I've been tracking this down for the last couple of weeks. An intermittant sh*it sound coming from this vintage amp. You'd play then after awhile it would get all weak and distorted. SO After taking my time and undoing all kinds of mods done to the amp it still did this...intermittantly. AND Replacing tubes and finally choke and OT it sounded fine for a bit. THEN Got to the sh*tty intermittant sound again. It sounded like spitting....And I kept thinking some sort of short somewhere in the amp. FINALLY I found it. I took the amp back to the bench when it started makeing the spit tone again. I started sticking it with one of those STICKs and get what I figured out? On the B+ from the standby switch, it had been arching to the middle board of the amp. The middle board is the board that insulate the main eyelet board from the chassis. When I lifted the eyelet board and pulled the middle board out from under the eyelet board, the middle board had about a silver dollar sized circle of carbon on it. Right where it would cause both channels to should like sh*t...underneith the final coupling caps and at all the b+ junctions. Three other techs had gone through this amp and tried to fix it. None did. They had tried replacing trannies, wiring, and components. I got lucky and happend to start poking the eyelet board when it started making noise again. It happened over in the PI area of the board, so as I tapped it, it started arching. A lot of times you have to be patient and wait for the problem to crop up again.