Thanks for that.... have had the amp 18 years just used Boogie valves , no real problems just price . Would be grateful if you could tell me who supplies JJ's , understand about the bassman pre amp design . Harma's in my opinion emasculate the amp somewhat ....lower output , reduced bottom end . I refitted the Boogie valves , and feel I can make these comments , but the Boogies are in need of replacement . Thanks for yor help .