• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Strange Hum

I am building a simple one 6688 tube per channel headphone amp. It was working fine on test board. When I put it into a wood frame case, there is vey loud hum from one side. It is not bad tube or wiring mistake. It plays music fine on both side and tube roll does no help. But when I put my finger on the tube, the hum stops. I guess it is oscillating on that side. I need to get my scope out tomorrow to check it. Or maybe it is a loose tube socket? Before I change the socket, I want to see if anyone has had similar experience.

Thanks.