Biasing a Blackstar HT Studio 20 Amp

Okay, so this was a weird one. My buddy asked me to throw some new power tubes in his living room amp real quick. I figured this would take 30 minutes tops. But this being the first Blackstar amp I have had on the bench I was in for a headache that would last about a week.

IT TURNS OUT THAT BLACKSTAR AMPS WILL NOT PULL FULL IDLE CURRENT WITH NOTHING PLUGGED INTO THE INPUT JACK.

There ya go. If that's all you need to know then head out into the world, bias between 60 - 70% and dial in for tone from there if needed, and be excellent to each other.

If you wanna know a bit more, here we go... First off, these amps have a PCB that sits about an inch below the chassis, with PCB mounted tube sockets. There is no way to get a typical bias probe into these things without having tube socket extenders. Which, I was not prepared for in the moment and did not have. Next up, this thing still pulls SOME current at idle with no input. Just enough to make it seem like everything is wrong, but maybe right, but no that can't be right that is WAY too low even for whatever is going on. Anyways. The schematics are also hard to come by (I had to use the Wayback machine).

Click here to download Schematic