A tubescreamer has an active bypass and always acts as an active buffer (emitter follower output) regardless if engaged or not.This is the weird part, when I put a tubescreamer between the amp and guitar, the whistle is gone. Regardless of whether the tubescreamer is engaged or not!
Its almost like the pedal being put before the input acts as a passive buffer, preventing the howl.
Seems the whistle depends on amp input termination impedance. Does the amp work as it should with the TS?
Does the whistle also stop with guitar volume at zero (without TS)?
Please post schematic.