What’s the secret to biasing a Blackstar ht40?

[Helmholtz]

So the idle current is actually sensed by the shared 1R cathode resistor in parallel with D27 (which is not conducting in normal operation).

[pdf64]

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.

Having seen the schematic, I wonder if you’ve misunderstood something?
50mV across a shared 1ohm cathode current sensing resistor averages to 25mA cathode current per EL34. Which seems a reasonable, conservative level.

Im not sure that the tech info has been officially published into the public domain? So it’s only intended for techs, and hence the balance setting method seems reasonable.

[JZRepair]

Having seen the schematic, I wonder if you’ve misunderstood something?
50mV across a shared 1ohm cathode current sensing resistor averages to 25mA cathode current per EL34. Which seems a reasonable, conservative level.

Im not sure that the tech info has been officially published into the public domain? So it’s only intended for techs, and hence the balance setting method seems reasonable.

I get that. But you're relying on a resistor to be averaging the two 6L6GC's current. It's much better to actually read what each tube is using rather than going off a resistor. I get it, but even still, 25mA across each each tube is severely underbiased. These are 30W tubes. I want to bias tubes at 60% for a class AB amp. I'm going to be closer to 40% at 25mA. The amp is going to sound like garbage, because the tubes aren't running hot enough. As for balancing, yes, 1kHz signal and looking for matched clipping makes sense, but in all reality, matching the tubes with current is a much better practice. Driving any amp at full volume, with a solid 1kHz signal for extended periods of time is going to damage the amp. That's constant, and will strain the output. Not to mention, who wants to be listeing to a 1kHz sine wave at full volume while they're trying to get this all balanced? I know you can use a dummy load, but, that's going to have to be one hell of a dummy load to handle the full power of the amp for the amount of time it takes you to adjust things.

All I'm really saying is, I don't like it. I don't agree with this procedure. It may not be "public domain" but the fact it's on this forum means it's public domain and anyone can find it now. So yeah, I think there are some concerns with this procedure and I wouldn't recommend it.

And by no means am I implying I know it all. I welcome the other view points from this, because it's all a learning process. So thank you to all who have put in your two cents. You make very valid points. I don't want to undermine that or sound condescending in any way. I simply want to understand more than from my own perspective.

[Helmholtz]

As for balancing, yes, 1kHz signal and looking for matched clipping makes sense, but in all reality, matching the tubes with current is a much better practice.

I tend to disagree.
Tube characteristics vary and equal idle currents doesn't imply equal signal currents at full power.
Regarding max. clean headroom the Blackstar method is best.

Driving any amp at full volume, with a solid 1kHz signal for extended periods of time is going to damage the amp.

No. If we're speaking of minutes not hours
And yes, you should use a suitable dummy load.

[g1]

I think Blackstar (and other manufacturers like Fender) equip the amps with matched sets of power tubes and assume they will be replaced as such. Some of the fender amps also use the shared sense resistor for bias setting.

The idea that there is one single % diss. number that applies to all amps is a myth. Some amps are designed to run power tubes hot, some cool. Many, many vintage Fender amps sound just fine at 40%, definitely not garbage. Some of the music man amps idled at about 5%, should that even work? You can not judge what an amp is going to sound like at some idle % without even hearing it.

I generally find that amps can be biased a lot colder than I would think before it starts to have any negative impact on the tone. But that is not true for all amps.

[pdf64]

The different valve rating systems used should be borne in mind too.
For a given valve type, 70% of its design centre rating will roughly equate to 60% of its design max rating.
A 6L6GC isn’t really 20% more beefy than an EL34, it’s just that they’re rated under different systems.
Design max ratings are intended to be used in worst case condition calculations, whereas design centre to be used in nominal condition calculations.
Unfortunately these things get conflated, no distinction ever seems to be made, and there’s pretty much universal use of design max ratings for nominal condition calculations, eg every bias calculator on the web.

https://www.one-electron.com/Archive...on%20Tubes.pdf

[Mick Bailey]

I bias tubes on the cool side. There's no right or wrong %, and for me tube life and reliability are important. I've never had one single customer complain that their amp sounded bad due to cool biasing. How cool depends on the particular amp, as well as the tube type. From the factory, many amps are biased very cool and this doesn't affect their reputation for being good sounding amps.

It's rare for me to balance tubes for maximum clean headroom, but doing this at the onset of clipping not only takes the tube curves into consideration, but also any other asymmetry or non-linearity that may be present in the rest of the circuit.. My dummy load will stand 2.5Kw continuous (dual fan cooled for PA testing) so guitar amp output hardly gets it warm.

[JZRepair]

I tend to disagree.
Tube characteristics vary and equal idle currents doesn't imply equal signal currents at full power.
Regarding max. clean headroom the Blackstar method is best.

I'm not disagreeing with you, I don't know enough about it. Can you provide an article to back this? It helps to see data.

No. If we're speaking of minutes not hours
And yes, you should use a suitable dummy load.

True, minutes is less likely to cause harm, but it will still cause some harm. Though I do agree it should be negligible, it's still not a practice I like. When I got my start in audio, my boss insisted that everything be tested full bore. This ultimately caused stress on the systems and shortened the lifespan of the product. Knowing this is why I do not like the idea of testing anything at full volume, with or without a dummy load. You can test at low volumes and get the same results you would see at higher volumes and it's much safer/gentler on the system under test.

