Diagnosing a friend's HRD. I'm handy with a soldering iron and have experience swapping components on other PCB fenders. But I've since moved on to vintage stuff. Anyway, friend of mine loves this hot rod but it sounds kinda buggy lately. Quieter than normal and really bad breakup on the clean channel. Having owned like 5 hot rods before I instantly recognized it as sounding like a dead power tube (or a loose power tube). The problem is the tubes are fine. I swapped in a known good set and it made no difference. However, when I yank the right power tube in the amp there's no change. When I pull the left power tube (and leave the right side in) the volume drops to almost nothing. I assume it's a resistor stuck open that feeds signal to/from that tube? Where would I begin?