What the heck is bias anyway?


I'm getting a new tube amp that will need to be biased. The process of doing it seems fairly straight forward, but I am curious: what are you actually doing when you bias an amp? In english, for us technical Ludites.
grimace
Here is some reading for you :

http://www.lynx.bc.ca/~jc/bias.html

For more info, google it
Bias is the power put into a tube to push the electrons across the vacuum in the tube. Too little and the tube is weak, too much bias and the tube may be damaged.
(and for you experts out there, STOP! he wants real simple explanation. not your twelve page essay.)
The real tubes needing a careful watch on bias are power tubes.
Preamp tubes usually can be OK with a fixed bias. And almost never need to be checked for bias.
So you DO have to learn how to adjust the bias IF you are buying a tube amp without any autobias.
Or you will have to pay regular visits to your local tube guru.
I'm biased against tubes, does that count?
Bias is simply the amount that the device is turned on(a % of max current) without signal(music) passing through it.
So it's basically adjusting the current to the tube to make sure the appropriate amount of juice is getting in and out of it. Not too much, and not too little. I think I have it.

Thanks Elizabeth!
Bias is the power put into a tube to push the electrons across the vacuum in the tube.

I hate to keep picking on you; but once again a technical explanation that is just too wrong to ignore. It is sort of, kind of, talking about what is happening but it is in a nutshell... wrong. I don't understand why you continue to jump in offering technical explanations about things you obviously don't completely understand.

It doesn't require a 12 page essay but it does require something that is correct, The following one sentence explanation is correct.

Bias is simply a voltage applied to a tube to control how much current flows when it is idling.

To say "power put into a tube to push the electrons across the vacuum" is flat out wrong. First of all it is not power, it is a voltage; they are not the same thing; and it is the voltage from Plate to Cathode that pushes the electrons through the vacuum not bias; bias controls how much can get through.

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Wow! Elizabeth posts a totally incorrect response - and Grimace grasps the main concept anyway.....
Severnapark19,

Wow! Elizabeth posts a totally incorrect response - and Grimace grasps the main concept anyway.....
05-18-11: Severnapark19
Heman posted after Grimace.

and Grimace grasps the main concept anyway.....
05-18-11: Severnapark19
Maybe you should reread Elizabeth's post and then reread Herman's post.
Stanwal - that one made me laugh :-) and I'm with you man
Bias is never having to say you're sorry or like solid state.
Geez Herman, no need to go off on Elizabeth.

She even stated that she was making a “simple explanation,” from which Grimace was able to grasp the general concept.

I can see what she meant. Perhaps if she would have changed a few words, you would not have taken such offense; i.e.: “Bias predetermines the amount of power (i.e.: voltage), to be directed to (applied to) the vacuum tube, to push the electrons through.”

In general (“simple” terms), people tend to think of current and voltage as “power.” Elizabeth wrote: “Bias is the power put into a tube to push the electrons across the vacuum in the tube.” In general, that’s not completely different from saying something like, “the bias predetermines the voltage (“power”) that is made to flow to the tube, and required to push the electrons through.”

Intelligent people can comprehend both complex and simple concepts. I have read that extremely smart folks (i.e.: geniuses) have a hard time with simple concepts. Herman…you genius, please don’t pick on us simple folks who have “our own way” of communicating!!!!!

(All in fun!)

_Ben
Hey Stanwal, transistors have to be biased too... and for the same reasons.
I was wondering where Ralph was.
Grimace what tube amp will you be biasing?
Cary v12. Apparently you can use a meter, or just turn the screws until the lights come on. Seems easy enough.
HEHEHE. Seems to be a few sublimal messages going on here.

PBNaudio. Stanwal's message was rather cute however Atmasphere's was even better. Hey,I hope you guys know how rebias those output transistors when required.
Grimace nice amp;what output tubes will you be running?
Yeah, i enjoy posting simplistic nadderings so the 'real' smart folks can jump in and save the day!! (I just had a feeling someone would pop in and trash my overly simplified explanation!!)
Makes me all smug and happy, makes them all smug and happy, so we are all happy! and the Op is happy too. (but is he smug???) What a wonderful world we live in!!
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It wasn't "oversimplified." It was wrong. There is a difference. It is possible to put things in layman's terms without being wrong.

