Biasing for KT88/90

[greiswig]

Hi, all,

My build is currently (no pun intended) running EH KT-90s, which I like. My plate voltage is about 525V, with a little less on the screens. Right now, I have them biased with 48ma on them, which should be about 25W at idle.

With 6L6's, I'd always biased them fairly cold at idle. Hotter than about 36ma at 420V seemed to make them sound tubbier than I cared for, relatively.

But I'm not sure whether that same "bias cold is best" philosophy applies to other tube types. Is anybody else running KT-88s or 90s, and if so what kind of bias current and voltages are you using that you found optimal? Looking at Weber's bias sheet, the closest I can get to equivalent to what I'd bias 6L6s is the "Average" bias for a 6550, which would be more like 40ma. But with the 50W plates on the KT-90 monsters, 70% would be more like 65ma.

[heisthl]

I have an amp model running 4 KT88's at 500v that bias at 184 (46ma per tube) - love the sound of kt88 tubes.

[talbany]

My Low Plate Classic runs 520V biased 45ma running a pair of JJ-KT-88's

Tony