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 …
 Look at R221 and connect a multimeter, then adjust the bias pot. You must read 11.5V
 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 !