GE 7600 and 7700 'Stereo Classic' tube amplifiers

MonoMania

Member
[Update: I found a damaged tube socket, set the 7600 aside, and acquired a model 7700 with 6973 output tubes - haven't gotten back to the 7600 yet. Comments and questions about the 7700 are on page 2 of this thread]

Ladies and gentlemen of the forum,

For years now I have suffered under a terrible curse. For the better part of a decade I have been trapped in the 1950s, and fear I might never escape! My all-original MONOPHONIC late-50s Hi-Fi system breathes such life into my 78s, 45s, and microgroove LPs that I do believe Elvis is alive and well - in my living room!

The trouble is, I can't seem to break into STEREO, no matter what I do. The technology itself defies me at every turn, and each new stereo amplifier soon takes its' place in the "to be repaired" pile. :sigh: A shame when nearly half of my LP collection is STEREO.

Right now I'm working on a General Electric 7600 "Stereo Classic" integrated 6v6-driven amplifier from 1959. I love its' styling, the qualities of its' sound, and the way it complements my GE cartridges, so I have alot of hope for this piece - but alas - I can't seem to get it working properly as a STEREO system!

Would you folks be willing to help me troubleshoot? If so, please read on.

I do have some basic electronics knowledge, a copy of the 1953 Radiotron Designer's Handbook, a good DMM, soldering experience, etc - so your advice will not fall on deaf or uncomprehending ears. I'm no expert, but have been tinkering with tube audio for quite a few years so not [quite] a beginner either.

BACKGROUND:
I acquired this amp with very few modifications - someone had replaced the diodes (which rectify 12ax7 heater supply and 6v6 output bias supply), and a few of the fixed capacitors seemingly at random. After delivery I replaced all of the electrolytics and the rest of the fixed capactiors with the exception of a few hard-to-reach ceraamic and mica caps which I left in place (keeping channels symmetrical). No discrepancies from the Sams Photofact folder found during recap. I also replaced the grid resistors on the output tubes, and for good measure a couple of resistors on the phase inverters which made a loud hum when touched with a chopstick. None of these were fauilty - I just figured it couldn't hurt. All other resistors are the original carbon comps, but I've tested most of them and they appear to be within spec.

PROBLEM:
On powerup all seems well at first. Tubes glow normally, all inputs deliver signal to both channels, mode selector switch does its' job, bass and treble controls are active, and I can turn the loudness control up to what feels like the full 20W per channel without any major hum or noise.

So what is the problem? The two channels don't sound the same - or even similar enough to be acceptable. At first I didn't notice, except that it didn't sound 'correct' - but when I tried a monophonic input source and switched between left-speaker-only-mono and right-speaker-only-mono modes the difference became obvious. Same difference is observed in stereo mode (with mono source) by throwing the balance all the way to the left or right.

The best way I can characterize the difference is to say that the right channel sounds (to my ear) full and balanced, while the left channel sounds more-trebly and much less bassy than its' counterpart. There seems to be a slight difference in volume also, but that is probably just the Fletcher-Munson effect playing tricks on my ears.

The channel which sounds "fuller" also has some slight audible hum (way below the music), while the channel that sounds "tinnier" has the gentle rushing of air sound and no audible hum to speak of. With the GE 7600's "contour control" complicating the picture it's difficult for me to say definitively which of the two channels is actually the abnormal one.

Yes, I've tried different speakers, swapping speakers, and moving speakers around - I am certain that it is neither the loudspeakers nor the room causing this effect.

I tried reversing (btwn channels), and then replacing the tubes (except for the phase inverter tubes of which I don't have spares) - the differences kept their original channel orientations, so I'm pretty sure this isn't a tube issue.

Also tried various mono sources. Not a source problem either.

So the imbalance in tone is occuring somewhere withinin the amp - but where?

I checked the physical calibration of the knobs for the tone control pots - they are designed in such a way that fine adjustment to individual channels is impossible (knobs have square holes), so electronic calbration is the only option.

WORKING HYPOTHESIS:

The only significant parts I haven't replaced are what the Photofact sheet calls "compenent combination" custom tone control modules (one for each channel). These are made up of a number of resistors and small-value (mica?) capacitors which share a single ceramic case/body. When the amp is turned on and I touch these with finger or chopsticks they hum loudly, and the "tone" of the hum differs noticeably between them. I could build new tone controls on terminal strips, but I remain skeptical that this is the source of the problem - I wouldn't expect major drift from these kinds of components. Does anyone here have specific experience with these?

My feeling is that the problem is elsewhere, but... I don't know where else to look! That is what brings me here, with some desperation, hoping to learn from the experts, old-timers, and audio esotericists who frequent this esteemed venue.

