1969 Gibson GA5-T Amp PS Caps

[Phil_S]

Red
267 VAC
291VDC

Green
276VAC
206VDC

Blue
248VAC
270VDC

The DC voltage looks great. The AC readings really bother me. The report is that the HT secondary supplies 262-262 to the rectifier. These AC readings suggest to me that it is just flowing across.

Either texstrat is measuring it wrong or there is a short from the HT secondary to the filter caps, bypassing the rectifier. I'm open to other suggestions to explain this. That's what I can come up with.

I think I'd like to see a couple of good pictures of the rectifier tube socket. I'd also like to see clearly the center tap ground.

Martin, any ideas?

[texstrat]

Either texstrat is measuring it wrong or there is a short from the HT secondary to the filter caps, bypassing the rectifier. I'm open to other suggestions to explain this. That's what I can come up with.

Ground to chassis and positive probe to each lead coming out of the capacitor on the positive end.

I think I'd like to see a couple of good pictures of the rectifier tube socket. I'd also like to see clearly the center tap ground.

Which is the center tap ground?

[martin manning]

I don't know what to make of the high AC on the power supply nodes. If it were really that high I think there would be an intolerable hum. Is it possible that your meter isn't capable of measuring it on top of all the DC?

The hum is back and the tremolo seems to be working like it was earlier. I did do a search for Gibson tremolo sound clips and mine amp does sound similar to what I am hearing with other vintage Gibson amps. As soon as I turn on the tremolo, I can hear the oscillation through the speaker before I strum the guitar strings, is this supposed to be?

When you say the trem is working like it was earlier, do you mean the funky whump-whump or do you mean it's better now? If there is some hum in the signal path, then the trem will modulate that. Is that what you're hearing when you switch it on with no guitar signal? Is it a loud or soft hum? It may be from the extra lead lengths on the pot, but if not we can find that later.

It is interesting to hear the difference using the pot as a variable resistor. With it off, the tone is muddier and as I add resistance, the tone has more clarity. The tone I like is with the pot set at 22K, almost half way. So if I understand correctly, the total resistance between V3 and V4 is now 3.9K? If I change the 330K to 220K what can I expect to hear from the amp?

This is excellent, and very good reporting of the results! The muddy tone is probably the result of V1b being down near cut-off and distorting heavily, so we are going in the right direction. Are the voltages you posted last the result with the pot at 22K? I see the green node voltage is just over 200 as planned, and the V1a voltages are indeed almost the same as before. If you go to a 220k across the 100k and recover the 200V on the green again, you will bias V4 and V1b further toward center, and I'd expect still more headroom and less distortion. I'd go ahead and try that to see if you like the result better before deciding. I get 3.87k for 22k//4k7, so you've got that right.

Here are two pictures showing how I connected things. The 330K resistor is left long until the correct value is determined, plus it is tight in there. Right now I have the 330K tacked on the 100K leads. Is this the same as soldering the resistor to the two outside lugs on the terminal strip?

Yes, looks good to me, use the terminal strip lugs if it's easier. You are right- wait until you decide what values you are going to go with before finalizing the connections. For the final layout you can either parallel resistors across the 100k and 4k7, or replace each pair with an equivalent single.

[Phil_S]

The picture of V3 shows that all of the grounds are tied to one of the unused pins. (The center tap is the red/yellow from the transformer.) I am satisfied that the rectifier is wired correctly. I don't see any shorts. I'll go with Martin on this one and blame the meter. I don't think you are seeing 270VAC at the filter caps for real.

I'm leaving you with Martin on the tweaking. He's much better at this than I am.

[texstrat]

I will recheck the voltages in the morning as I am at work right now. I probably measured them incorrectly.

When you say the trem is working like it was earlier, do you mean the funky whump-whump or do you mean it's better now? If there is some hum in the signal path, then the trem will modulate that. Is that what you're hearing when you switch it on with no guitar signal? Is it a loud or soft hum? It may be from the extra lead lengths on the pot, but if not we can find that later.

I still have the whump-whump, but only when I engage the tremolo switch and there is definitely hum in the signal path.

The tremolo sounds like it is working based on what I have heard in the You tube clips.

