6505+ Fault Microphonic squealing / whistling, but stops when a pedal is plugged into front end!

[GrantsV]

Hi,
I have a 6505+ amp and the Crunch Channel only whistles and howls like an old kettle. The pitch of the whistle changes as the gain knob is altered...

[NovTech]

My Marshall VS100 head has a problem...

Here are the symptoms;

Clean channel is fine, although it does have a slight...