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View Full Version : What's bias???


Jeremy520
11-21-2012, 10:45 AM
I've been looking at different amps and some of them have a bias control.

What is it and what does it do?

RupertB
11-21-2012, 12:39 PM
The first thing to become familiar with is the "bias threshold" which refers to the fact that "fixed bias" amps = adjustable bias (mostly) while "cathode bias" or "self-biasing" circuits are non-adjustable (fixed), except by changing resistor values in the circuit; usually at the cathode.

If this seems confusing, counter-intuitive, and unnecessarily complicated, you should pay a competent tech to take care of it & not worry about it.

If this leaves you facinated & hungry to learn more, procede to the Aiken Amps link that Brian posted. Congratulations, you have crossed the "bias threshold."

Good luck. :)

Peteyvee
11-21-2012, 12:40 PM
More like he's entered "The Bias Zone"...cue theme music. ;)

DT7
11-21-2012, 02:43 PM
...sure to receive many a biased response.


Biasing has, for a long time, meant adjusting the idle-point of a pair (or more) of power tubes in a Class AB power-amp for the purpose of allowing a minimum of crossover distortion while maintaining a maximum of clean headroom. Crossover distortion is due to the fact that, in class AB amplification, there is a period of signal reproduction (close to the cutoff point) that is very non-linear. In a good matched (for current draw) pair of tubes, the bias point can be set so that the second tube begins to turn on just as the first tube enters the more non-linear area of performance, close to cutoff. Ideally, this cancels out the non-linear region of signal production...reproducing the original signal with very little distortion. The reality is it's not perfect...so there is always going to be a little crossover distortion...though there is certainly an optimum point for a given set of circumstances (tubes, amplifier components, B+ voltage, etc.)

These days, it has come to mean (it seems) any idle point, regardless of amplifier performance, that satisfies some average for all tubes and all amplifiers, taking only a known B+ and tube current-draw into account. Anybody who knows anything about tubes, knows that such averages have little relevance...since tubes vary so much. But because some folks desire to be able to do something themselves more than they desire to understand what is actually going on (or not), they have bought into this new definition of "bias"...which is often supported by internet "gurus" who have something to sell them.

Believe what you want...just remember it's easier to be fooled than it is to become convinced that you were fooled.

For either definition, the idle-point is adjusted by changing resistors or adjusting a pot to give a certain voltage at the control-grid with reference to the cathode.

Class A power-amps (as with most pre-amp tubes) are not adjusted for elimination of crossover distortion because, ideally, there should be none...they are considered "self-biasing". Though, in practice, there is some tonal variation that can occur by adjusting the cathode resistor, which is resposible for setting the bias point by varying the voltage at the cathode with respect to the control-grid.

That's the short of it...now back to your regular TGP programming.

GT100
11-21-2012, 03:38 PM
...sure to receive many a biased response.


Biasing has, for a long time, meant adjusting the idle-point of a pair (or more) of power tubes in a Class AB power-amp for the purpose of allowing a minimum of crossover distortion while maintaining a maximum of clean headroom. Crossover distortion is due to the fact that, in class AB amplification, there is a period of signal reproduction (close to the cutoff point) that is very non-linear. In a good matched (for current draw) pair of tubes, the bias point can be set so that the second tube begins to turn on just as the first tube enters the more non-linear area of performance, close to cutoff. Ideally, this cancels out the non-linear region of signal production...reproducing the original signal with very little distortion. The reality is it's not perfect...so there is always going to be a little crossover distortion...though there is certainly an optimum point for a given set of circumstances (tubes, amplifier components, B+ voltage, etc.)

These days, it has come to mean (it seems) any idle point, regardless of amplifier performance, that satisfies some average for all tubes and all amplifiers, taking only a known B+ and tube current-draw into account. Anybody who knows anything about tubes, knows that such averages have little relevance...since tubes vary so much. But because some folks desire to be able to do something themselves more than they desire to understand what is actually going on (or not), they have bought into this new definition of "bias"...which is often supported by internet "gurus" who have something to sell them.

Believe what you want...just remember it's easier to be fooled than it is to become convinced that you were fooled.

For either definition, the idle-point is adjusted by changing resistors or adjusting a pot to give a certain voltage at the control-grid with reference to the cathode.

Class A power-amps (as with most pre-amp tubes) are not adjusted for elimination of crossover distortion because, ideally, there should be none...they are considered "self-biasing". Though, in practice, there is some tonal variation that can occur by adjusting the cathode resistor, which is resposible for setting the bias point by varying the voltage at the cathode with respect to the control-grid.

That's the short of it...now back to your regular TGP programming.

No one that knows whats going on biases guitar amps with a scope/ crossover distortion. Bad tone and even meltdown are possible that way.

