Question about preamp bias

S

Stadanko

Inspired
Been looking through Cliff's posts about the subject and had one question.

How is the harmonic ratio odd/even affected above and below zero. In other words: below zero does? Above zero does?

I understand that farther away from zero has a more pronounced affect, just want to be sure about directionality.

I've also gotten the impression that even harmonics sound resonant and good or complimentary, where as odd harmonics sounds more harsh and clashing or go against the grain.

Thank you for your consideration :ugeek
 
Hi

Center- or positive biased preamp stages produce a more warmer tone, because of symmetrical clipping. Too high biased gain stages become to fuzzy IMO
Negative bias (so called cold bias gain stages) were used in most higher gain amps were sounding more crunchier.....

Cheers
Paco
 
Thanks Paco,

I'm wondering how that translates to the preamp bias control in the Axe. Does warm mean more even order harmonics? What happens when you turn the knob +0 vs -0?


I'm guessing more even order above Zero and less below? Again I'm trying this to relate to the knob in the Axe.

I've been reading your thread on Cathode Following. Can't understand a lot of it due to my lack of fundamental comprehension, but it makes me want to understand it better. At least I now have a relative idea of why its there.
 
Symmetrical clipping gives odd-order harmonics only. With asymetrical clipping, there are some even-order harmonics, too. Even harmonics add perceived warmth and musicality.

But wait, there's more! The clipping you get from higher bias is softer than the clipping you get from lower bias. That's either a good thing or not, depending on what kind of sound you're going for. Too much bias either way can introduce unpleasant levels of distortion.

It's called "hot bias" and "cold bias" because when you raise the bias, the tube dissipates more power, and it runs hotter. When you lower the bias, it runs cooler.
 
Thanks Rex. That sounds familiar to some of the bias concepts I've been introduced to in the past.

I guess my question is what zero represents in the preamp bias knob. Perhaps normal which equates to symmetrical clipping? Above zero is hot and below zero is cold? Are even order harmonics introduced either way?

Not sure sure if I'm on the right track here, but I'm tenacious if not super smart.:pride:
 
Im assuming (which you should never do really - so I could be wrong) "0" in the bias represents whatever the modelled amp runs at - consequently it could be said to be "hot" OR "cold" depending on the amp model.
 
My understanding is that the zero point is the default bias as Cliff modeled that particular amp. Above zero is hotter than the way he modeled it; below zero is colder.
 
It will never clip symmetrically since a tube doesn't clip symmetrically. Zero sets the operating point of the virtual preamp tube halfway between the clipping boundaries. It's exactly analogous to setting the operating point of a preamp tube except that the logic is inverted. For example, if a preamp stage has a minimum output voltage of, say, 100V and a maximum output of, say, 400V and is biased such that it's quiescent operating point is 250V then this would be equivalent to setting the bias to zero. Increasing the bias is analogous to lowering the tubes quiescent voltage. IOW, negative bias moves the operating point towards cutoff, positive settings toward saturation.
 
Rex said:
My understanding is that the zero point is the default bias as Cliff modeled that particular amp. Above zero is hotter than the way he modeled it; below zero is colder.
Click to expand...

No, each model defaults to the appropriate bias value for the given amp. There are multiple stages in each model but only the last bias point is exposed as this is the most audible and exposing all of them would be too much.
 
Thanks Cliff,

It was my greatest hope that you might chime in. To be clear. If you increase the bias in your example, then the quiescent voltage which was 250V, would drop and vice versa?
obviously there is more than meets the eye. For people like us, it is counter intuitive to think of to think of less voltage meaning more saturation. hmmm...

And if there's any way to correlate, do even/odd harmonics play a part either way you go hot/cold?

I swear if you ever wrote a book, you'd make a fortune and many of us would have a clue! I'd buy it...

Even if you didn't have time to focus on it, you could just turn on a recorder and babble away at random. Then hand that off to a writer to put it all together! Magic I Tells ya!

You could call it: "Cofessions Of a Tubeophile" :lol
 
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FractalAudio said:
It will never clip symmetrically since a tube doesn't clip symmetrically...
Click to expand...
Good point. Am I right in thinking that, as the bias moves away from zero, the relative amount of even-order harmonics increases because of increased asymmetry?
 
FractalAudio said:
No, each model defaults to the appropriate bias value for the given amp. There are multiple stages in each model but only the last bias point is exposed as this is the most audible and exposing all of them would be too much.
Click to expand...
Thanks for the clarification.
 
Rex said:
Good point. Am I right in thinking that, as the bias moves away from zero, the relative amount of even-order harmonics increases because of increased asymmetry?
Click to expand...

Cliff posted this in another thread

The further you move away from (roughly) zero the more even harmonics are introduced. It's an asymmetric transfer function so you have to experiment.
Click to expand...

To me that means going above or below zero will add more even harmonics & zero would be the most uneven. Is that right Cliff?
 
Our ears produce even harmonics as a natural result when hearing loud sounds. Tube-amplifiers produce these when pushed. That's why a clipping tube-amp "feels" louder than a transistor-amp going into clipping (generating mostly third-harmonics). And then they sound harsh and non-pleasing.
I have to look into how Cliff digitally resembles and twirks a typical input-stage, driver-stage and all the many designs around out-put tubes, fixed or variable bias, class A or AB or the out-tranni wounding with respect to the speaker-load.
With all respect - I really can hear the work going on.
 
FractalAudio said:
It will never clip symmetrically since a tube doesn't clip symmetrically.
Click to expand...

Well not entirely symmetrical clipping regarding signal forming , but when the grid gets positive of a strong signal this would induct grid current limiting, on the other side it gets limited by the plate voltage, this what I call symmetrical clipping in a tube gain stage. This has been rarely seen in guitar amplifiers, cause if their latest gain stage is cold biased, it goes into cut off before any grid current limiting would be reached . What I called symmetrical clipping is more common in microphone preamps, where maximum of possible headroom is needed - but it's possible to introduce limiting in positive and negative side of the signal. And since your axefx can do things, what normal guitar amps can't, I thought this would work for your device :star: .....
 
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