When playing my Mark IV, the volume surges briefly

[fairfield2002]

Hi I was playing my mark iv the other night on Lead and had the master volume set at about three when all of a sudden the volume surged for a second or so then went back down to the previous volume. All of the tubes have been replaced in the last six months. Does anyone have any suggestions on fixing this? I dont want it to happen during a gig. Thanks!

 

[cradlefish]

tap each tube and listen for any weird noise to compare each one. It could also be your phase inverter going bad, which is the tube closest to the your power tubes.