Tremoverb Feedback

[masterofsparks]

Anyone have an idea why my tremoverb would produce a very high pitched feedback with the guitar volume turned all the way down? It only tends to happen with the gain turned up, I think past 12:00 or so. Seems odd. . . Microphonic tube?

Thanks.

 

[Platypus]

Microphonic preamp tube

 

[masterofsparks]

Thanks. Hope it's not the old Mullard I have in there. . .