My spider valve 212 has doubled its output volume since the last time i used it. Basically I didn't mess around with the channel volumes, so when i jammed at my buddy's place i'd have the amp around 10-11 oclock on the master volume and it was plenty loud. Now a few days later, the same volume is achieved at 8-9 oclock.

Also, when the master is turned all the way down, I can still hear my guitar through the speakers, and sometimes it sounds like its repeatedly going up then down in volume. Also, if i tap the master volume (literally tap it on the face of the knob), it crackles and does the up and down volume. Sometimes it crackles when turning it up normally as well.

I don't know what it could be, but is it that my master volume knob is going bad? The tubes are fine and all, so I have no idea.