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.


  1. I don't know if I can expect an answer here (I've not so familiar with blog culture!), but I though it couldn't hurt to ask. What should the bias mV be on this amp? and where do I test the level? I know I have to move the PR2 but where should I put the tester to measure the current and what value should the have? thanks a lot!

  2. Well, I use a bias adapter that I made from one of Hoffman Amps kits. There may be a test point on the board somewhere but I wouldn't know where it is and unfortunately although I had the same amp in for a new set of tubes this week it's gone now. The plate voltage was about 440v, so you can use the Weber bias calculator online and get a number. As these were new tubes I set them at about 38 ma each. You balance them with the PR1 pot and set the level with the PR2. A nice setup.

  3. Blackstars service tech said to bias the ht100 you set ur tester to 200 mv dc and measure across a 2 pin connector located on the board where the power tubes are.reading should be 100mv with all knobs full ccw,all buttons out on front and back of amp under load with plug in input of amp.balance adjust should be set to middle of the road.hope this helps

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    2. PR1 is the balance pot right? what do you mean by middle of the road?

  4. That's a good question. What IS the middle of the road? Using that method all you see is four tubes at one time with no insight as to whether they are balanced or not. Balanced tubes make for a quieter and better sounding amp. In my case I first measure the plate voltage and then use the handy dandy Weber bias calculator, I look at each tube individually and then bias off the one that draws the most current. I use a pair of dedicated multimeters and Allen Amps direct reading adapters.

  5. Not sure what he meant by it.Was hoping my amp tech will understand that part of it.

  6. This amp's back again. Someone spilled a beer inside it and barbecued the tube board. Bear in mind it was not I who said "middle of the road" but some Blackstar person. My own personal take on it is once you figure out what the cathode current (plate + screen current) draw should be and you compare it to what the maximum ought to be (about 70 per cent dissipation) then the middle of the road ought to be to back it off a bit to leave some room for further adjustment as the tubes age.