If I post something that is incorrect I hope someone jumps in and corrects it. People come her to share and learn. That is impossible if we allow errors to go unchallenged.

If Elizabeth enjoys posting misinformation as she says and that makes her feel smug then this forum is worse off for it.

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Following on this, is I put more voltage to the tube, how the sound will be impacted? Will I get louder? Or does it allow to get louder? Will I get more bass? Does it related to the power rating in the specification? Like when it say 7 wpc, is it at a particular voltage?

Thanks
Here's the classic simplified answer: Bias is to an amplifier what the engine idle speed adjustment is to a car. Set too low, there are performance issues. Set too high, and many cars and amplifiers also have performance issues, and additionally pay a price in efficiency and component life.

Now I'm admittedly somebody who has a penchant for verbose technical ramblings . . . but really, was it that difficult to answer the OP's question?
I think of the bias current in a tube amp as being similar to the fuel required to keep an engine idling. The engine must be running (idling) prior to applying throttle to accelerate (voltage). Maybe over simplified but works for my mechanical engineering brain.

I have a Cary SLI-80 that I must use a meter to set the bias current and it is very easy to do. I wouldn't freet it and it doesn't sound as if you are. Enjoy your new amp it is a beautiful piece to behold.

Chuck
Well, whatever the hell it is, it sounds really nice!

Thanks for the answers and food fight! :-)
Ecclectique - yes I got that one covered :-)
I had a Cary V12R before the CAD 120's. You need to use a meter, not just depend on the red led's. They run hot, but are a great amp. The led's show that the tubes are good. Happy listening.
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How often should you check the bias on a tube amp?
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how the sound will be impacted? Will I get louder? Or does it allow to get louder? Will I get more bass? Does it related to the power rating in the specification?

The bias sets the point where the tube idles. When you apply a music signal the amount of current increases and decreases. Lets say for max power the current needs to swing plus and minus 40 mA from the bias point. If the bias current is set too low (say 30 mA) then it won't be able to make a full swing down since it wants to go down 40 but will hit zero and clip before it gets there. Even if you don't hit the zero point the tube operates in a non linear manner as it approaches zero so anytime you get near there distortion goes up.

If you set the bias current too high you might hit the limit on the plus 40 swing and clip, but even if you don't the tube will run hotter than it needs to at idle and shorten its life.

So you want to set it where it can make a full swing in both directions without running the tube too hard or swinging down into the nonlinear area near zero.

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You know I always love your explanations, Kirkus, and I know you know all about bias, but I think the analogy of idle speed on is somewhat misleading to those who are trying to understand.

If idle speed is set low or high, the performance of the engine under load can still be as good as would be otherwise. Whereas, the same cannot be said with a tube amplifier. Maybe the better analogy is idle mixture? But, even that somewhat misses the mark. We could also add timing into the equation, and that might be closer - but uh, but maybe we're getting more technical than we were allowed here...

Briefly, an underbiased amp will sound suboptimal, or even flat out poor, and there's not much that will change that. An overbiased amp will actually sound good; most often, even better than one biased to the manufacturer's spec.

Mark O'Brien of Rogue Audio once told me in his opinion, the biggest reason today's tube amplifiers sound better than those from the golden age may be bias. Amps back then were biased with the intent of maximizing tube life, as opposed to best possible sonics.

I have another friend who holds some out of fashion opinions regarding tube amps, and he biases his tubes to the point where the plates begin to glow, and then maybe (I said, maybe) backs off a hair. Now, he's converted his amp to use non-audiophile/guitar player desired output tubes, which he came into many lifetime's worth (and that's with the plates lighting up a bit) for simply the trouble of hauling them out of the place he got them. It's definitely the value play, as you're essentially dealing with "trash". As I've seen with my own fixed bias tube amplifiers, he'll adjust bias in front of you; higher bias is clearly superior, sonically. By superior, I mean, warmer, more liquid and musical sound overall, with better, tighter, more powerful bass, and yes, even improved imaging.

To take a stab at it in real, yet simple terms, it's the flow of electrons from one side of the tube to the other, which can be flat out or none at all. The optimal bias lies somewhere between those two endpoints. Given that optimal point is not going to be stable through the life of the tube and/or amplifier, you periodically adjust to maintain it.

