My 68 bandmaster reverb has started to crackle. The sound comes from the speakers and is an intermittent type crackling sound. I thought perhaps something was wrong with the speakers but it does it in different cabs, does it even if I unplug the guitar cable. It seems to do it after about 20 minutes of playing then you just hear a white noise crackling sound coming from the speakers. I will yank some tubes and see if that helps-any suggestions?