What’s the secret to biasing a Blackstar ht40?

[Perkinsman]

Hi guys, first time biasing a Blackstar ht40. I replaced the power tubes & found the bias pot... but can’t get a reading at the TP’s 10 & 18. Can someone please step up & disclose the correct TPs for biasing this amp.

[nosaj]

Hi guys, first time biasing a Blackstar ht40. I replaced the power tubes & found the bias pot... but can’t get a reading at the TP’s 10 & 18. Can someone please step up & disclose the correct TPs for biasing this amp.

Here you go
http://dealers.korgusa.com/svcfiles/...0Procedure.pdf
nosaj

[drewl]

Remember, a cable MUST be plugged into the input!

It turns bias on/off.

Threw me for a loop the first time I saw one!

[nosaj]

Remember, a cable MUST be plugged into the input!

It turns bias on/off.

Threw me for a loop the first time I saw one!

Must be why it is Bolded in the attached document.
nosaj

[g1]

Have some mercy on those who can't download anything, like folks reading this in certain libraries or workplaces.

[nosaj]

Have some mercy on those who can't download anything, like folks reading this in certain libraries or workplaces.

Bias Procedure for Blackstar
The following will show the bias test points, correct bias voltage(s), and bias balance procedure for the
HT and Series One lines of Blackstar Amplifiers.
Before starting, make sure you plug a ¼” cable into the input jack
(**Except on the HT5**)
Also, all volume and tone controls should be set to “ZERO” and all switches should be in their “OUT”
position
HT5 – Adjust bias control for a reading of 46mV across D20
HTSTUD20 – Adjust bias control for a reading of 12V across R221
HTCLUB40C – Adjust bias control for a reading of 50mV across D36
HTSTAGE60C – Adjust bias control for a reading of 50mV across D27
HTSOLO60C – Adjust bias control for a reading of 50mV across TP1 and TP7
HT100H – Adjust bias control for a reading of 100mV across D27
S145 – Adjust bias control for a reading of 50mV across CON1
S1100 – Adjust bias control for a reading of 100mV across CON1
S1200 – Adjust bias control for a reading of 100mV across D22
S1104EL34 – Adjust bias control for a reading of 100mV across D22
S11046L6 –Adjust bias control for a reading of 100mV across D22
Bias Balance Procedure
1) Set amplifier to CLEAN CHANNEL
2) If there is a tone control(s), set these to 12 o’clock
3) Set Channel and Master Volume controls fully clockwise
4) Using a 1kHz input signal, drive the output waveform into saturation (full clipping)
5) Adjust Bias Balance potentiometer so that the output waveform clips symmetrically

[Perkinsman]

Thank you for that, perfect.

[JZRepair]

Bias Procedure for Blackstar
The following will show the bias test points, correct bias voltage(s), and bias balance procedure for the
HT and Series One lines of Blackstar Amplifiers.
Before starting, make sure you plug a ¼ cable into the input jack
(**Except on the HT5**)
Also, all volume and tone controls should be set to ZERO and all switches should be in their OUT
position
HT5 Adjust bias control for a reading of 46mV across D20
HTSTUD20 Adjust bias control for a reading of 12V across R221
HTCLUB40C Adjust bias control for a reading of 50mV across D36
HTSTAGE60C Adjust bias control for a reading of 50mV across D27
HTSOLO60C Adjust bias control for a reading of 50mV across TP1 and TP7
HT100H Adjust bias control for a reading of 100mV across D27
S145 Adjust bias control for a reading of 50mV across CON1
S1100 Adjust bias control for a reading of 100mV across CON1
S1200 Adjust bias control for a reading of 100mV across D22
S1104EL34 Adjust bias control for a reading of 100mV across D22
S11046L6 Adjust bias control for a reading of 100mV across D22
Bias Balance Procedure
1) Set amplifier to CLEAN CHANNEL
2) If there is a tone control(s), set these to 12 oclock
3) Set Channel and Master Volume controls fully clockwise
4) Using a 1kHz input signal, drive the output waveform into saturation (full clipping)
5) Adjust Bias Balance potentiometer so that the output waveform clips symmetrically

This can't be legit, can it? I've got an HT Stage 60 on my bench and I did a bias on it by using some bias probes. I monitored D27 and it didn't really alter much from 66mV bias on the amp was WAY hot at 102%. I dialed it down from 66mA across the plates to ~34mA. And the bias balance procedure? Really? All that pot is doing is balancing the current between the two output tubes. Unless I'm missing something here, these directions are super screwy.

