I'm trying to figure out what is wrong on a friends amp. A cheap carvin vintage 33. It has a very decent clean tone but has a low hum that makes it unusable. I tried replacing all tubes with known good ones and poking around with a chopstick. I've checked the bias . It does it on both channels. He's had it to the tech 3 times over the past few years and he's tired of soldering on the flimsy circuit board Thats why I'm messing with it. I'm not a trained repairman but can usually poke around and figure obvious things out. I know how to discharge the amp to not kill myself with it. Any help will be most appreciated!