Hello all -- I picked up an Ampeg svt 3 that was barely producing sound and followed @ejschultz fantastic instructions for biasing the amp in this thread; Mine was reading less than 1mv and I successfully biased it up to spec around 25mv. Amp sounds great but there is an issue with the tube preamp section. When the tube gain is all the way down it makes a buzzing, then a whump as you turn it up and the amp sounds good with tube gain at 10, but I think because it is out of the circuit and not functioning at 10. Using the preamp output, it sounds good, but the tube gain does nothing. I tried swapping out all the tubes for known goodies, and tapped them with a chopstick looking for bad sockets. Could anyone advise on possible weak links here -- the potentiometer for tube gain, or something else? That pot is hard to get contact cleaner into. I'm not sure that the amp is producing all the power it should -- I have the master at 10 and use the gain to adjust volume -- but maybe it is. I'm not used to cranking the master, but it clips with the solid state gain at about 6. Any input would be welcome. edit: while I'm here, is there a known, quieter replacement fan?