Tomorrow I will insert the 220K resistor and check voltage values. Once the correct resistor value is determined, I will replace them with one that is closest in value.

Again, thanks to both of you for your help. The best way for me to learn is to ask a lot of questions

[martin manning]

Texstrat, I think maybe you should try a 150k (instead of a 220k) paralleling the 100k dropping resistor for your next test. That will drop the resistance down to about 55k. This decrement would be about the same as the first, since with the 330k you had ~77k for the combination. If you don't have a 150k, just add the 220k to what you have now- that will make 57k.

Thanks for the vote of confidence Phil, but keep looking on please; there is still some work to do.

Reading back over the thread, the hum was absent after the tubes were replaced, and later it returned, a little at first, and then a bit more, is that right? The no-hum state was also the point at which an extremely low ripple voltage was measured. Perhaps there is a poor connection somewhere? V3's pin 2 is serving as a ground point... Make sure that the wire going from there to the tremolo switch is well connected all the way back to the input jack, and make sure that the jack is making good contact with the chassis. Is the jack grounding the input cable properly? Try shorting the plug's sleeve to the ground lug on the jack.

[texstrat]

Martin/Phil:

I got to playing around with the current setup, 320K resistor tacked to 100K, and initially I got a good tone from the guitar at 22K, the pot, but a warbly tremolo and a lot of hum.

So, I reconnected the pot leads directly on the lugs versus on the resistor leads and that addressed the hum. I cracked the tremolo switch and slowly brought the pot up until I could hear the tremolo kick in. I dialed in the pot until I got a tighter sounding sounding tremolo. Checking the value on the pot I get 3.4K resistance, or 2K total resistance between V3 and V4. I rechecked voltages and got the following:

V1
Pin 1 - 166VDC (186VDC)
Pin 2 - -10.0mV (0.0mV)
Pin 3 - 1.65VDC (2.09VDC)
Pin 6 - 136VDC (140VDC)
Pin 7 - 0.0mV (0.0mV)
Pin 8 - 1.21VDC (1.25VDC)

V2
Pin 2 - 16mV (16mV)
Pin 3 - 8.0VDC (7.9VDC)
Pin 7 - 270VDC (268VDC)
Pin 9 - 270VDC (296VDC)

V3
Pin 1 - 259VAC (257VAC)
Pin 6 - 260VAC (257VAC)
Pin 7 - 290VDC or 260VAC (290VDC or 248VAC)

V4
Pin 1 - 182VDC (209VDC)
Pin 2 - 63mV (68mV)
Pin 3 - 1.64VDC (2.11VDC)

Rechecked ps cap values and got the following:

Red
290VDC (291VDC) schematic value is 260V, 11% high

Green
196VDC (206VDC) schematic value is 200V, 2% low

Blue
270VDC (270VDC) schematic value is 250V, 8% high

When I tried to check the AC values for the ps caps, I could not get a steady value. Not sure what is going on there.

So clipping the leads to the lugs eliminated the hum/noise and a smaller value resistor vastly improved the overall tone of the tremolo.

Should I go ahead and try the 220K resistor?

[martin manning]

Great! Sounds like you're having som fun with this. I'd try adding the 220k to what you have now to get another 20k or so off of the power string dropping resistance (put it on the terminal strip lugs maybe) and see what change that makes for better or for worse. You need a third data point to determine where the optimum is likely to be. I guess I wouldn't worry about the AC voltage on the power supply nodes since things seem to be working as expected.

[texstrat]

Great! Sounds like you're having som fun with this. I'd try adding the 220k to what you have now to get another 20k or so off of the power string dropping resistance (put it on the terminal strip lugs maybe) and see what change that makes for better or for worse. You need a third data point to determine where the optimum is likely to be. I guess I wouldn't worry about the AC voltage on the power supply nodes since things seem to be working as expected.

I'll give it a try and see what happens.

