bias circuit not adjusting HT Club 40

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defy555
Posts: 1
Joined: Fri Jan 31, 2020 10:02 am

Fri Jan 31, 2020 10:19 am

Hi guys, new member here...and apologies if this has been covered somewhere. I did search but found nothing...

Trying to bias up a pair of tubes in a Blackstar HT Club 40. I work on tube amps a lot, so the general procedures and principles are well known to me, however, in this case I am getting -72 volts on the screens (pin 5) - effectively switching off the tubes - and adjusting the bias pot has no effect on this. (Measuring 0mV at the test point, too) I suspect a fault in the bias circuit, or in some part connected to the PIC chip, which seems to turn on the bias circuit?? Anyone out there with knowledge at this level?

Cheers,

Defy

Rybow
Posts: 6
Joined: Thu Apr 08, 2021 5:19 am

Thu Apr 08, 2021 6:43 am

Hope you find an answer to your problem. I have a stage 60 that had a blown cap which cut power to the plates. After replacement I could get sound with no headroom. Adjusting the bias did increase the headroom, but would start to cook resistors. I could not get any reading from the bias test point. Wish I had a schematic or at least an idea of what voltages I need to see on the tubes

panservid
Posts: 1
Joined: Thu Dec 15, 2022 9:20 am
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Fri Dec 16, 2022 8:38 am

I have some issues with the HT CLUB 40 bias circuit. Apparently all is working, but the BIAS_ON signal in not present. I check voltages and apparently find the problem on pin 23 of IC10 (PIC16F57 microcontroller) where the BIAS_CNTRL signal is missing. get-vidmate.com instasave.onl

mrtoad
Posts: 9
Joined: Tue Jan 08, 2019 12:57 am

Sat Jul 08, 2023 7:54 pm

I had that same problem and it turned out to be a bad screen resistor. Don't know what caused it though, maybe a bad tube.

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