Tuesday, October 1, 2013
Biasing a Blackstar HT100
This amp arrived for a basic tube checking and rebiasing after three years of heavy service three nights a week. A couple preamp tubes were pretty well shot but the power tubes were fine. When I went to check them with my handy dandy cathode current checkers I found no cathode current but -95 volts on the grids. Whaaaaaaa?
Bear in mind that Blackstar don't think you or I know nuthin' bout these yere ampolifires, Lester. You either Jeeter. We've had this conversation before.
Doing a little research I discovered a reference to the fact that something must be plugged into the input jack to make things happen. It seems that with nothing in the input the amp acts as if it is on standby. I can't think of a reason for that unless you want to make people think that the amp is a lot quieter than it really is.
Once that was done the cathode current came up nicely on my two Harbor Freight dedicated multimeters. The procedure then is to adjust the bias balance control marked PR1-which is the upper one in the picture-to equalize the bias current and then adjust the level with PR2 which is the one in the bottom of the picture. These pics were taken from the rear of the amplifier.
As I understand it the HT60 is adjusted the same way.
Now you know.