| Sounds like a parasitic oscillation, more commonly known as motorboating. Check the tubes first by swapping in known good ones, then find out if any of the front panel controls change the sound. It would be a volume or treble pot that would do this. If the tubes are good and you hear a change in the sound when turning a knob, then you need to check all the caps starting with the filter caps. Sometimes a bad ground on a cap can cause this. |