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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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.
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)
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…