Should I rebuild these 'component combination' tone controls? Or am I barking up the wrong tree? Feel free to ask for measurements, photos, schematics, etc. As a new poster here I'm not sure if copyrighted material (i.e. from Photofacts) is permissible.

Thanks in advance for all your advice! I promise to not waste your time, and to not abandon the thread or project until a solution is reached and documented.

I won't be satisfied until this amp is running perfectly, so I will likely have more questions along the way. With a bit of luck and help I hope to have my first real STEREO system (was formerly using a pair of low-wattage mono amps) and finally join the 2nd half of the 20th century! :music:

P.S: My first goal is restoration to original operating condition as intended by the GE engineers, but would afterwards be open to mods/improvements. Already planning to add switchable cartridge loading (stock value is 62k not 47k), and adjustable bias pots.

P.P.S: This is my ever first post, so forgive me if I seem at all out of step.
 
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Sounds like your amp uses what is known as a PEC, or packaged electronic component. Its the bundle of caps and resistors all in one blob, often as part of a tone control circuit and sometimes the phono preamp circuit. They can and do go bad, and the only way to fix it is to rebuild the circuit with individual parts on perf board and wire that in. I've had them fail before.


Do you have a schematic for this? I'm wondering if there may be some quick and dirty method to get a signal in downstream of the tone controls to at least narrow down the area thats causing you grief.
 
I have a schematic (copyright has expired on Sams before 1964), but too large to post. Does you problem occur on all inputs, or just phono? Does the contour (adjustable loudness compensation) work equally on both channels? Does the rumble filter have equal effect on both channels?

You can connect a preamp (or a source with adjustable output, like an iPod) to the center terminals of the balance control. This will be past the tone controls, so if both channels sound alike, you've narrowed it down. If the difference remains, we can start looking at the power amplifier section.
 
They can and do go bad, and the only way to fix it is to rebuild the circuit with individual parts on perf board and wire that in. I've had them fail before.

Do you have a schematic for this? I'm wondering if there may be some quick and dirty method to get a signal in downstream of the tone controls to at least narrow down the area thats causing you grief.

Thanks for the quick replies! You both seem to be on the [same] right track.

I do have the (paper version) Photofact, plus the schematics from the original manual that came with the amplifier. Can upload photos if needed.

Does you problem occur on all inputs, or just phono? Does the contour (adjustable loudness compensation) work equally on both channels? Does the rumble filter have equal effect on both channels?

You can connect a preamp (or a source with adjustable output, like an iPod) to the center terminals of the balance control. This will be past the tone controls, so if both channels sound alike, you've narrowed it down. If the difference remains, we can start looking at the power amplifier section.

To answer Tom Bavis' questions:

1. Problem occurs on all inputs
2. The contour control works on both channels, but it's hard to judge if the effect is equal because the net result of the tone filtering is not.
3. Rumble filter seems to have negligible effect on either channel - attached to it are the few ceramic capacitors I didn't replace - but I don't expect to employ the rumble filter at all.
4. Test you recommended shows that problem is before the balance control.

I followed your advice and connected a stereo source directly at the balance control, bypassing the tone controls. While I wasn't able to test the speakers in isolation [Maybe I plugged in the wrong way - Balance control was inoperative once signal was patched in], the bass came back to the left channel - so it seems that the problem is indeed before the balance control.

Would your best guess be that those tone control PECs need to be rebuilt? Or could this still be a preamp issue? Or something else entirely?

I will rebuild the tone modules regardless [will take me a week or so to get the parts] and report back. Would you recommend performing any other tests in the meantime?

I am much obliged to you both for your comments and suggestions. Just the kind of common sense that I needed - seems obvious once pointed out :)
 
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Sounds like PEC failure is a good guess. I would also check the resistors in the preamp section just in case one has gone seriously off value.
 
For rebuilding those PECs, would 1/2 watt metal film resistors be suitable? I think I have some 1/4 watt around too...
 
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Probably either would be fine. There should be no real amount of power dissapation going on there. You're just moving the audio signal around.
 
In addition to the PEC's make sure your speakers are in phase.

Use 1% metal film resistors and match them side to side.
 
Yeah, my first thought was speaker mismatch. I hastily rebuilt the crossovers before realizing the problem was in the amp. Oh well, they were due for replacement anyway. And yes, they're definitely wired up in phase.

I've already mail-ordered the parts - including 1% metal film 1/2W resistors as recommended. Those of you who can buy components locally don't realize how lucky you are. Since the death of (Canadian) RadioShack there isn't a resistor or capacitor for sale anywhere in my (small) city. Not even a repair shop in town. Moral of the story: Support your local parts dealer!

I'll report back in a week or so when I've rebuilt the PEC tone controls.

Thanks again for all the advice!
 
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In addition to the PECs, you should check all of the resistors in the preamp. An in-circuit test should be good enough for most of them since you're mainly looking for differences between channels. It's also common to see volume and tone pots that don't track well between channels.
 
