Gibson GA-100 amp is a troublesome critter...

[Britt Rossman]

Hello folks,

I'm a long-time lurker (read, "researcher") and first-time poster. Thanks for having me here! I hope I don't ruin my welcome with what I'm about to ask...

[EDIT] I'm posting this here in hopes that someone will find this useful someday. I have not found much of anything on these amps after months of crawling around the web. From what I can tell, Gibson only put out about 140 of these, so that's part of the reason why there is so little on them.

So I have this old Gibson GA-100 I'm working on for a friend. I was told he did not like the sound and wanted to change it to suit his taste, so I said OK. But, when I got the amp and plugged it in, it immediately turned on and produced no sound. (Lets for now forget the whole fact that I never heard the original "sound" he didnt like)

I checked the OEM speaker and it is fine. Next, I attached a new 3-prong cord for diagnosing issues. I come to find that one of the I/O wires to the power switch is melted through and shorting lugs. I pull the wire away and no more auto-powering issues!

Now, I turn the amp on and read input 125 VAC input, and the rectifier is pumping 480 VDC. I get a nasty low-speed motor-boating (?), but not per the typical, more like disctinct "whump-whump-whump" kinda thing happening. Also, in acts of desperation (and time saving), I have shotgunned all coupling caps, and about half of the resistors were way off. (maybe I should swap them all? Gosh, am I in deep here...)

I replace large electro filter caps, only to later find I'd install one backwards, which I swapped for a new one in the correct config (I know, shoot me... I'm NOT a tech, mmkay?). NOW, my caps are rated at 450VDC cuz the schematic I have tells me I should be getting 430V out of the GZ34 recto. I've been using a variac to bring the input down to 105VAC, but I'm still getting ~410VDC out of the rectifier (which is now a 5V4 I swapped in for testing, though the GZ34 acts the same).

Other notes:

- 6L6 tubes are OK
- 6BD6 drivers have been swapped for JAN-6AU6-WC, which I had on hand
- one of these is reading strangely high on plate voltage (160V) where the other is 110V (i checked the associated plate resistors and all check OK.

HERE comes my question... Can anybody help me determine the source of issues here? Here are the major symptoms:

- Abnormally high output voltage,
- 0A2 regulator drops ~150v from first node instead of 75V stated on schematic,
- Voltage drop on OPT primaries gives me ~2W of plate dissipation per 6L6 with a -33v bias voltage,
- Turning volume of amp up only increases HUMMMM, not actual amp volume,
- At low volume, there is what seem to be a high freq (15khz?) coming from the speaker; oscillations somewhere???

Everything I can think tells me it's the OPT, but I can't be sure. If you need more data, I'll post - just ask.

Thanks so much in advance for you help. Anything y'all can contribute would be much appreciated... I'm about to pull my hair out.

- Britt

P.S. - Here's the schematic:

[Britt Rossman]

I heard that Enzo might have some input on this? I was told to put all the information I can here, but I wonder if it's just too overwhelming? I'm about to just take this thing to someone else...

[Enzo]

Not sure how I got to be the guy, this amp is like any other. But here goes...

0A2 drops 150v? Well, look at the schematic, it calls for an 0C2. 0A2 is a 150v glow tube, while 0C2 is a 75v glow tube. I suspect there is nothing more to it than that.

Now look at the output tubes. Do you not now have much lower screen voltage than specified? That causes the tubes to conduct less, meaning low current. Restore proper screen voltage and that ought to clear. And low power tube current removes much of the load on the B+ supply, which will rise then. SO with proper tube current, your high B+ should drop.

I can think of no reason to suspect the OT of anything.

[Britt Rossman]

Hey All, just thought I'd give you an update on this...

I replaced the regulator with the correct OC2 tube, but still no change in the amp. I'm still getting a loud hum as I turn up the volume. There is also a slight ticking, as if a ticking tremolo, but there is no tremolo - is this a bad tube oscillating?

I've finally got the time to take another crack at this thing, so I'll come back with more when I get it.

[Britt Rossman]

UPDATE:

I have 425VDC coming out of the GZ34, and I'm dropping almost exactly 75v with the OC2, but still have these issues... I still have a mismatch of voltages on the phase inverter. One has 116VDC on the plate, while the other is at 146VDC - schematic says they should be the same. All of the resistors seem to check out, and these tubes tested fairly strong on my meter... Is this where my issue is?

EDIT:: the variac puts me at 114VAC of input for 425VDC out of the rectifier. ALSO, strangely, since the last time I sat down to work on the amp, and the firest time since getting it, the pilot lamp has turned on...odd.

[ThermionicScott]

UPDATE:

I have 425VDC coming out of the GZ34, and I'm dropping almost exactly 75v with the OC2, but still have these issues... I still have a mismatch of voltages on the phase inverter. One has 116VDC on the plate, while the other is at 146VDC - schematic says they should be the same. All of the resistors seem to check out, and these tubes tested fairly strong on my meter... Is this where my issue is?

EDIT:: the variac puts me at 114VAC of input for 425VDC out of the rectifier. ALSO, strangely, since the last time I sat down to work on the amp, and the firest time since getting it, the pilot lamp has turned on...odd.

If you swap the 6BD6/6AU6's around, does the plate voltage follow the tubes or stay put?

The volume control is upstream of the PI, so it sounds like you may have multiple unrelated issues going on -- I'm not as suspicious of the PI as you are. Cleaning and tightening all socket pins would be a good thing, even if that doesn't fix an intermittent connection.

Have you replaced the electrolytic cathode bypass caps and the one in the bias supply?

[nickb]

Hey All, just thought I'd give you an update on this...

