Supro Thunder Bolt Cutting Out

[brsmith2121]

I've got an old Supro Thunder Bolt that sounds great until it gets hot (about 15 min.) and then cuts out and plays with very low volume and lots of distortion. The volume comes back momentarily when I turn it off, but that's it. I've replaced the tubes, cleaned the sockets, replaced the electrolytic caps and screen grid resistors. Any ideas where to look next?
Thanks!

[Jazz P Bass]

I would imagine that testing the coupling caps would be next.
Plate voltage on one side, nothing on the other side.

[brsmith2121]

Thanks! I'm getting 379 on one and 378 on the other. This doesn't change even after the amp cuts out and the sound distorts.

[Jazz P Bass]

I do not know what you are measuring.
Plate voltage of the output tubes?
I was leaning towards the first tube, V1.
The plate sends the ac signal through a .05 capacitor, while blocking Vdc.
That is what I would test.
Is the cap blocking the dc voltage to the next stage.
Beyond that, you could monitor the cathode resistor (200 ohm) of the output tubes.
See if there is any change when the problem occurs.

[brsmith2121]

Yes, that was the plate voltage at the output tubes. I'll check the plate voltage at v1 and make sure the .05 cap is not leaking. Thanks!

[Jazz P Bass]

You have the right idea, though.
Look for something, anything, that changes when the problem occurs.
That will be the clue.
Unlike a totally failed amp that exhibits the symptom all the time, as your amp works & then does not, you have to catch the failure.

[brsmith2121]

Thanks for the help! I'm currently waiting for it to fail. The .05 on v1 is steady and is blocking all of the dc, but I've noticed that the dc at the other end of the .05 coming off the plate of v2b is creeping up from .12vdc when I started to .16vdc now. The other one (v2a) has been steady at .06vdc.

[Jazz P Bass]

Ooh!
Here is schematic with a voltage chart.

[brsmith2121]

I replaced the .05 that was letting more dc through and it seems to have fixed the problem. I'll check it some more later when I can play it for a while and make sure there isn't anything else fishy. All the voltages look good compared to the latest schematic you shared. My output tube plate voltages were a little low, but then I remembered that I had a 5R4 instead of a 5U4. I put the 5U4 in and they're almost spot on. Thanks!