For more of a silly explanation than that, so I can have someone kick me in the teeth, it could be likened to a pot of water on the stove. If the heat is off or too low, you're not going to be cooking, or perhaps at least, optimally. If it's too high, the water will boil over, and you'll wind up with a problem on your hands. You find the place where you get a nice boil without making a mess, that's where you bias your tubes.
If you overbias the tube (set it so that there is too much current flowing) the tube will run hotter and distortion is sometimes higher. This would be particularly true of any class A amplifier.

In a class AB amp, if the bias is set to high you can roast the tubes. Its a good idea to set it correctly according to the specs or procedure.
"Lets say for max power the current needs to swing plus and minus 40 mA from the bias point. If the bias current is set too low (say 30 mA) then it won't be able to make a full swing down since it wants to go down 40 but will hit zero and clip before it gets there.”

I have to say, Herman, that I think your rebuke of Elizabeth was undeservedly harsh. Good intentions deserve better. That said, I’m not new to tubes, but I’ve never felt like I really grasped the importance of "bias” until I read your explanation. Thank you.
Yes, I think if the signal current(music) exceeds the bias current, then there is no sound or just tons of distortion. Is that how they make the Ibenz tube screamer?

Solid state needs bias too, we just dont roll transistors too much. :)
I dunno Trelja, I'm going to have to stick by it. Amplifiers and and cars are certainly different, but the main, general pragmatic, neophyte-level points between bias adjustment and idle throttle angle correlate well:
- They both determine the level of energy dissipated by the system while it's performing no work, but in an active state waiting to be used
- The proper setting is one that is neither too high nor too low, depending on the machine's design and intended application
- A misadjustment of either rarely results in a catastrophic condition, but results in reduced performance and/or increased stress on the system
- Both are traditionally adjusted by turning a little doo-hickey with a screwdriver, in a manner hidden from the casual end-user
- A similar level of mechanical/electrical aptitude and tools are required to adjust either to a factory-specified setting
- Deviation from the "stock" setting is ill-advised, unless you have the technical competence to evaluate the many interrelated system parameters

Above all, I think this analogy gives a good insight to one who is unfamiliar, whether it's something he/she should be comfortable twiddling with.
Lesse... the setting of the bias has *no effect* on the power output. It affects the distortion and the heat of the tube (or transistor).

I think if the signal current(music) exceeds the bias current, then there is no sound or just tons of distortion.

Once properly set, if the audio signal exceeds the bias value there are no worries. That's kind of the point of being able to amplify.

Distortion pedals used for guitars use a variety of techniques to make distortion and that is a subject for a different thread.
Kirkus, where the analogy of idle speed falls apart is with an engine, regardless of whether I set the idle speed at 500 (for the sake of example - too low), 800 (recommended), or 1650 (too high) RPM, performance will be identical at 2500, 3500, or 6000 RPM.

With a tube amplifier biased at 20 mA (again, for the sake of example - too low), 50 mA (recommended), or 75 mA (too high), its performance will be different in readily apparent ways when playing music.
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How often to you have to set and check the bias on a tube amp?
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My injectors are class D but I only run them in class A. I generally bias with IAC adjustments but sometimes fuel pressure.
Mitch4t, this depends on the amp. Some amplifiers have very stable bias settings and can be checked maybe once every 6 months. Others seem to need constant observation. So I would go with the manufacturer's recommendations.
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Thanks for the reply Atmasphere. I really would like to try tube amps, but I'm not mechanically inclined at all. I'm a plug and play kind of guy, which is why I have solid state amps. I'd consider tube amps as long as they don't require much fiddling around with. I think I read somewhere on Audiogon about self-biasing amps. If that technology exists, why don't all tube mfgs use it. Is self-biasing expensive to implement? Is it as accurate as a manually biased amp?
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Mitch4t, automatic bias as a feature varies from amp to amp. With some systems it is quite simple and others rather complex. The accuracy varies too. For the most part what autobias means is that the most 'fiddling around' will have to do with initially setting up the amp, and after that dealing with the occasional bad tube. None of that is particularly difficult. You can run into issues with any audio product that might be equally as easy or perplexing.