[Delta362]

This can't be legit, can it? I've got an HT Stage 60 on my bench and I did a bias on it by using some bias probes. I monitored D27 and it didn't really alter much from 66mV bias on the amp was WAY hot at 102%. I dialed it down from 66mA across the plates to ~34mA. And the bias balance procedure? Really? All that pot is doing is balancing the current between the two output tubes. Unless I'm missing something here, these directions are super screwy.

There are two pots for the bias circuit on the HT Stage 60. One pot (PR1) is the bias balance pot, while PR2 is the bias pot. Have the guitar plugged in to enable the output section. Then adjust PR2 for 50mV across D27 (which is across R207 - 1 ohm 1W resistor in the combined cathode line for the power tubes).

[JZRepair]

There are two pots for the bias circuit on the HT Stage 60. One pot (PR1) is the bias balance pot, while PR2 is the bias pot. Have the guitar plugged in to enable the output section. Then adjust PR2 for 50mV across D27 (which is across R207 - 1 ohm 1W resistor in the combined cathode line for the power tubes).

Yes, I understand the procedure, but this will not properly bias the amp and is verifiable through the bias probes. PR2 controls the bias current, true. PR1 controls the balance of available current through the tubes, but actual current through the plates and voltage across the plates are what matters, not this misleading voltage across D27. This is an incredibly lazy way of biasing an amp and is going to cause a lot more tube failure. The balancing of PR1 is visibly seen with bias probes, as the balancing of current across the plates to cathodes in each tube. Adjusting the pot one way or the other simply adjusts the available current (available current being set by PR2) across the plate and cathode respectively in the output tubes, meaning the PR1 pot simply negates the need for matched tubes. Maybe I'm wrong, but it shouldn't have anything to do with distortion when a 1kHz signal is put through the amp at full volume unless you're already overbiasing the amp, which is going to shorten the life of the tubes.

[Delta362]

Yes, I understand the procedure, but this will not properly bias the amp and is verifiable through the bias probes. PR2 controls the bias current, true. PR1 controls the balance of available current through the tubes, but actual current through the plates and voltage across the plates are what matters, not this misleading voltage across D27. This is an incredibly lazy way of biasing an amp and is going to cause a lot more tube failure. The balancing of PR1 is visibly seen with bias probes, as the balancing of current across the plates to cathodes in each tube. Adjusting the pot one way or the other simply adjusts the available current (available current being set by PR2) across the plate and cathode respectively in the output tubes, meaning the PR1 pot simply negates the need for matched tubes. Maybe I'm wrong, but it shouldn't have anything to do with distortion when a 1kHz signal is put through the amp at full volume unless you're already overbiasing the amp, which is going to shorten the life of the tubes.

Yes, like you, I also understand the purpose of the two pots and the pitfalls of the 50mV across D27. I myself use individual bias probes. But, from a manufacturer's standpoint, they have chosen that number to limit warranty claims by running the tubes quite cool.

[JZRepair]

Yes, like you, I also understand the purpose of the two pots and the pitfalls of the 50mV across D27. I myself use individual bias probes. But, from a manufacturer's standpoint, they have chosen that number to limit warranty claims by running the tubes quite cool.

I get what you're saying, but doing it this way just seems incredibly silly. Adding the scope and signal generator just appears to be a way of "scaring" the general public from servicing their own amps, thinking they need all this special equipment to do what you could do without it and an understanding of the basic principles of electronics. It's sad that we went from a time where schematics were given out with every electronics product, but now aren't, because of liability and stupidity. Oh well, it is what it is, but these instructions really shouldn't be continued like this. Blackstar should really get away from this practice.

[Helmholtz]

Not getting the problem here.
I think it's beneficial to have both a bias level as well as a bias (idle current) balance pot.
Adjusting for symmetrical clipping guaranties max. clean output.
In Fender amps the balance is adjusted by ear for minimum hum.

Could someone post a schematic (no dealer, don't want to register)?
What's the function of D27?

[JZRepair]

Not getting the problem here.
I think it's benficial to have both a bias level as well as a bias (idle current) balance pot.
Adjusting for symmetrical clipping guaranties max. clean output.
In Fender amps the balance is adjusted by ear for minimum hum.

I'm not saying it's not. I rather like the balance feature. I'm simply complaining about Blackstar's procedure. It was a great idea to have a balance pot, because then you don't have a need for perfectly balanced/matched tubes, slight imperfections are negligible.

[Delta362]

Here you go, Helmholtz. From post #2 of this thread (since I haven't figured out how to post a direct link to a specific post number yet.

https://music-electronics-forum.com/...t-stage-60-amp