KECL82 debugging

[Tumwad]

In the early 2000s, I built the KECL82 valve amp kit from World Audio Designs. It worked fine for a while, but then it started to show partial muting in one of the channels. It would be fine at switch-on, but after a few minutes, one channel would reduce in level, giving an unbalanced sound. It was always the same channel, but I can't recall which.

There's four valves in it, and I swapped them around to see if the problem changed sides, to no avail.

I checked all the test voltages at various circuit points as per the assembly manual, and these were all at or very close to what they should be.

It got tucked away in a cupboard shortly after this, but has anyone got any suggestions for next-stage diagnosis? I was out of ideas when I tucked it away.