I bought this used from a reputable musician and it's finally starting to fade. The other night the main channel started to fade volume wise in and out. I switched over to one of single amps instead of the center bridge it performed fine the rest of the night. Once I got home I checked the tubes - not much else to jiggle and shake in there. Anyway, are these amps repairable? One thing that I thought was peculiar was it use to whine (like a cap charging up)during the first 30 sec of warm up. Was this normal or a potential problem?