Troubleshooting a Gibson GA-5t

[Strat Matt]

Hey guys,
Hopefully one of you has experience with repairing amplifiers and is willing to help me out.
I recently purchased a non-working 1966-67 Gibson GA-5t amp for cheap in order to practice fixing such things. I've modded my own amps (grounded plug, new filter caps) with success, but never a full repair. Anyways, on to the problem...

Here is a schematic for reference: http://www.0rigami.com/gg/ga-5t_skylark_white-panel__back_qual6.jpg
The only change is I've added a three prong power cord and removed the death cap (C15).

Symptoms: No guitar signal passes through to speaker. I still get a wobble when turning up the trem. I measure voltage within spec everywhere except at pin 5 of V2 (300vdc) and at pin 7 of V2 (0vdc). Also all resistors measure within spec.

I believe the problem lies with V2. I have tried a few different tubes in there with the same results. I also removed the cathode bypass capacitor to check if it was faulty, but experienced no change in any measurements. There could also be a problem with the Phase inverter transformer. I fear this possibility the most as it would be expensive and invasive to replace. I also don't have an accurate method of testing transformers.

Any help is greatly appreciated. I know this isn't an amp or electronics forum, but I've been a member for several years and I know the collective knowledge here is expansive.

Thank you,
-Matt

 

[Cagey]

Testing transformers is fairly simple with just a multimeter. Set it for resistance. There should be no connection to ground, and the individual windings will read basically as shorts from the start to finish connections for each winding, with no connection between them. This makes sense, as it's basically just a coil of wire and you're measuring DC resistance. You can't check impedance, winding ratios, etc. with a typical multimeter, but if the windings aren't shorted/grounded/open, chances are pretty good there's no problem. When measuring, be sure there are no connections to other components that could interfere with your measurements.

 

[Strat Matt]

Thanks Cagey,
I disconnected and tested the transformer tonight. Here are my results:

Primary side shows about 750 ohms end to end.
Secondary side shows about 3 kohms across with 1.5 kohms and 1.9 kohms from each end to the center tap. No shorts to ground and no shorts between windings

The difference between the two sides of the secondary is strange, but I don't know why that would cause no sound or 0 voltage at the cathode of V2.

I did find a discrepancy between the schematic and my amp tonight. My amp has C13B wired in parallel with C14A. I will switch this back, but again I don't think this is causing my problems.

I feel like this should be an easy problem. "No voltage here so do this." But I can't figure it out. Am I right to accuse the preamp if I hear the trem oscillations?

 

[mayfly]

Depends on the type of tremolo.  Some gibsons, especially cathode bias ones, modulate the power tube bias to achieve the effect.

I'm at work right now so I can't dive into this, but are you hearing any hiss at all from the speaker?  Are you sure you have your primary and secondary the right way around (Primary - to the tubes, secondary - to the speaker).  750 ohms sounds high to the speaker, but it should do something.  3K sounds not unreasonable to the tubes.  An unbalanced center tap is a bit of an eyebrow raiser (as in 'wow Gibson used some crappy transformers!', but even like that you should get a sound.

 

[mayfly]

just had a quick look at the schematic.  Wow!  a transformer as a phase inverter! 

 

[mayfly]

Hold it!  zero volts DC at pin 7 of V2?  Either R11 or C7 is shorted out.  Probably the cap actually. 

Replace them both.

 

[Strat Matt]

R11 reads 1.047 kohms and I've removed C7 completely with no change. I'll find replacements for these and other electrolytics as soon as I can.
There is a loud hum from the speaker.

 

[mayfly]

Hmmm.  well, the tube is not conducting, because if it was you'd have ~8V at that point.  That's definitely the problem.

Either it's

1 - an open somewhere at that node
2 - a short to ground at that node caused by some other means (wiring?  bad insulation on a wire?)
3 - tube problem (an open or a short - kinda the above except in the tube)

Checked the socket yet?  the insulation on the wires yet?  Have access to a tube tester?

 

[Strat Matt]

I do have a tube tester. No shorts in the tubes and they test strong. I'll check for shorts in the socket tonight.

 

[Strat Matt]

Things just got weird. I was testing for shorts when I noticed a low resistance in the filament for V2. So while leaving my test leads on pins 3 and 4, I turn the amp on and test for voltage. The hum gets really loud and goes up with the volume knob. So I plug in a guitar and hear it coming out of the speaker! The volume fades to zero when I remove the meter.

Does anyone have any clue what's going on? I guess I'll be investigating the filaments more closely now.

Edit: I have not even able to recreate this again. I'm guessing that by attaching the clips I moved one or both of the pins and temporarily fixed the open/short. May just need new sockets. They're all pretty loose, but they're riveted in and I checked continuity from side to side.