Help Needed - Troubleshooting Gibson GA30RVT

[guitarrock04]

Hello all, hoping for some help in diagnosing this issue. Been scratching my head on this one for a while.. Amp was previously working fine, and has now developed a hum at all times amp is on, and a very odd harmonic distortion that is present at all volumes. The amp was recapped and working well prior to now. All voltages measure as expected when amp is powered.

I did notice that as the amp is switched off, the problem goes away for the brief time period that the amp still passes sound. I figure this may be a clue.

Any thoughts on where to look are greatly appreciated. Sound clip here:

[shooter]

pull V 1 n V2 does it still hum?

[guitarrock04]

Yes, the hum is unchanged with all preamp tubes removed - hum still present when running only power and regulator tubes.

No sound is passed when power tubes are removed..

[66Strat]

Maybe one of the output tubes is bad. Do you have spare output tubes that you could swap in?

The hum sounds like it's 120 hz which points to the power supply. Is the OA2 regulator tube good?

[guitarrock04]

While I was pulling tubes this morning, I swapped in a known good pair of 7591 power tubes, and no change.

I am uncertain as to how to test the voltage regulator, but can verify that it glows when the amp is taken off of standby, and it regulates to 150v DC..

I also suspect power supply. All caps and diodes were replaced - which doesn't mean something couldn't have prematurely failed, but I don't think it likely. Perhaps the choke..? Not sure what symptoms of a choke on the fritz would be.

[66Strat]

Try pulling the regulator tube. I wouldn't think that the choke would have failed without some prior symptom

[DummyLoad]

did you check the bias supply electrolytics? link to schematic below - C28 and C29. 

https://el34world.com/charts/Schematics/files/Gibson/Gibson_ga30rvt.pdf

[66Strat]

When the electrolytics were changed, was the cap can replaced?

[guitarrock04]

Thanks all for the help and suggestions.

I replaced all the cap cans with modern discrete electrolytics. Cheaper and easier. Glue and zip tie them together, and put right back where the cans were originally.

I replaced R55 with a 50k trim pot some time ago. I inadvertently omitted C29 altogether following this modification. However, it's been that way for many years and functioned normally in the past. Replacing both C28 and C29 has entirely resolved the concern. The amp is now working properly.

A bit more troubleshooting to do on the reverb now.. Driver circuit isn't working - always something.