Man, this one is vexing. I tried your suggestions above, no luck.
I thought it would be the send/return jacks for a minute there, but it's not. I should have tested this sooner, but the Send jack from the preamp section is clean - no hum.
Then, audio from the power amp section is also squeaky clean -- if using an external source at the R1.
Given unity gain, noise floor at send (or return) jack is about -70db. This is baseline normal. At R1 it is about -45db with the hum, when the preamp section is coupled to it as it normally is. That's what I'm hearing from the speaker, and it's no bueno.
The whole output of the preamp section is clean when I used it as a preamp with an external power supply.
I thought maybe this all might have to do with the 15v power rail. I haven't thoroughly eliminated that possibility.
I tried moving around those wires on the lead that plugs the preamp into the power amp. I'm not finding voltage on the chassis.
preamp👇
How's the hum with Master (R26) at 0? And how's hum with Master at max and Volume (R9) at 0?
The three that wires that connect preamp to power amp appear to be +15, -15, and audio. How does analog ground connect from power supply to preamp?
Curiosity: what are the Send and Return jacks for?
The three that wires that connect preamp to power amp appear to be +15, -15, and audio. How does analog ground connect from power supply to preamp?
Curiosity: what are the Send and Return jacks for?
@BSST The Master volume does not effect the hum level at all - the is hum later in the signal chain..
The power amp section (these are two separate PCBs) grounds to the speaker jack in the back, while the preamp grounds at the input jack - which is traditional for guitar amps I guess. I have tried floating the input jack and grounding it to the back of the chassis - had no effect.
I've gone as far as to do this experiment here:
Send the +15/-15 from power amp to preamp board. But instead of the normal signal routing, I'm flying the send/return jack to the power amp board. That signal is acceptable at the output, with hum at -65db, noticeable but I could live with it. But that's skipping a gain stage at the final opamp of the preamp. If I do that 'flyover' from R54, the last resistor on the preamp, the hum is loud again.
Either the hum happens at that last stage, or is earlier in the preamp and amplified greatly in that last stage. What bugs me is that this final output of the preamp was quiet with a separate (switching) power supply. Whatever this is, is simple but maddening.
The power amp section (these are two separate PCBs) grounds to the speaker jack in the back, while the preamp grounds at the input jack - which is traditional for guitar amps I guess. I have tried floating the input jack and grounding it to the back of the chassis - had no effect.
I've gone as far as to do this experiment here:
Send the +15/-15 from power amp to preamp board. But instead of the normal signal routing, I'm flying the send/return jack to the power amp board. That signal is acceptable at the output, with hum at -65db, noticeable but I could live with it. But that's skipping a gain stage at the final opamp of the preamp. If I do that 'flyover' from R54, the last resistor on the preamp, the hum is loud again.
Either the hum happens at that last stage, or is earlier in the preamp and amplified greatly in that last stage. What bugs me is that this final output of the preamp was quiet with a separate (switching) power supply. Whatever this is, is simple but maddening.
Was preparing post:
I neglected to explicitly thank you for the preamp schematic--- it's very helpful.
I'm still trying to understand grounding for whatever insight that might add. I've yet to confirm any direct wire delivering power supply common to the preamp board; I'm not arguing this is outrageously bad practice, but a suspicion. The PCBs use the triangle ground symbol and chassis ground uses the fork symbol. I see the two ground symbols tied together at the left side of the preamp schematic and also at the right side of the power amp schematic. Does preamp ground follow the I/O jacks to reach the power supply? i.e. if all jacks were floating, would preamp common also be floating re power supply?
I neglected to explicitly thank you for the preamp schematic--- it's very helpful.
I'm still trying to understand grounding for whatever insight that might add. I've yet to confirm any direct wire delivering power supply common to the preamp board; I'm not arguing this is outrageously bad practice, but a suspicion. The PCBs use the triangle ground symbol and chassis ground uses the fork symbol. I see the two ground symbols tied together at the left side of the preamp schematic and also at the right side of the power amp schematic. Does preamp ground follow the I/O jacks to reach the power supply? i.e. if all jacks were floating, would preamp common also be floating re power supply?
@BSST
Okay thanks, you have it right as far as the schematic goes. Before I answer those questions, I noticed something here. I think the hum has to do with proximity to the transformer, but what to do about it?
When I ground power amp R1 to chassis, it cuts all hum*. Then when I ground preamp R54 (last one) to chassis, it cuts maybe 80% of the hum, but not all. But there there is nothing between these two except some lead wire on the pcbs and between them 🤔. So I found that if I ground R1 right above the transformer I can get it to hum a little. I may be confusing interference through the alligator clip wire, but I don't' think so. It varies by proximity to the PT.
Is this some kind of leakage from the PT? Can I isolate it? Is this a "bad" PT? There's not a lot of room for moving AC wires around, but I can make the lead from PT primary to switch shorter. Moving that wire closer to the preamp PCB I can make the problem worse...
Say I wanted to replace the PT. I measured 44v on the secondaries. Is that a standard output, and if not, what is the wiggle room above or below that I could use?
Okay thanks, you have it right as far as the schematic goes. Before I answer those questions, I noticed something here. I think the hum has to do with proximity to the transformer, but what to do about it?
