So I have one amp, a custom built unit based on a 5F11 that has something strange going on. Plugged straight into the amp, when I turn the volume knob on my guitar up and down, coming through the amp, it sounds like a dirty volume pot!! Guitar plugged through any other amp I have, doesn't do it,.. so I'm sure it's the amp and any guitar that I plug in, same issue,.. turn the guitar's volume up and down, sounds like a dirty volume pot on the guitar. Plugged into two other amps I have, no problem. Any ideas? oh,.. all new tubes.