I brought one of my combos to the recording studio this past week to give an artist some choice for a variety of sounds. Maddeningly, the amp seemed to have a microphonic tube and rattled and rang as a result. After spending all day swapping tubes and rewiring the pre-amp circuit and not fixing anything, I took out the 12AT7 I was using as a preamp tube and replaced it with a 12AX7. Problem solved. It is working but I will have to finalize component values and do some more soldering. All of the ATs that I installed, including Marconi NOS, old pulls of various brands, new EH and Sovtek all had the same problem with being too microphonic to use as an input tube. Lesson learned; 12AT7 cannot be used in a high gain situation. The amp would actually feedback through the tube and speaker, even with the chassis out of the cabinet but placed nearby!
Don