Originally Posted by amplifier

Sure thing buddy ol' pal. When a tube goes microphonic you can get bad feedback (edit even with your guitar turned down)or certain frequencies will continue to ring out, like when FiliphSlim mentioned it sounds like it has a reverb effect on. You can also see if it's going microphonic by tapping the tube w/the eraser end of the pencil slightly. You'll hear it through the speakers but if it continues to ring it's going bad.