As for dummy loads, I don't like them, because I can't hear what is truly going on. There are little things that can be detected audibly that I find much more helpful than using a dummy load.

[JZRepair]

I think Blackstar (and other manufacturers like Fender) equip the amps with matched sets of power tubes and assume they will be replaced as such. Some of the fender amps also use the shared sense resistor for bias setting.

The idea that there is one single % diss. number that applies to all amps is a myth. Some amps are designed to run power tubes hot, some cool. Many, many vintage Fender amps sound just fine at 40%, definitely not garbage. Some of the music man amps idled at about 5%, should that even work? You can not judge what an amp is going to sound like at some idle % without even hearing it.

I generally find that amps can be biased a lot colder than I would think before it starts to have any negative impact on the tone. But that is not true for all amps.

Again, I don't know enough about running them at lower dissipation levels. Everything I've read states that optimal performance is at 60% for class AB, 70% for class A, and up to 100% for cathode biased. Granted, I will concede this is stated as "safe operating levels" an nothing that I can recall has stated they can be run much lower without issue, other than it's up to the user and their preference of tone. That being said, I've found that keeping things at the suggested levels have kept my customers much happier. If we consider tone is subjective, then you really can't say it sounds better at certain dissipation levels compared to others, since you may hear it differently and prefer it differently. This is why I generally ask before I set the levels as to whether the customer is looking for a warmer or colder tone.

[JZRepair]

I bias tubes on the cool side. There's no right or wrong %, and for me tube life and reliability are important. I've never had one single customer complain that their amp sounded bad due to cool biasing. How cool depends on the particular amp, as well as the tube type. From the factory, many amps are biased very cool and this doesn't affect their reputation for being good sounding amps.

It's rare for me to balance tubes for maximum clean headroom, but doing this at the onset of clipping not only takes the tube curves into consideration, but also any other asymmetry or non-linearity that may be present in the rest of the circuit.. My dummy load will stand 2.5Kw continuous (dual fan cooled for PA testing) so guitar amp output hardly gets it warm.

I guess I can agree with you on the "no right or wrong %". I do the same as far as considering tube life. I always encourage my customers to keep in mind with tube amps that running them a bit cooler with extend their life, because let's face it, most musicians aren't made of money. The extra cost of constantly changing out tubes and needing a rebias isn't worth it.

[Helmholtz]

I don't know enough about it. Can you provide an article to back this? It helps to see data.

Based on own measurements since around 1980.
I like to confirm power output using scope and dummy load. Easy, quick and reliable.
Sometimes reveals a flaw (like oscillation) that would not show otherwise.
Doesn't harm the amp to run at full power.
A class AB amp puts most stress on power tubes plates at medium output.

Balancing (or matching) at idle only covers one DC operating point. When tube curves are not identical, AC signal operation will not be balanced.

You can test at low volumes and get the same results you would see at higher volumes.

Not true in my experience.
This said, balancing at idle is typically sufficient for a guitar amp.
After all some asymmetry at full power may even sound good.

As for dummy loads, I don't like them, because I can't hear what is truly going on. There are little things that can be detected audibly that I find much more helpful than using a dummy load.

That's simple: If you need to hear, connect a speaker in series with a 100R resistor across the dummy load or use headphones with a voltage divider instead.

[JZRepair]

Based on own measurements since around 1980.
I like to confirm power output using scope and dummy load. Easy, quick and reliable.
Sometimes reveals a flaw (like oscillation) that would not show otherwise.

In your observations, what percentage of the time would you say this has shown you flaws?

Doesn't harm the amp to run at full power.
A class AB amp puts most stress on power tubes plates at medium output.

Each amp might be different, but there is a characteristic curve. Maybe this is only present in solid state. I don't have as much experience with tubes as I do working with solid state. This is the whole reason I question all of this, because it's all a learning process for me. I've only been working with tubes for a little over a year now. I have a love/hate relationship with them as I do appreciate solid state much more, but again, tubes are a different beast.

Balancing (or matching) at idle only covers one DC operating point. When tube curves are not identical, AC signal operation will not be balanced.

This is a very valid point and I never really put that into any sort of consideration. Thank you for that. However, from everything I've read, a slight imbalance may actually work in the favor of the listener.

That's simple: If you need to hear, connect a speaker in series with a 100R resistor across the dummy load or use headphones with a voltage divider instead.

Interesting. I wouldn't have thought of this. However, there is still the cost of the dummy load. These are not cheap by any means and even a DIY load is quite expensive. I'm not opposed to them, but I still prefer the idea of actually hearing it first hand without going through a dummy load. I will definitely keep this in mind, though.

[pdf64]

To me, through a guitar type speaker, a constant tone is intolerable at more than micro watt power.
Hearing is a consumable, I’m not wasting what little I’ve still got on that stuff.

[JZRepair]

To me, through a guitar type speaker, a constant tone is intolerable at more than micro watt power.
Hearing is a consumable, I’m not wasting what little I’ve still got on that stuff.

When I worked on hearing loops, I got used to all the tones. I have witnessed many people lose their mind with them, and honestly that makes me laugh a little.

[Helmholtz]

However, there is still the cost of the dummy load. These are not cheap by any means and even a DIY load is quite expensive.

It shouldn't cost more than 20 to 40 USD (depending on max. power) to build a decent dummy load.