Bias is best set by monitoring tube current and plate voltages (static plate dissipation) -along with listening to the amp being played. Some folks actually like the sound of crossover distortion btw.

Class A amps can be fixed (adjustable or not) biased or cathode biased or both.

Bias is a way of keeping the operation of the tube within its safe limits/ parameters of design.
It may appear to be setting the idle current but it actually controls the flow of electrons even when a signal is present.

If there was no bias voltage present (fixed or cathode) the tube would melt down in short order.
If somehow the tube didn't melt and the rest of the amp held up the sound produced would be awful- bad distortion and even a loud hum.

Lloyd

cap47
11-21-2012, 04:03 PM
:hide2Not again!:jo

DT7
11-21-2012, 06:00 PM
Bias is a way of keeping the operation of the tube within its safe limits/ parameters of design.


That's setting an idle point. Biasing is based on performance of the tubes in the circuit...not how much current is drawn. And unless you have the data sheet on the tubes in question...not a generic "one-size-fits-all" sheet...you have no idea as to what is and isn't safe. Period.

Class A amps can be fixed (adjustable or not) biased or cathode biased or both.

All such an answer does, with no additional information, is confuse the OP by clouding the issue.

No one that knows whats going on biases guitar amps with a scope/ crossover distortion.

Tell it to the guys who were designing and building amps since before you were born.

DT7
11-21-2012, 06:01 PM
:hide2Not again!:jo

You love it and you know it. ;)

Shiny_Beast
11-21-2012, 06:15 PM
Correct me if I'm wrong, I'd also add that the general concept has to do with adjusting the circuit and operating conditions so the amplified wave generated by the tube(s) isn't offset too far positive or negative. More intuitive to understand for a single preamp tube than for a pair of power tubes running in AB, but at it's core it's pretty much the same thing, no?

cap47
11-21-2012, 06:28 PM
You love it and you know it. ;)
I guess the secret is out ! :)

jekylmeister
11-21-2012, 07:08 PM
I'd look up "thermionic", but I still don't think I could work it into casual conversations.

59Vampire
11-21-2012, 07:35 PM
Welcome to a lifetime of learning

geetarplayer
11-21-2012, 07:38 PM
When a mod deletes your thread and not someone else's.

GT100
11-21-2012, 08:18 PM
Tell it to the guys who were designing and building amps since before you were born.

Those same guys thought that distortion was something to avoid...

Lloyd

GT100
11-21-2012, 08:24 PM
...once more the "old" guys/gals pass along knowledge to the "young" guys/gals about thermionic electronics.

This "young" guy also explains to these "old" guys the positive aspects of distortion and running tubes in their non linear range...

Lloyd

DT7
11-21-2012, 08:41 PM
You have no choice but to operate in a non-linear range...too many imperfections; especially in guitar amps. But you must first understand what it is you're doing in order to understand what results to expect when you deviate from the ideal condition (or as close to it as you can get). Otherwise you will never be able to find a repeatable process for acheiving what it is you hope to achieve. Measuring the current-draw and the B+ isn't going to get you there.

GT100
11-21-2012, 08:50 PM
You have no choice but to operate in a non-linear range...too many imperfections; especially in guitar amps. But you must first understand what it is you're doing in order to understand what results to expect when you deviate from the ideal condition (or as close to it as you can get). Otherwise you will never be able to find a repeatable process for acheiving what it is you hope to achieve. Measuring the current-draw and the B+ isn't going to get you there.

Those old guys were trying to minimize distortion. Since then we now know that distortion is the "secret" to good tone - even with "clean" tones.

Once you turn the pot enough to eliminate the cross over distortion (monitoring with a scope) you have no idea how far past you went. Considering how touchy most bias pots are you can go way too far very easily.
If you think using a scope is going to get you safe operation and the best tone good luck....

Lloyd

zzmoore
11-21-2012, 08:50 PM
You have no choice but to operate in a non-linear range...too many imperfections; especially in guitar amps. But you must first understand what it is you're doing in order to understand what results to expect when you deviate from the ideal condition (or as close to it as you can get). Otherwise you will never be able to find a repeatable process for acheiving what it is you hope to achieve. Measuring the current-draw and the B+ isn't going to get you there.
Not sure I understand what you are saying.
Don't all class AB amps operate (a little) in the non-linear part of their graphs by nature.?
Os is that your point.? :confused:
Thanks

DT7
11-21-2012, 11:47 PM
Those old guys were trying to minimize distortion. Since then we now know that distortion is the "secret" to good tone - even with "clean" tones.

There are many kinds of distortion...some are good...some not good. There wouldn't be such a hunt for tone, and boards like this probably wouldn't exist, if all distortion sounded equally good.

Once you turn the pot enough to eliminate the cross over distortion (monitoring with a scope) you have no idea how far past you went.

