I have an older Trace Elliot head from the mid 90's that hums (60hz) intermittently when I roll the solid state/valve knob to the valve side. The ground lift doesn't change anything. I tried switching out the tube with no difference, and I have gone through and re-soldered the usual suspect connections on the main board and power supply(ie: the tube socket and most of the transistors that look like they get hot, some caps too). Has anyone else ever had this issue and know what was done to fix it? I would take it in, but I have a 3 month old and wife isn't working, so cash is short for a couple months yet. It works fine on the solid state side, but I'd like to be able to use the tube side as well. I have lots of experience tinkering with old 70's silver face stereos, but this is the first time trying to fix an instrument power amp.