Bias question

[Simpson]

Yesterday I replaced the power tubes on a Blackstar HT40.

The amp has one 1R resistor on the cathodes and the voltage across it must be set to 50mV.

So I let it warm up and set it to 50mV and left the amp sitting there at idle while I was doing something else.
After, maybe 30m i was checking the bias voltage again and it was already on the 100mV range.


Is that a common issue on this method of biasing or should i consider a bias drift problem?

[Helmholtz]

Bias should always be set after some warm-up time. If it still climbs over time there's some issue with tubes or amp.

[vintagekiki]

Warm-up time is usually 30 min.

[Jon Snell]

It is common with this amplifier to find the MosFet drivers, Tr1 & Tr2, go noisy and produce inaudible oscilations to the output valves, this has the effect of increasing the bias current.
Fit IRF830APBF for added reliability. Some have an odd chinese device fitted to them.
After replacing the Fets, re bias and then check again after half an hour.
Never purchase from fleabay!!!

[TomCarlos]

Just in case you are referring to the Blackstar HT Club 40 Amp... schematic is attached.

If not, let me know and I will remove this post.

[Simpson]

It is common with this amplifier to find the MosFet drivers, Tr1 & Tr2, go noisy and produce inaudible oscilations to the output valves, this has the effect of increasing the bias current.

Fit IRF830APBF for added reliability. Some have an odd chinese device fitted to them.
After replacing the Fets, re bias and then check again after half an hour.
Never purchase from fleabay!!!

I will try that. I will get some from Farnell.

Just in case you are referring to the Blackstar HT Club 40 Amp... schematic is attached.

If not, let me know and I will remove this post.

That's the right one

[J M Fahey]

Just in case you are referring to the Blackstar HT Club 40 Amp... schematic is attached.

If not, let me know and I will remove this post.

Thanks and even if itīs not, do not remove it!!! , may/will be useful to others in the future.
Worst case just edit post to add a warning label stating it does not match OPīs amp.
Thanks again.

[Mick Bailey]

The main problem I've found with these amps is the board becomes conductive between the MOSFET driver pins. It starts as noise, but as the board becomes more conductive the voltages go further astray. Bias increases and output drops, worse when the amp is warm. When I sectioned a board the centre third was carbonized in this area. It is possible to fix them to a certain extent, but how long the board will last before the problem spreads to somewhere else is another matter. I've also seen voltage leaking across the 2nd preamp tube pins of one amp.

[Simpson]

Unfortunately i'm not able to work on the amp due to the lockdown.
But I already have some Mosfet to replace and after replacing them I will check that and give some feedback.

Strange thing was the Technical Support of Blackstar telling me that was an unknown issue.

Last month I had two dsl100 with a conductive Pcb and I don't want to go through it again.

[Mick Bailey]

When I started regularly getting these amps in for repair with the same issue (one just a few weeks out of warranty) I contacted Blackstar. No response whatsoever. Before replacing the MOSFETs, remove both of them and check for excessive voltages on the pads.