This is a weird one. A dealer gave me a call saying his vintage Ampeg GS12 amp was humming. Went and had a look at it, and when you turn the volume control all the way down the amp hums. Turn the volume up and at about 9:00 position the hum suddenly stops and the amp is quiet. So I bring it back to the shop thinking bad cap, bad pot, bad ground, any of which may cause this. And this is where it gets weird. Fire it up, and no hum, no matter where the volume control is. I tried changing the grounds, removing the death cap, grounding the chassis and a few other things. I could not get the humming to return. Bring it back to the dealer and the hum returns. It's like a switch too. Reach a certain point on the volume control and it suddenly stops completely. He's going to try the amp himself at a different location and see what happens. Anybody else seen this happen?