KT 77's in a HT 50?

[gbsmusic]

My friend put a set of KT 77's in his HT 50 and while it sounds incredible he didn't bias it, just put them in. How bad is this and anyone know where the #'s should be for biasing the amp for the 77's? Thank you.

[thephantum]

There is no set number. KT77's have the same plate dissaption rate as an EL34 (25 watts), so the bias process is exactly the same:

• - Measure the plate voltage
  - Do the math
  - Measure and adjust bias current
  - Rinse and repeat until dialed in

The math is: Maximum Plate Dissipation [W] / Plate voltage [V] = Bias Current [A]

Maximum Plate Dissapation is not 25 watts (if you do that it will burn the tubes up). It's the maximum desired plate dissapation and it should be some percentage of that number. I personally start at 70% (17.5 watts for EL34/KT77) and then dial in by ear from there.

Theoretically, it should live somewhere between 35mA and 40mA per tube, but that's a guess. I know I wouldn't want to hang the health of my tube amp on a guess.