I dont know if its my effect loop or what.. but ill be playing.. then randomly the volume will raise or something.. maybe its my effects loop w/ maximizer cause i noticed at times when i have the effects loop plugged in.. its really quiet and i unplug the cable from the loop and it gets back to normal volume.

Any ideas what this is? And when should i change tubes? Never done it before..

peavy 6505 tube head