You know where to stop if you know what you're doing. Explaining it is somewhat ambiguous...I've tried to explain before. In this case, pictures are worth a thousand words...and actually watching somebody else do it, ten-thousand words.

Considering how touchy most bias pots are you can go way too far very easily.

Not completely false. It requires a fine touch near the best bias point, but I've always managed to get there without having to install something like a multi-turn pot.

If you think using a scope is going to get you safe operation and the best tone good luck....

Lloyd

Done it many times...used to do it for a living. Invariably, the optimum point for tone falls within a volt (plus or minus) of the bias point of least distortion...and that point is subject to many different variables, including (and especially) the tubes themselves. That's why measuring current-draw and B+ doesn't work...it just makes people feel good. But make no mistake...setting the bias using a scope is an attempt to get the best tone; not the safest operating point for the tubes. There are certain amps where some tubes (especially modern-production) won't take it. But if the safest operating point is what you want, turn the bias pot as far negative (voltage-wise) as it will go and be done with it...that's the "safest" point for a Class AB power-amp.

DT7
11-21-2012, 11:49 PM
...visit the http://thermionic.info/ website and look up the author McCaul!


Looked it up. No mention of tone or amplifier performance.

DT7
11-21-2012, 11:59 PM
Not sure I understand what you are saying.
Don't all class AB amps operate (a little) in the non-linear part of their graphs by nature.?
Os is that your point.? :confused:
Thanks

That was one of my points...there's always going to be a little distortion of some kind.

The other point is that using a scope to find the optimum point for minimum distortion gives you a starting point to work from. It's going to vary tube to tube...amplifier to amplifier even of the same type, for that matter. No other method allows you to find this starting point. Just a little too far, plus or minus from that point, and you begin to lose performance in one way or another. Too hot, and articulation, punch, dynamics and higher frequencies are lost...the amp sounds mushy. Too cold and you lose clean headroom...in fact, you lose the ability to get a good clean tone completely, for that matter, as even the smallest signal will produce distortion. And the distortion you get there will be cold and harsh. What you're looking for is the right blend without sacrificing other desireable aspects of the amps tone and dynamic qualities.

GT100
11-22-2012, 06:32 PM
Done it many times...used to do it for a living. Invariably, the optimum point for tone falls within a volt (plus or minus) of the bias point of least distortion...and that point is subject to many different variables, including (and especially) the tubes themselves. That's why measuring current-draw and B+ doesn't work...it just makes people feel good. But make no mistake...setting the bias using a scope is an attempt to get the best tone; not the safest operating point for the tubes. There are certain amps where some tubes (especially modern-production) won't take it. But if the safest operating point is what you want, turn the bias pot as far negative (voltage-wise) as it will go and be done with it...that's the "safest" point for a Class AB power-amp.

The current / plate voltage method I mentioned uses your ears to decide what sounds best -as I said.
The measurements are to make sure you are not running anything too hard such as the power supply and tubes.
Do you really think your scope knows the tone I like better than me or anyone else?
Also as you may know some amps sound like crap if you bias just enough to eliminate crossover distortion. The Marshall Major comes to mind.

Just cause you are good at avoiding Turing the bias pot too far doesn't make the scope method a smart way to to bias consistently -compared to actual measurements with a meter...

Lloyd

DT7
11-22-2012, 11:44 PM
The current / plate voltage method I mentioned uses your ears to decide what sounds best -as I said.

An unrepeatable process that can waste a lot of time, since the point is different for every set of tubes. Every last one. But if you've ever used a meter, you already know this...and you also know there's no magic plate dissipation percentage that's going to get you there. Use a scope, find the point immediately, and based on your experience adjust in the direction you know you want to go, and you're there. It's like starting with the same set of tubes every time. You can't tell me your method is more reliable and repeatable than that.

And if some guy new to the inner workings of amps didn't have his guitar/pickups adjusted when the amp was properly biased to begin with, and doesn't have the experience to know what it is he's listening for, it could take him days to find it...if ever.

Do you really think your scope knows the tone I like better than me or anyone else?

That's not the question. The question is how much extra time do you spend fooling around because you never start at the same performance point twice in a lifetime.


Also as you may know some amps sound like crap if you bias just enough to eliminate crossover distortion. The Marshall Major comes to mind.


I prefer folks understand what's going on...and you don't teach them that by highlighting the exceptions. The rule in explaining something is to explain the rule first...then you can go on to explain the exceptions.

DT7
11-22-2012, 11:55 PM
Just cause you are good at avoiding Turing the bias pot too far doesn't make the scope method a smart way to to bias consistently -compared to actual measurements with a meter...

Lloyd

Performance, and therefore performance points, cannot be measured by current-draw. What do you think a scope does, anyway?