What would be normal and what acceptable for a solid state or valve/tube stage amp. ? Well you would not want the gain/ output audio to go up over an hour of use with a constant input. Testing a solid state amp that had a problem that certainly affected the FET gain killer protection circuit so (falsely) cutting the output over an hour to close to zero. But checking to see if there was any other gain drop problem but then realised I don't know what the norm is. Amp rated at 200W (music power) which I've taken as 100W (continuous RMS if you could). Running a constant source of 1Kz through amp to give a continuous 1/5, 20W in a dummy load , which with heating itself is not necessarily constant. Result of testing over 40 minutes was a 9 percent drop in V rms into the load so about 20 percent drop in equivalent audio watts. No fans on this amp and the heatsink settled at 68 degrees C after about 30 minutes. The output was still dropping after 30 minutes but very much slower and decided to cancel after 40 minutes
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on