to tame! that being said i now have 3 (theyre that fun to work on and play on!).
I find that they always have quite a bit of hum, but changing out the electrolytics help. The first 2 i replaced with values TWICE the schematics/original can cap values. The third i kept as close as i can.

but heres something i found that helps A LOT – replace the non-electrolytic POLARIZED caps under the chassis. Theres 2 of them, i think. one is close to the 6EU7’s and another is right on one of the EL34’s.

The rest of the non-polarized caps can be left alone, in my opinion. i kept them intact on the third. i replaced ALL of them in the first using orange drops, and the second one i replaced them all with polyprop caps. Again this is just my preference then, and now im going back to restore what i can. Although Paper-in-oil caps is an interesting alternative…

Definitely look for those POLARIZED (non-electrolytic) caps and replace them.  I used the same values. Not sure what tonal difference or effect it will make if the values are greater, but im more than satisfied with the greatly reduced hum.

i recommend also going to the Kalamazoo Amp Field Guide Site, by Miles O’Neal. Its THE bible for the kalamazoo amps. I gathered most of what i knew from there.
http://www.netads.com/~meo/Guitar/Amps/Kalamazoo/

December 08, 2009, 02:39:05 PM
Of the 3 Bass 50s i have, one of them ive turned into a head also. I picked this one since the OT started to go. I was able to locate an OT replacement from www.tubesandmore.com (with a selector for 4/8/16 ohm taps), and its been fine since. Im using it to switch from EL34 to EL84 with the help of yellowjackets. A lot earlier breakup as expected. Gives me this real nice fizzy/buzzy/industrial top end sound (while still retaining a good bottom end) when i place a combined dano coolcat transparent overdrive and metal stompbox in front of it (using a 95 P-Bass Special “cowpoke”). EL34’s are just too dark to produce that sound.

I used an old empty Gibson head i found on ebay. I forget the model of the gibson head (it was i think for a solid state amp), but it was perfect since its handle matched the Bass 50’s original handle (those handles are HARD to find – no one makes a replica of them anymore!). There was even a rectangular hole cut in the bottom to accomodate the bass 50 cab’s protruding handle. Its about the length of the Bass 50, making it look like it was made for the Bass 50 when it sits on top of it.
Tolex was different though – the head used what looked like a basket weave pattern/texture. Im still looking for a matching grill cloth, or at least similar shade/color. The head was constructed out of MDF board (yeah, it wasnt even thick plywood or solid wood). Pretty sturdy though.
What i liked about it most is that the ease of access to the chassis and tubes now. Plus it now has the height to accept ANY EL34. JJ’s dont fit in the original cab, i had to find EH tubes to make it barely fit (as im sure youve experienced). A little long compared to the Bass 50 chassis, but the head give it plenty of space now, even for the standard sized yellowjacket converters with EL84’s.

Ive toyed with the idea of dispensing with the flip top control panel, as the harness (as you suspect also) can add hum. But i do have one Bass 50 thats relatively quiet (it starts to do so only when the treble is turned up at higher volume levels) so im keeping the harness for now (less of a project, plus i kinda like its fliptop quirkiness). Im putting the control panel on the front this time, but im debating if i should keep the fliptop action or just have the panel permanently set facing forward. that should allow me to shorten the harness, less chance of wires shorting or having a break due to a constant fliptop action.