Blackstar HT60 bias circuit

[lowell]

Well, got one of these in with massive hum. Got this email previously, when working on one that wouldn't bias.

Thanks for contacting me,

The best course of action would be to speak to Korg who handle our distribution in the US with regard to service centre accreditation

Can you out line the problem you have with the Stage 60, if you are trying to set the bias on the amp be advised there is a protection system on the amp that may be active,

To set the bias:

All volume tone and gain pots fully CCW, select clean channel

Ensure you have a dummy load or speaker attached to the amp, also insert a shorted jack plug into the guitar input, this will now release the protection circuit,

If you look at the output valve pcb you will see a 2 pin connector that is not used, this is the bias measuring point, (alternatively measure across D27 if that’s easier)

Set your DVM to its 200mvDc range and measure across the 2 pins, adjust the bias adjust pot PR2 to read 50mv, leave the balance pot PR1 in its mid way position,

That will set the bias,

If you need more help please come back to me and we will help as much as possible

Very best regards

Phil

Doesn't help much being that you all have figured this out already. I pulled the first 2 preamp tubes, and the hum remains, but when I pull the 3rd it goes away. I measured voltages around this tube and it doesn't have voltages similar to an LTP PI. It measures like your typical common cathode gain stage. I am seeing about 600mv of AC on the cathode when the hum is active. This tube has series DC filament wiring, and has 6Vdc for each filament (pin4=12v, pin9=6v, pin5=0v) Those voltages are good. There is no DC on the grids and there is indeed about 220k from grids to ground. Yes I tried a new JJ ecc83

edit: there is actually 600mv of DC on the grids. It appears each of these grids are fed by a capacitor (1uf 450v) from a MOSFET.

[Chuck H]

Have you checked for any AC on the filament circuit? I don't know how the amp is constructed, is it possible there's a loose or dirty pin connection via a cable? Does the chassis mounting put pressure on any component leads? I've had problems with a few 'not vintage' products that rely on board mounted, chassis mounted pots as part of the board mounting stability. Add the popular trend of tube swapping and board mounted sockets and you can get lifted board pads or failing lug contacts inside the pots or on the board.

[lowell]

Ok folks so I can say, the PCB common circuitry connects to Chassis/Earth via 3 parallel components. 2 diodes (opposite polarity) and a 100hm resistor. So it makes sense that if I was measuring from the Chassis as common, as is TYPICALLY fine for such measurements in any guitar tube amp, that there is 600mv on the grids. It's actually zero in reference to the main PCB common.

[wizard333]

Thank you guys for this thread, you just saved me a lot of time.

Looks like it biases up around 55% PD at idle with this method, not really all that cold. Perfectly fine for a Fender, and not nearly as cold as the typical Mesa.

[Jazz P Bass]

Well, got one of these in with massive hum. Got this email previously, when working on one that wouldn't bias.



Doesn't help much being that you all have figured this out already. I pulled the first 2 preamp tubes, and the hum remains, but when I pull the 3rd it goes away. I measured voltages around this tube and it doesn't have voltages similar to an LTP PI. It measures like your typical common cathode gain stage. I am seeing about 600mv of AC on the cathode when the hum is active. This tube has series DC filament wiring, and has 6Vdc for each filament (pin4=12v, pin9=6v, pin5=0v) Those voltages are good. There is no DC on the grids and there is indeed about 220k from grids to ground. Yes I tried a new JJ ecc83

edit: there is actually 600mv of DC on the grids. It appears each of these grids are fed by a capacitor (1uf 450v) from a MOSFET.

Just wanted to clear up the confusion on this post.

As noted, V3 is set up as a cathode follower.
This is not the LTP section.
That is accomplished by the two mosfets.
V3 is simply a buffer after the LTP.

Blackstar HT Stage 60-100 LTP.pdf

[clintronics]

Thank you for posting that about input switch

[bonitec]

I have the solution to the fault I want to comment on them if it still serves ..

[nosaj]

I have the solution to the fault I want to comment on them if it still serves ..

So what are you waiting for an invitation?
nosaj

[g1]

I have the solution to the fault I want to comment on them if it still serves ..

Yes please comment. Even though the original post was 8 years ago, your information can still help others with similar problems.