Blackstar HT-5R MKII Bias Hack

[Moony]

Hi,

does anyone of you know, if there's a simple way to bias the 12BH7 in the new Blackstar HT-5R MKII like there was on the older model (voltage across the cathode resistor R33)?

Best regards!
Moony

[Jazz P Bass]

I would suggest that you contact Blackstar.
They usually respond.

[Moony]

I would suggest that you contact Blackstar.
They usually respond.

Nothing against Blackstar, I really like their amps - but they usually don't help with something like this at all and tell you to bring the amp to a Blackstar service center.
Even the screws of the spring-retainer for the power amp tube are sealed with this red paste.

If you take a look into the Blackstar forum on their webpage, you will see, there are countless threads about several topics and even there no one responds. It's such a shame.

That's why I'm asking here if there is already known a simple method without messing around with 1 ohms resistors at the cathodes. I'm very sure, there is a trick for biasing the HT-5R MKII as with the original HT-5R so please feel free to share.

[nevetslab]

Nothing against Blackstar, I really like their amps - but they usually don't help with something like this at all and tell you to bring the amp to a Blackstar service center.
Even the screws of the spring-retainer for the power amp tube are sealed with this red paste.

If you take a look into the Blackstar forum on their webpage, you will see, there are countless threads about several topics and even there no one responds. It's such a shame.

That's why I'm asking here if there is already known a simple method without messing around with 1 ohms resistors at the cathodes. I'm very sure, there is a trick for biasing the HT-5R MKII as with the original HT-5R so please feel free to share.

I haven't been inside the HT-5R MK II model, but, if the power tube board is wired anything like the Model 200 Series One, which also has a common 1 ohm cathode for the four power tubes, I resorted to using a DC current probe to sense the plate current of each power tube (Tektronix A6302 probe/AM503A probe amp). Current read on the scope, sensitivity set for 5mA/Div or 10mA/Div.



I left the DMM across the 1 ohm common cathode resistor for further reference as I adjusted the bias voltage pot. There is a method measuring DC voltage drop across between the O/T Primary C/T and the plate of the tubes....it will show the total voltage drop for both tubes on each half of the primary. I don't use that method myself, but others here on the forum probably do and can add more details in that method.

[Moony]

Thank you for your reply.

I now have found out by myself when taking a closer look at the pcb.

Problem solved!