Volume fade

Just as a followup, I replaced the rectifier tube and it is fine now.

if it's in both channels, it's probably in the preamp. I assume you have a diagram of which tubes do what, and there are almost certainly tubes that affect both channels - change those tubes and see if it fixes the problem. if you're going to own tube gear, you'll want to have a full set of replacement tubes anyway, so this experiment won't really cost anything.

with tubes, diagnosis is pretty simple: identify the location of the problem (both channels? line or phono or both? if it is in one channel does tapping on that channel's tubes lightly affect the problem?), then switch out the suspect tube(s) - if that doesn't do it, you'll need to look at the next most likely suspects: things like bad capacitors; but with tube amps those are usually easily locatable and not too pricey to fix. good luck