Tube Amp Archive

I just finished upgrading a Kay 703c. I put in an isolation transformer and 3-prong power cord specifically to avoid the issue you mention. The chassis is connected to AC ground and one leg of the isolation transformer output. I also replaced the electrolytics. Here's a link to photos http://www.nat...

Tubes are the first thing to check, and dropping them isn't good.

The next suspect is electrolytic caps in the power supply. Sometimes they can do this when beginning to fail.

I've also found coupling caps to do this, but that's fairly rare.