When I ground power amp R1 to chassis, it cuts all hum*. Then when I ground preamp R54 (last one) to chassis, it cuts maybe 80% of the hum, but not all. But there there is nothing between these two except some lead wire on the pcbs and between them 🤔. So I found that if I ground R1 right above the transformer I can get it to hum a little. I may be confusing interference through the alligator clip wire, but I don't' think so. It varies by proximity to the PT.
Is this some kind of leakage from the PT? Can I isolate it? Is this a "bad" PT? There's not a lot of room for moving AC wires around, but I can make the lead from PT primary to switch shorter. Moving that wire closer to the preamp PCB I can make the problem worse...
Say I wanted to replace the PT. I measured 44v on the secondaries. Is that a standard output, and if not, what is the wiggle room above or below that I could use?
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I believe your suspicious of transformer magnetic fields is well placed. A toroidal transformer would probably have lower radiated fields but maybe it's possible to improve the present situation.
A tedious experiment would lengthen the secondary leads so that the transformer could be positioned at a distance from the enclosure. I can't see the details of the transformer, but perhaps the transformer could be rotated 180 degrees to position leads farther from the preamp board and closer the the walls of the enclosure. Make sure the leads are closely grouped, even twisted if possible. The objective is to minimize the "loop area" of the connecting wiring. Maximizing distance from the amplifier boards will help with coupling of both magnetic and electric fields into the high gain circuits. Be sure to consider both primary and secondary wiring; secondary will have higher currents than primary, but voltage fields will be the reverse.
A tedious experiment would lengthen the secondary leads so that the transformer could be positioned at a distance from the enclosure. I can't see the details of the transformer, but perhaps the transformer could be rotated 180 degrees to position leads farther from the preamp board and closer the the walls of the enclosure. Make sure the leads are closely grouped, even twisted if possible. The objective is to minimize the "loop area" of the connecting wiring. Maximizing distance from the amplifier boards will help with coupling of both magnetic and electric fields into the high gain circuits. Be sure to consider both primary and secondary wiring; secondary will have higher currents than primary, but voltage fields will be the reverse.
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Well speaking of tedious, I went and floated the whole preamp board outside the chassis with the amp on, and that pretty much conclusively showed that it has to do with 'radiation' from the PT. Moving some of those wires around and twisting per your suggestion, I was able to reduce, if not totally eliminate the hum, to a level I can live with for now. I will maybe look at replacing the transformer with a toroidal, and start researching that.
Thanks a million for the brainstorming, it has been enormously helpful. I'm literally learning something every day here, which is the agenda. 😎
Additional question on this amp, if @BSST or anyone could help me understand this;
On the back of the amp, there are two speaker jacks, and it says "8ohms"
I would take this to mean you can run two 8ohm cabs in parallel, although normally it would say "minimum" one way or the other.
In any case, from the schematic and the topology of the amp, how does one determine whether or not it "can" load down to 4 ohms?
I'm also thinking of adding capacitance to the power supply because I happen to have an extra four of those 470uf value; later GK amps seemed to use more.
On the back of the amp, there are two speaker jacks, and it says "8ohms"
I would take this to mean you can run two 8ohm cabs in parallel, although normally it would say "minimum" one way or the other.
In any case, from the schematic and the topology of the amp, how does one determine whether or not it "can" load down to 4 ohms?
I'm also thinking of adding capacitance to the power supply because I happen to have an extra four of those 470uf value; later GK amps seemed to use more.
Adding extra C on the power supply sounds like a fine idea.
Re the speaker load impedance: if we assumed 50V peak across 8 ohms, power out would be 156 Watts RMS, and twice that with 4 ohms. The schematic legend describes the amp as a 100W design and it doesn't have any protection limiting--- not even glass fuses. I'd be very cautious about using two 8 ohm speakers. Two speakers might help with sound distribution, but trying to get double SPL seems like asking for trouble. But you're practiced in repair... 😉
Re the speaker load impedance: if we assumed 50V peak across 8 ohms, power out would be 156 Watts RMS, and twice that with 4 ohms. The schematic legend describes the amp as a 100W design and it doesn't have any protection limiting--- not even glass fuses. I'd be very cautious about using two 8 ohm speakers. Two speakers might help with sound distribution, but trying to get double SPL seems like asking for trouble. But you're practiced in repair... 😉
Lol, tongue planted firmly in cheek. After all this saga, I'm not eager to roast the thing. Although I did learn to diagnose damn near everything that can go wrong with this particular amp, and will snap up a blown 800RB next time I find one cheap - and do this whole thing over, but faster.
I actually measured the power output by measuring voltage from one of the speaker outputs while running a sine wave just under clipping with a speaker cab load on the other out, and got about 110 watts as expected (@ 30v*30/8ohm). I also measured with volume cranked and got precisely the 156 watts you specified. I just wonder what Bob Gallien was "getting at" with the two jacks. Would be weird if he meant it to be two 16ohm for 8ohm minimum. Not trying to push my luck. It says something that the very next year he designed the 400RB which is exactly the same amp but with four output transistors, for 125(8)/200(4) watts.