Hey so I just bought a Marshall vs100 head the seller said it had loud buzz but when I turned it on it wasn't bad until the pre-amp tube warmed up I would turn the volume up (on any channels) then the sound from cab would pop really really loud and cut out for a few seconds and come back on and pop then come back on, I swapped the mesa tube that he said he swapped out to have made it sound better with the jj12ax7 tube and it doesn't pop in the overdrive channels but the clean channel still pops and cuts out like its distorted any ideas where should I start looking? Thanks.