New tubes . . . now my Subway Rocket sqeels! . . . Help!

[makeuacaster]

I just put new Mesa tubes in it. . . all of them. Except one 12AX7, which is slightly used out of a Marshall. I also took the 10" JBL out and put in a blue framed alnico speaker.

My problem: I get a microphonic feedback on RHYTHM channel, but not on the other two positions. No guitar plugged in. Gain on zero. Starts to squeel when master is at and above 4. Also, you can tap the chassiss and get that "ting" like you get when you tap a tube.

I notice when I run to an external speaker, there is no feedback. Could it be the alnico speaker?

HELP!

 

[makeuacaster]

Forget it . . . microphonic pre-amp-tube

 

[recto-robbie]

With your amp on, lightly tap each preamp tube with a pencil and listen for the tapping. When you find it,,, replace it and you should be good to go.