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!
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.
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!
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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