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Author Topic: CORDOVOX CG TUBE AMPLIFIER  (Read 22925 times)
chosenfool
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« on: September 14, 2009, 12:01:04 PM »

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 12x3 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 completey 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 paralle 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 1x12 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!  Grin

(pictures to come!)
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chosenfool
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« Reply #1 on: January 10, 2010, 06:13:28 PM »

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:
Quote
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.

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chosenfool
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« Reply #2 on: January 16, 2010, 03:57:11 PM »

i got another CG Amp and Generator off ebay, in black. came to about a third cheaper than the first CG set i got (shipping included). The tone generator came first today, the amp and speaker is scheduled to arrive on monday.

Simple inspection shows different branded preamp tubes than the first set. This second tone generator shows mostly US-made RCA 12AX7 tubes, and one korean made realistic brand 12AX7. Glass construction of it looks suspiciously like old mullard 12ax7 tubes - has a more square top, with the cross-sectional ridges. Nice feature of this one tube: has gold-looking pins! copper maybe? I put the tubes back in their sockets so i can plug in the amp when it arrives and test it sonically.
After which ill gut it and isolate the preamp and make a SECOND standalone CG amp! Smiley
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chosenfool
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« Reply #3 on: January 22, 2010, 11:22:29 PM »

an update: the amplifier cabinet came in last monday. Its friday now and i just hooked it up to the tone generator, and plugged it in to a power outlet.
For about 5 minutes there i thought the system was a dud - no sound! pulled the power tubes, resat them. no light from the jewel lamp. Turned off then back on.... then that unmistakeable hum! Normally hum isnt good, but it brought a smile across my face - IT WORKS!
I went and grabbed a control panel i got (also on ebay) that was pieced out of a CG tone generator. I took off its lamp and put it in as a replacement. turned it back on, and the light came on as well. SWEET!

for about 30 minutes i tried it out, alternating between the 3 guitars i have out - a squier butterscotch tele (stock pickups), a blond squier '51, and a lime green danelectro baritone.
Sonically, the amp sounds dark. Even not having a tone control, only the tele was able to produce a bright sound with its stock single coil pickups, and it sounds best when played thru the bridge pickup. It gets muddy at the neck and middle setting.
It does have the promise of a great crunchy overdrive when the volumes are dimed on the amp, using the Squier '51's GFS-brand humbucker bridge.

Im revising my estimate on its power output - i think its closer to 20W rather than the original 30W i stated. It doesnt make sense to put in C12R jensens rated at 25W each in an amp that puts out more than that, regardless if the speakers' peak power is rated double (50W). I was basing my initial estimate at the comparison to the Kalamazoo Bass 30 (30W, using 2 7591 power tubes). In hindsight, the Bass 30 had C12N speakers i think (rated 50W RMS). Also a closer inspection of the output transformer shows it to be a bit beefier than a 15W bell & howell's OT, but nowhere near the OT of a Bass 30. Ive been looking at how huge the power transformer is, which led me to believe it can put out 30W. I now realize this is to power the 50 some odd tubes that the tone generator had.

Regardless whether this is a 20W or 30W amp, it. Is. LOUD!!! again, that grin doesnt quit my face easily. Smiley

Cosmetically this set is a little more beat-up and shows some more wear and tear compared to the first CG set i got. No casters or rollers were included, and no footswitch as well (which really isnt needed for my purposes). A closer inspection shows the black front grill cloth is dyed on! There were spots that werent completely covered, still showing bits of the tweed color that these amps originally had. I may use parts form the other CG cabinet to use for the amp integration i plan to do on this set. WHEN FINALIZE THE FIRST SET FIRST!

a bad case of too many projects, so little time. Tongue.
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sticksandpicks
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« Reply #4 on: December 25, 2010, 11:38:12 PM »

Hello,
I recently came across a Cordovox CG myself, and I am attempting the same integration.  Did you ever get around to shortening/eliminating the 6 foot black cable that runs from the amp to the preamp?  Also, does eliminating the tone generator circuitry affect the power supply? (e.g. B+ too high)
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