I went over all the resistors in the pre-amp section (and the rest of the amp), and found everything in tolerance, but often near the boundaries of their 5% and 10% ratings. I will plan to replace these, but I have two questions:

1. Is there any place in the preamp or phase inverter that I should be using metal oxide instead of metal film - or anything higher than 1/2 watt? If I'm reading my schematic correctly everything stays under 300VDC.

2. If the engineers specified 5 and 10% tolerances, is replacing with 1% likely to bring about an audible improvement? Or is it just to be seen as a beneficial side-effect of upgrading from carbon comp to quieter and more stable metal film?

Either way I will replace the resistors in the near future, after I rebuild and install the PEC tone controls. :)
 
Use metal oxide in power supply only - not as linear or quiet as metal film. 300V is the voltage limit for smaller metal films - sounds like you're OK there. 1% won't make a difference - though 10% MIGHT be audible in the phono EQ components - that's 0.8 dB.
 
I tend to use a lot of carbon film 5%, but not for any special reason. I have no real preference. If you want to get fancy about it, you can hand match values channel to channel but in reality I don't expect you'll ever hear it if you're using 5% resistors already. Most modern production resistors are much tighter than their marked tolerance and I expect they're probably better than what was in that GE when they were brand new.
 
Well, I rebuilt those tone control PECs, but unfortunately that did not solve the problem, so I'm hoping you'll all be willing to help me through a 2nd round of troubleshooting.

I installed my new homemade PEC tone controls, turned the amp on, tested through the AUX input, and got the exact same result: Right channel full and clear, Left channel tinny, subjectively quieter, and severely lacking bass. My first thought was that I must have screwed up the soldering, or that one of the fragile resistor leads had broken - not so - I pulled them back out of the circuit and tested with an ohmmeter - they read exactly the same. So, the PECs are fine [I think] and not the cause of this problem.

I stuck them back in the circuit again and rolled the tone control pots individually (one channel at a time).. I discovered that one channel is entirely unresponsive to the bass control, while on the other channel the effect is quite obvious. I'm embarassed to say that this was probably the case all along, and that I must have failed to notice it the first time around.

This discovery led me to believe that the Bass potentiometer had gone bad, so again removing the tone controls I tested the pots with a multimeter - they don't match very well and probably do need to be replaced, but unless I am misinterpreting the data [posted below] these readings don't seem to explain the pot having no audible effect at all on one channel. If anything the readings indicate that the bass pot is healthier than the treble...?

I tested as follows:

TOTAL RESISTANCE ACROSS POTS:

......................LEFT..........RIGHT..........SPEC'D.....
BASS..............1.05 Meg....1.15 Meg........1Meg.....
TREBLE...........583k..........510k..............500k.....


RESISTANCE READINGS BETWEEN WIPER AND CONTACTS:

..............MIN.....MED.......MAX .....
LBASS.....1M.......500k......35 ohms.....
...............1M.......526k......38 ohms.....
RBASS.....1.1M.....617k.....54 ohms.....
...............1.1M.....597k.....54 ohms.....

LTREB.....587k.....294k.....15 ohms.....
..............351k.....302k......24 ohms.....
RTREB....308k.....256k......19 ohms.....
...............515k.....245k.....15 ohms.....

(I tested from center to each side because the readings differed, thus the doubled sets of readings seen above)

"min", "med", "max" = wiper positions, with "med" being 12 o'clock.


I'm just as bewildered as when I started, and at this point don't know how to proceed. I do want to replace all the pots if I can find exact replacements, but will that solve the problem? Or will be back at square one again?


Questions I'm hoping to have answered:

1. Could the problem be something we've not yet considered? If it isn't the power amp section, or the phono preamp, or the capacitors (I replaced the last few ceramics last night), or the resistors (carbon comp but in spec), or the tone controls, or the pots, then what could it be?

2. What are the chances of sourcing the correct potentiometers in the 21st century? These are stereo linear pots with two concentric (inner & outer) split shafts attaching to two different knobs (one for each channel) which lock together once in place. Sams Photofact gives GE and Centralab part #s. Keeping the original aesthetic (knobs, shaft length, etc) is important to me. Also which brands offer the best quality and longest life?

3. Having followed the advice to wire a signal into the circuit after the tone controls I've been hugely enjoying listening to the GE as a power amp - so much so that I plan to wire it as a permanent input! The amp sounds absolutely beautiful, but... I still hear some low-level hum when (and only when) I have a source plugged in. Is this cause for concern? Or is it more likely coming from my cheap signal cable plugged into my cheap computer soundcard? I have no other line level source to compare against at the moment.

If photos, schematics, or readings are needed just ask and I will post them.

