I changed the old V2 tube to a new one a couple of days ago because the old was microphonic and the amp feedbacked when I turned the volume up a bit. It feedbacked even if I turned everything but the amp volume down (including my guitar's volume knob and amp's gain knob). But no problems any more because I bought a new Electro Harmonix tube and it improved the sound a bit so my amp sounds even better than before.
