My Vox AC10 has a mild buzz or hum ( hard to describe) when I engage the gain knob to activate the reverb while playing my Gretsches. If I place my fingers on the strings, or if I turn the gain knob back to zero, this stops. It is also occurs when I play my Tele, but placing my fingers on the strings in this case has no effect. I replaced the power tubes last year thinking this might be the prob, but no happiness. Amp is 3 years old. My practice space does not have flourescent lighting, so that isn’t the cause. But what might it be? And what to do?