Volume dropping over time while playing SE 6AQ5 amp

[dogburn]

Happy holidays! I've got a tiny vintage Japanese amp that's a single-ended 6AQ5 with 12AX7 preamp and diode rectification, and when I first got it a few years ago, it seemed to play fine. I did a few mods, including NFB switch, in-line power fuse, and three-prong cord, and it kept working fine after that. But recently, when I play guitar through the amp, it starts out fine at about the volume I expect, then after a bit of playing (ca. 5-10 minutes), the volume drops and it's rather quiet.

Any clues as to what could cause that? Could it be a capacitor going bad? I replaced the filter caps a few months ago, but that did not fix the problem. So could it be a coupling cap or bypass cap?

I've attached a schematic with values as far as I could determine them - it's point-to-point wiring in a very tiny chassis, so hard to see everything. Anyway, it's sort of like a 5F2a circuit with a single diode rectifier and a 6AQ5 power tube. It sounds great when first playing through it, and I was inspired to build a 6AQ5 5F1/5F2a along the same lines (and they sound quite similar except for the major difference of 6" vs. 10" speakers).

[astronomicum]

If the amp is using Suzuki or Atlas oil caps, especially those rated 400V, they are most likely bad and should be replaced. I would also replace the power tube cathode bypass cap if you have not already done so.

Edited: Nippon also made them.

[dogburn]

Thanks - I will check those out.

[tubeswell]

And you swapped out the 6AQ5 already?

[dogburn]

Yep, already tried different 6AQ5 and 12AX7 tubes (should have mentioned that at first).

[shooter]

have you monitored the cathode bias VDC and plate VDC of the PA when things are good and not good??

[tubeswell]

And test for dc-leakage on the coupling caps (although that would cause volume loss all the time, not just sometimes)

[dogburn]

I have not done any voltage measurements yet - it's quite tricky with the cramped little chassis. But these suggestions give me some direction, and maybe today I'll have the chance to open it open and check things.

[dogburn]

Update: The coupling caps were leaking DC, so I replaced those and now the amp is behaving normally. The originals were branded Zeus, and must be ca. 60 years old, so not a big surprise they were bad. Turns out I'd already replaced the power tube cathode cap in the last year or two, so that one was fine.

I've included some pics of the chassis and caps so people can see a bit more of this amp. It is fun to play through, so I'm glad it's back to form.

[astronomicum]

I have come across this particular cap but not the Zeus brand. Thanks for the pics.