Fx loop help - squealing

[a.train77]

Hey guys, on my 2x12 tremoverb, with the fx loop on, once I turn the volume and gain up past half way i start getting a feedback, that sounds like microphone feedback. My set up is Strat, amp, line 6 m13 in the fx loop. This is my first tube amp and I got it about a month ago, so feel free to say anything that I could be doing wrong.

 

[bendo]

Replace the V4 tube. It has gone microphonic.

If you bought the amp used, re-tube the entire thing. It's a bit expensive, but you won't run into any issues like this for a while.

 

[a.train77]

Okay. I know the schematic so I know where it is, but how do I check just to be sure?

 

[bendo]

Remove the retaining cap on V4 (if equipped). Turn the amplifier on, take off of standby (make sure the speaker and everything is still plugged in) and tap the V4 tube with the eraser end of a pencil. If you can hear the tapping through the speaker, the tube is gone. Make sure the FX mix is up.