I've enjoyed spending a week listening to FLAC files through the power amp section of the GE... But I really need to get back to my LPs - specifically my STEREO vinyl which has been out of rotation for years.

Needless to say any and all help has been and will be immensely appreciated!
 
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Did you ever figure this out?

I also have noticed that both channels do not match perfectly in my GE amps (the 6973 ones). Tom suggested the low-cut filter switch, which can get stuck on or off one channel if dirty, so it will appear to have little effect when switched but is actually just doing the same wrong thing in both positions. I've had this fight with mine before, but still there's always some imbalance due to old pots and PECs. I just suck it up and adjust the left and right tone and balance controls until it sounds "close enough". And, they do sound good when used as power amps (modifying the tape output so that external preamps are properly loaded).
 
I have run into the same problem when working on integrated amplifiers. I have used some as just a power amp and they sounded great! I would try changing the inverter tube, as thats the only tube you have not replaced. In the past when I am hearing a scratchy or rush sound out of the amp with no signal it was a carbon comp resistor, when I changed the plate and cathode resistors the noise went away. Most likely a cathode resistor early on in the circuit.
 
@jon_s Sorry, didn't see your respond to this old thread. I have the thread "watched", but for some reason didn't receive an email that someone had posted.

My GE 7600 amp is still gathering dust, I had figured out that I had a bad preamp tube socket and just haven't gotten around to replacing it yet. Not sure if that could have been the source of the problem all along... I'll keep the low-cut filter switch in mind when I eventually do get back to it.

In the meantime I acquired a GE 7700 (the 6973 version) which I much prefer, and after a rebuild has worked perfectly - it's been my main amp for a few years now.

You said "they do sound good when used as power amps (modifying the tape output so that external preamps are properly loaded)." Could you share how you modified the tape output? I'm still wired straight into the balance control, which seems to have worked out well for me for digital audio - to my ear it sounds perfect.
 
@jon_s
Sorry, didn't see your respond to this old thread....
In the meantime I acquired a GE 7700 (the 6973 version) which I much prefer....
You said "they do sound good when used as power amps (modifying the tape output so that external preamps are properly loaded)." Could you share how you modified the tape output? I'm still wired straight into the balance control, which seems to have worked out well for me for digital audio - to my ear it sounds perfect.

No problem - And, I'm not surprised that the 6973 amp is the better sounding of the two! Those are great tubes, but more importantly the GE's output transformers are phenomenal. You can tell they are something "different" just by the laminations, which have some interesting epoxy impregnation, but there's also a detailed engineering article (somewhere, I now can't find it...) that explains how they made the amp extra stable by using an asymmetrical (relative to each push-pull half) winding technique to control the resonance peaks. My amps measure well to ~85kHz (definitely flat past 60kHz) and produce gorgeous square waves when driving the output stage directly (rolled off just past ~20kHz if using the preamps). And the second power supply for the screens, combined with the robust driver stage is probably why they have such impressive bass (I recommend 6BA8 over 6AW8, if you have 'em). On the other hand, due to the pentodes they are somewhat noisy (a bit of hum and hiss), but I found they absolutely rock with Large Advents, A25s, or similar speakers.

Re: using as a power amp, just inserting the signal before the balance control (not after, unless you want to add resistors) is probably the easiest way to do this. I suspect you have it figured out! :)
 
@jon_s If you ever find that article again I'd love to see it!

I've wired directly into the terminals of the balance control; I'll review the schematic and look at wiring in *before* the balance control, which makes more sense.

The 'tape out' I still use for digitizing vinyl, so I've been planning to wire up the 'tape head' input as my 'input to power amp'. For now I just have a cable hanging out the back of the amp with female RCA ends - messy, but it works.

One thing I've noticed about this amp is a hum through the speakers when I turn the amp on, which fades away over the course of a few seconds. Is this normal for the 7700? I've wondered if it could be related to the selenium rectifier which I've bought parts to replace, but haven't done yet...
 
Oh, wiring it “after” (on the wiper) is fine as long as you always keep the control in the center. So I wouldn’t worry too much!

The hum during warmup is totally normal, and probably due to the three supplies warming up at different rates. But, I’d still replace the selenium bias rectifier with a modern part. Hard to believe now, but I was able to get the complete bias supply at a Radio Shack (about 10 years ago).

Here are the pics from when I finished my 7700. I have an MS4000, too, which is the same but without left-right controls for tone. The 7700’s caps were different and already had a fourth hole, so I made some upgrades.

0C9CC2DD-D405-40F7-8AD4-865FA34FEE15.jpeg 43E640BE-CE82-4868-8C46-6ACAD444FFB4.jpeg B41A70D8-793D-4D2A-B6D6-B8FD010CF8B7.jpeg A800A064-922A-4184-9292-1114236F69DB.jpeg
 
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