Last night on way to a gig, I drove over a huge bump. The giggage in back of truck went airborne a few inches, then dropped back into place. Got to gig and set up, turned power switch on SVT tube head, waited for a few minutes, then turned off standby. Immediately got a very noticeable loud hum, sounding just like a bad ground. Changed elec outlets, got same result. Switched the polarity back and forth, no difference in hum. After a few minutes with power on, the hum reduced, then disappeared completely. I'm thinking that when the head was bumped during the drive, it knocked a tube or something loose. Any ideas what it might be? Oh, and I checked it again today. Same thing happens with the hum.