Mesa Rec bad feedback?

[SwambA]

I just changed my tubes to jj's about 3 months ago. But I've been noticing high pitch feedback at pretty low levels. Today I turn on my amp and getting about 12:00 on my master with no feedback what so ever, that's what got me thinking, I was like there is no feedback...its loud... this amp sounds great! Played for about 1 hour then the feedback started again. Had to turn down the master to 9:00 and still had feedback. Any thoughts?

 

[Sloan]

Sounds like a microphonic tube, tap preamp tubes with pen/pencil and see if one reacts louder. Or you can swap them around and try to track a problem tube. Try cranking the amp without an instrument plugged in and see if it does the same thing - if so it's def a tube.