blues Junior amp makes a squealing sound after being on for ten minutes even with no connection
That could be a dead preamp valve. Buy one 12AX7 (try to get a balanced one) and systematically try it in each position. A phase inverter can make it do that, so can others but my money's on the PI.
Gilchrist custom
Yamaha SBG500
Randall RM100 & RM20
Marshall JTM45 clone
Marshall JCM900 4102 (modded)
Marshall 18W clone
Fender 5F1 Champ clone
Atomic Amplifire
Marshall 1960A
Boss GT-100

Cathbard Amplification
My band
Last edited by Cathbard at Jun 30, 2010,
I had a similar problem with my hot rod deville..... it was a bad pre-amp tube.