How to bias a Blackstar HT Studio 20

A nice Blackstar HT Studio 20. My customer asked for new tubes.

I do not have the schematic, but what I see is that this amp has a mix cathode / fixed bias system … so bias must be adjusted.

Edit feb 2017 : another Studio HT 20 to deal with. This time, I got the Blackstar HT-Studio 20 schematic !

First of all, connect a speaker cab, then plug in a jack in the input : input jack is used as a standby : no jack = standby on (no high voltage), jack inserted => standby off (high voltage applied to the tubes).

Bias and bal pots …

[1] Look at R221 and connect a multimeter, then adjust the bias pot. You must read 11.5V

[2] You can check the « bal » adjustment to have the same voltage for grid1 (pin5) for both EL34. On my customer amp, I can read :

  • Pin3 (anode) : 333V
  • Pin5 (grid 1) : -14V
  • Pin8 (cathode) : 11,5V

Hope this helps !

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3 Responses to How to bias a Blackstar HT Studio 20

  1. andrew breckill says:

    This doesn’t help, when adjusting (2) balance where are the probe located? I can see in the bias where they are connected but not in the balance adjustement.

    • t-geek says:

      Sorry, I don’t remember exactly (as I don’t have the amp in front of me).
      Maybe I measured the voltage drop between each anode and central tap of the output transformer (beware, high voltage!) … (or, more simply, grid 1 voltage, assuming the tubes are appaired)

    • Vinic says:

      Hey TG!
      I have one on the bench right now for new tubes too.
      The measures are made between ground and pins.
      Personally I get :
      - Pin3 (anode) : 333V
      - Pin5 (grid 1) : -12,8V
      - Pin8 (cathode) : 11,8V
      This is the second one I get on the bench too, and I have to say these are great sounding amplifiers, and they are quite well built! And REAL 20W, with 2x EL34 and not 2x EL84 which are limited to 15W push/pull. Sadly, Blackstar returned to 2x EL84 on the new series…

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