DJPhil - Thanks for the ideas and the schematic. It helped a lot. When I tore the amp open and looked at it, I could see two diodes (CR25, CR26) that have overheated at some point. On the back of the circuitboard, the solder to the negative sides of these diodes had come undone, and this is what was causing the hiss. I re-soldered the board, plugged the speaker in, and it was quiet again. Now I'm wondering what caused the overheating in the first place, and whether it will repeat itself over time. With the amp plugged in, the two resistors nearest these diodes (R94, R95) get hot fast. They are 330ohm 10% resistors, and are reading 310 and 319 when metered. That seems to be within spec. What kind of heat is acceptable or too much?

Attached is a photo showing the overheated diodes and resistors.

I've been searching the boards, but haven't found a post with the same problem, so any help would be appreciated.

I've got a single 12" 8ohm speaker mounted in a cabinet, wired to a mono jack, then plugged into the 8ohm output of the amp with a speaker cable. With no instruments plugged into the amp and all volumes on all channels at 0, I get a loud hiss. With an instrument plugged in, the hiss is still there at any volume. I've tried replacing the speaker, replacing the wiring from the speaker to the mono jack, replacing the jack, replacing the cable from the speaker jack to the amp, and nothing helps. This problem has developed over the last few weeks, and gotten progressively worse. This amp does have an effects loop (preamp out, Power amp in) and two inputs. Not sure if it helps to know this or not, but throwing it out there.

Thanks for any suggestions...