I replaced the regulator with the correct OC2 tube, but still no change in the amp. I'm still getting a loud hum as I turn up the volume. There is also a slight ticking, as if a ticking tremolo, but there is no tremolo - is this a bad tube oscillating?

I've finally got the time to take another crack at this thing, so I'll come back with more when I get it.

The hum could be for several reasons -

Miswired or bad bypass cap - that's the 10uF one. Try soldering another in parallel with and note any difference. Bigger won't do any harm.
Bad V1 - heater cathode leakage
Poor wiring and or grounding on input jacks - at that age they might no longer be making good contact with the chassis.
Is the hum there with no instrument plugged in?

On the ticking - does the volume control affect it?


The difference in voltages on the drivers V2/V3 is not a concern unless you happened to know the tubes are matched.

[Britt Rossman]

@ThermionicScott

- I read the voltages on the OEM 6BD8 tubes as 143V and 128V. Switching their positions and letting the amp settle again gave 121V and 151V respectively - very strange. Both tubes are matched according to their mutual transconductance.

@nickb

- I purchased another NOS 6EU7 to replace V1, but my tester tells me it is actually weaker. Rolling these only gives a difference in timbre and dynamics, notably that the one I purchased to replace the original is more dull sounding...
- Right now, the amp is an octopus on the bench, with the input jacks floating off the chassis. Regardless, there is a ground wire running across to the controls and to the amp chassis - no loops.
- As far as the ticking goes, it is there when the volume is up and at its clearest point - about 2 ticks/sec. steadily - faint, but definitely there.

@Both

- I have replaced all the electros (every one) but may have to do so again... The first time I did so, I accidentally installed one of the 10uF 450V caps backwards ... I replaced THAT one, but none of the others - Maybe my HUM source?

[g1]

I don't think a transconductance match of the 6BD8's will necessarily translate to equal idle current under circuit conditions. Your measurements seem to prove that.

[Britt Rossman]

@ g1

If it was only the tubes being mismatched, then why am I not experiencing the same measurements flip-flopped? It seems there is something else creating imbalance, no?

[Britt Rossman]

Here's something interesting... to me at least!

I swapped in "matched" JAN 6AU6 WGC's and got readings of 207V and 170V - still ~30V difference between the two, but higher overall due to lower plate resistance of 6AU6. I swap in a mismatched tube of identical make (second position), and my voltages become 241V and 134V. NOW, I swap those two tubes and my readings are 245V and 145V (almost the same)... weird.

Swapped in another "matched" pair of 6AU6's and got matching plate voltages of ~193V. Way high for that spot, but still reasonable for those tubes... AND the first matched voltages I've seen there. I could change the plate resistor's to have these match the schematic, but they aren't they same tubes, so...

Regardless, I still have a LOUD hum when increasing volume, while actual volume does not increase...

[Jazz P Bass]

Late to the show here.

If the Volume control affects the hum, then the problem (or one of them) is before the Volume control itself.

Not much there.
A plate coupling cap & a tone stack.

[ThermionicScott]

Here's something interesting... to me at least!

I swapped in "matched" JAN 6AU6 WGC's and got readings of 207V and 170V - still ~30V difference between the two, but higher overall due to lower plate resistance of 6AU6. I swap in a mismatched tube of identical make (second position), and my voltages become 241V and 134V. NOW, I swap those two tubes and my readings are 245V and 145V (almost the same)... weird.

Swapped in another "matched" pair of 6AU6's and got matching plate voltages of ~193V. Way high for that spot, but still reasonable for those tubes... AND the first matched voltages I've seen there. I could change the plate resistor's to have these match the schematic, but they aren't they same tubes, so...

Regardless, I still have a LOUD hum when increasing volume, while actual volume does not increase...

So it's looking that the idle voltage difference is partly due to the tubes and perhaps partly due to a little drift in the surrounding components though. I'll reiterate -- if the hum goes away with the volume turned off, those 6AU6's you're fixating on are NOT the problem.

Try pulling the 6EU7 entirely and seeing if the amp still hums when turned on.

[nickb]

@ThermionicScott

- I read the voltages on the OEM 6BD8 tubes as 143V and 128V. Switching their positions and letting the amp settle again gave 121V and 151V respectively - very strange. Both tubes are matched according to their mutual transconductance.

@nickb

- I purchased another NOS 6EU7 to replace V1, but my tester tells me it is actually weaker. Rolling these only gives a difference in timbre and dynamics, notably that the one I purchased to replace the original is more dull sounding...
- Right now, the amp is an octopus on the bench, with the input jacks floating off the chassis. Regardless, there is a ground wire running across to the controls and to the amp chassis - no loops.
- As far as the ticking goes, it is there when the volume is up and at its clearest point - about 2 ticks/sec. steadily - faint, but definitely there.

@Both

- I have replaced all the electros (every one) but may have to do so again... The first time I did so, I accidentally installed one of the 10uF 450V caps backwards ... I replaced THAT one, but none of the others - Maybe my HUM source?

Given the 'octopus' arrangement, it may simply be that you are picking up hum and/or interference from the input jack wiring. The ticking is probably interference from outside the amp, possibly conducted through the mains or picked up by the input wiring. A grid stopper located right on the grid of V1A might help. Tidying the input wiring by screen or twisting the signal and ground together might be worth a try.

Did you try adding a cap across the 10uf to see if that helped?

PS: To match tubes to need to measure in a few different places, just gm alone isn't enough to ensure identical DC operating points. Also the plate resistors will have a tolerance. But this isn't a HiFi amp so it doesn't matter. It certainly is not your problem.

[Britt Rossman]

So I pulled V1, and the hum stopped. I swapped the caps in the tone stack, but no help. I will try another across the 10uF later. I don't suspect the coupling caps to V1 plates...