Blackstar HT Studio 20 Bias Setting?

[Bobwith2Bs]

Anyone have info on these amps?

I have a Blackstar HT Studio 20 on the bench. Just needs some power tubes swapped but the tube sockets are recessed so far below the chassis that it is impossible to get my bias probe in there.

Took forever to track down a schematic for this one (will post it on the thread in a bit).

I saw on post say that -40VDC at TP13 is what we are looking for on these amps, but hoping someone here has confirmation or a suggestion on a better method for biasing these up.

[xtian]

Standard approach for all fixed bias amps that don't have convenient cathode bias sense resistors installed: With amp off and discharged, measure resistance of each leg of OT primary (example, red to brown, red to blue). With amp running, note plate voltage at power tubes. Measure voltage drop across each leg of OT primary. Use Ohm's Law to get current. Check bias calc <https://www.amplifiedparts.com/tech-art ... calculator>. I prefer to aim at about 50% of max dissipation.

[Phil_S]

Standard approach for all fixed bias amps that don't have convenient cathode bias sense resistors installed: With amp off and discharged, measure resistance of each leg of OT primary (example, red to brown, red to blue). With amp running, note plate voltage at power tubes. Measure voltage drop across each leg of OT primary. Use Ohm's Law to get current. Check bias calc <https://www.amplifiedparts.com/tech-art ... calculator>. I prefer to aim at about 50% of max dissipation.

The alternative to what Aaron says is the shunt method, where you measure the actual current. I don't think anyone actually likes to do that. I don't like the idea of it. Much safer to measure the voltage. Lots of us use the R value of the OT primary to calculate bias.

[johnnyreece]

I don't know how much room there is in the cab, but maybe something like this would help "extend" your tester socket? https://www.amazon.com/AmplifiedParts-8 ... B005IYPLH0

[Bobwith2Bs]

Thanks everyone!

I guess i should have mentioned that the transformer winding measurement was my backup plan, but was hoping someone knew the bias test point voltage that is expected on these.

I will dive back in and report back with where that voltage lands in case anyone else needs the info in the future.

[Stevem]

Go to the antique electronics site and order up two 9 pin socket savers.

Open them up and , open the cathode pins and then drill holes to extend out wires so you place your meter set for milliamps in series.

I wire in cut off clip leads for easy connection to the meter.

I have this one set up to extend heater test points to the lip of the saver so I can exactly set my testers heater voltage.

[Bobwith2Bs]

Yeah, the compu-bias i typically use has a similar setup but wires come out the side of the tube socket adapter...

With the Blackstar amps the tube sockets are recessed low enough below the chassis that there is no room to get them in there.

[Stevem]

To me -40 sounds cold for a EL84.

[Bobwith2Bs]

To me -40 sounds cold for a EL84.

This amp has EL34s.

[sluckey]

Took forever to track down a schematic for this one (will post it on the thread in a bit).

Where's the schematic?

[Bobwith2Bs]

Took forever to track down a schematic for this one (will post it on the thread in a bit).

Where's the schematic?

Just got home from work. Attached the schematic to the initial post.

[Stevem]

Thanks for posting the schematics but I don’t see one with the output section.

[sluckey]

Thanks for posting the schematics but I don’t see one with the output section.

The output tubes are on pages 3 and 7.

[Bobwith2Bs]

Finally got this resolved and had to close the loop on it in case anyone else comes across this and has the same frustrations that I did in solving this.

The short version:

THESE AMPS NEED SOMETHING PLUGGED INTO THE INPUT JACK TO PULL FULL IDLE CURRENT ON THE POWER TUBES

The longer, more frustrating version:

I emailed Blackstar asking for them to confirm the -40V at TP13 value I came across online and got a response from Korg saying they don't share info with people that aren't service centers. I asked to become a service center and they wanted me to fill out waaaay more paperwork than seemed worth it for my small time shop.

In the end the transformer bias method (measuring the resistance of each side of the OT primary, then measuring voltage to calculate idle current) worked just fine once I knew I had to have something plugged into the amp to activate the bias/idle current circuit. But I thought I was losing my mind since with this method and nothing plugged into the front of the amp I was getting tube dissipations of 3 - 4 watts.

Lesson learned on these I guess. Hope this helps anyone that comes across a Blackstar in the future and saves them from the headache this first one that I saw gave to me!

[frankdrebin]

procedure on this amp is to read 12v on the cathode resistor,which is 220 ohms.
then you can balance the amp at full power,checking with a scope that the clipping is symmetrical