I had performed at a gig with my two year old Marshall valvestate amp about a week ago and never used it until the weekend on plugging in i found i had no sound? All lights on and the amp doing everything it should but no output of sound? I checked all the usual stuff and nothing was found. However i suspected one of the tubes had gone? I replaced this tube 12AX7EH also known as (ECC83) I was carefull not to touch the glass of the tube so fitted this inside the amp. Switched on and brilliant i had sound once again!!! However i had the amp on for approx 7 mins so while playing the guitar it all went dead again? All lights still on but no sound this has just cost me £13.00 and i suspect the tube has blown again. Can any body tell me what i may have done wrong? was i supposed to have done something before playing the guitar through the amp and putting on the overdrive? Any help please Could this be something else blowing the valve?? I had no tale tale signs something was going to go wrong, no burning smells or no cutting out just went dead.