[texstrat]

Martin/Phil:

New voltages with the 220K resistor in place and there were some slight changes in the numbers:

V1
Pin 1 - 183VDC (166VDC)
Pin 2 - 0.1mV (-10.0mV)
Pin 3 - 1.84VDC (1.65VDC)
Pin 6 - 148VDC (136VDC)
Pin 7 - 0.0mV (0.0mV)
Pin 8 - 1.30VDC (1.21VDC)

V2
Pin 2 - 13mV (16mV)
Pin 3 - 8.0VDC (8.0VDC)
Pin 7 - 270VDC (270VDC)
Pin 9 - 269VDC (270VDC)

V3
Pin 1 - 259VAC (259VAC)
Pin 6 - 259VAC (260VAC)
Pin 7 - 292VDC or 289VAC (290VDC or 260VAC)

V4
Pin 1 - 195VDC (182VDC)
Pin 2 - 57mV (68mV)
Pin 3 - 1.85VDC (1.64VDC)

Rechecked ps cap values and got the following:

Red
292VDC (290VDC) schematic value is 260V, 12% high

Green
215VDC (196VDC) schematic value is 200V, 8% high

Blue
270VDC (270VDC) schematic value is 250V, 8% high

The pot value is 4.9K, so if I go this route then a 2.4K (2.2K) resistor between V3 and V4.

What next?

[Phil_S]

Those voltages look very good to me. +8% high voltage reading is consistent with modern day a/c supply. We get about 120VAC now. Back then is may have been as low as 110VAC. This translates very directly to the DC output.

I'm curious about your reported AC voltage at the rectifier. Do a little something to satisfy my curiosity? Set the meter to VAC, put the probes in a wall socket and tell me if you get around 120VAC.

You want to know what's next? I think you should let your ears decide what you like and go with that. Also, I'm not clear if you've go the tremolo running OK now and I think you were able to cure the hum.

[texstrat]

I'm curious about your reported AC voltage at the rectifier. Do a little something to satisfy my curiosity? Set the meter to VAC, put the probes in a wall socket and tell me if you get around 120VAC.

I get 121VAC at the outlet.

You want to know what's next? I think you should let your ears decide what you like and go with that. Also, I'm not clear if you've go the tremolo running OK now and I think you were able to cure the hum.

Dialing the pot I can get the tremolo to sound fat and sloppy or add more resistance and lean it up. I like the leaner sound. As far as the hum/noise, I cut that out when I clipped the pot leads directly to the tube socket lugs versus to the 4.7K leads.

I have learned much during this exercise.

[martin manning]

I'm not clear if you have the 330k and 220k paralleled acros the 100k dropping resistor now. In any case, does it sound better now at any pot setting than it did with the 330k alone? Keep the configuration you like best. If it's the one with all three resistors paralleled in the power supply string replace the 330k and 220k with a 125k, or if you remove the 100k too use a 56k. Once you get that in, you might want to do a final check with the pot to get the value for the shared Rk.

I think we all learned a thing or two going through this exercise ;^) When you get done with this, you can go read the triode gain stage and tremolo oscillator articles on Merlin B's web site: http://www.freewebs.com/valvewizard/ You have some great practical experience to relate the theoretical material to.

[texstrat]

I'm not clear if you have the 330k and 220k paralleled acros the 100k dropping resistor now. In any case, does it sound better now at any pot setting than it did with the 330k alone? Keep the configuration you like best. If it's the one with all three resistors paralleled in the power supply string replace the 330k and 220k with a 125k, or if you remove the 100k too use a 56k. Once you get that in, you might want to do a final check with the pot to get the value for the shared Rk.

Good idea. I will replace all three with a 56K resistor and recheck pot to be sure it is the tone I like.

BTW, I am picking up a new "project" amp, a 1995 Fender Blues DeVille 212 Tweed/Ox Blood......Could I have a problem if I have 6 guitars and 3 amps??

[Phil_S]

BTW, I am picking up a new "project" amp, a 1995 Fender Blues DeVille 212 Tweed/Ox Blood......Could I have a problem if I have 6 guitars and 3 amps??

Definitely a problem. Too many guitars, not enough amps. I recommend you get some balance in your life. You need more amps. BTW, maybe you didn't see the warning signs, but amp building/tinkering is highly addictive. It's way too late now. You're going to need an intervention stronger than a 12 step program to stop.