Amp gets quiet after a while

[Sydius]

My Bottlehead Crack with speedball kit has been working really well for months now, but it has just started having a strange problem: it's fine for ~20 minutes and then very slowly gets quieter and quieter. I can adjust the volume to compensate for a while. Turning the power off and on again fixes it for a while.

What might I check?

 

[Paul Birkeland]

This is probably a funky solder joint that is responding to the amplifier heating up.

If it's on both channels, I would be highly suspicious of the twisted pair of wires leaving power transformer terminals 4 and 5 going to B7/B8 and A4,5/A9.

-PB