I've been running a quad of these in my Dynaco Stereo 80 (90's NY manu) power amp and a pair in my home-made Marshall 1987x 50 watt plexi for about 4 years or so now. Both amps put over 500+ vdc plate. Tone in both applications is outstanding! Cranked tone in the Marshall is stupendous! I rechecked values last year on a Hickok 539C and they still tested like new for transconductance. These will last for years. A great buy no matter how you look at it...especially with the cost of NOS KT-77!!!!