My first guess was a bad tube but the channels are separately wired and the racket's the same on both channels and it does not pass an audio signal, so I'm guessing it's not a single lame tube and may be in the pwr supply or some other commonality. The caps all appear to be OK by visual inspection, though I've no real way to test them. The xformer looks OK, no bubbles. It uses 12XX7s in all four holes.
Might one of you be so kind as to steer me towards a diagnostic fix? Or should I just stuff it with new tubes?