Great tubes! I'm using them in my DIY PPPdiff mono-blocks. These amps use 4 output tubes in a "forced" class A push pull differential mode (the 2 pp halves are in series, nor parallel like class AB1 and cannot operate out of class A). Operating at 40% of plate dissapation as opposed to 85% with 6550s in these amps is a real benefit. I have experienced one instance where aKT-120 opened and stopped working requiring the replacement of a matched quad. That's better than the time a 6550 arched and turned a tantalum grid-stopper resistor to charcoal. ;~)
