How old is the amp? How long Have you had it? Were the Valves (Tubes to you yanks) replaced in the last 5 years? Were the filter caps changed in the last 5 years? Do you know a good tech? If no to the 3rd and 4th Q, change them now!!
Thats a new set of MATCHED valves and new high voltage filter caps....as matter of fact ...change all the electrolytic caps while you are in there. BE CAREFUL many good techs have died working on high voltage valve amps.
Good luck!!!
