Why do different bias settings result in different tone?

[Voxbox]

Uummm... the title says it all really.

Post Number Two in a short series of "Why are things the way they are?".

I know we can all accept things, but occasionally I wonder "Well, that's fine, but why IS that? What's going on?"

 

[jjasilli]

Bias affects:

* the amount of amplification.  More amplification means taller sound waves.  I.e., smooth sine waves approach the shape or rectangular waves: steep walls approaching 90°; shoulders approaching square instead of gently rounded; tops approaching flat.  Changes in the wave form = harmonic distortion which changes the tonal characteristics of the signal.  At some point, simply adding amplification makes the tone more "clear" then more harsh.  Depending upon bias setting & incoming signal strength, the wave will eventually "hard clip" -- the wave's peak literally shaves-off and becomes flat -- going from "clean" to a soft over drive tone, to a harsher form of distortion.

*  the impedance of the tube.  Impedance = resistance for AC.  Signal is AC.  Impedance presents a different amount of resistance to different frequencies.  Changing the bias setting affects the frequency response of the tube, in conjunction with values of the caps & resistors around it. 

[Voxbox]

Thanks jjasilli,

I'll go and check these things out with an oscillator and scope, however, subjectively, you can adjust the bias at low volume settings when presumably there is no clipping or signal distortion and still hear a change.

Let you know how I get on.

cheers

[HotBluePlates]

Maybe I'm the odd man out, but I've never heard any big change of tone with a change of bias. Maybe little more/less distortion, some change of highs, but no radical tonal changes.

That assumes the bias never gets so cold as to have strange distortion effects due to the tube cutting off on small signals.

[phsyconoodler]

Tonal changes?Not really,but there are perceived changes like 'fatness' of the tone.Some amps like different bias settings to bring out chime and bell-like tones that don't happen at lower current settings or too high current settings.So tonal changes?Yes I'd say so but not a drastic departure from the basic design of the amp.

Voxbox is on a quest.Maybe we need to answer some other questions like 'who invented liquid soap and why?'

[jojokeo]

I think the first thing thought of is biasing of power tubes. But all of the tubes in the amp are biased in various ways all contributing to the overall "tone" of the amp. In preamp tubes biasing does change the tone of the amp drastically. Power tubes, not so much as noted.
I always think of electrical properties in terms of water and/or plumbing. Changing bias to me is changing the port of the hose bib or valve or spigot...which ever term you'd like to apply. A small tweak to open and the thing is full open or fine tuning w/ a small opening and it can take multiple full turns to get full open. Now a bypass cap can be thought of like the hot water side...bigger cap = more hot water and vice versa.

[tubeswell]

Bias affects:

* the amount of amplification.  More amplification means taller sound waves.  I.e., smooth sine waves approach the shape or rectangular waves: steep walls approaching 90°; shoulders approaching square instead of gently rounded; tops approaching flat.  Changes in the wave form = harmonic distortion which changes the tonal characteristics of the signal.  At some point, simply adding amplification makes the tone more "clear" then more harsh.  Depending upon bias setting & incoming signal strength, the wave will eventually "hard clip" -- the wave's peak literally shaves-off and becomes flat -- going from "clean" to a soft over drive tone, to a harsher form of distortion.

Further to what jjasilli said

In an inverting gain stage, you can bias hotter or cooler, and this changes the type of clipping in different ways, which can affect the tone.

Biasing hotter eventually causes grid-current-limiting, where the positive voltage swing of the grid is clipped. This type of clipping is also affected by the source impedance of whatever circuit precedes the stage in question. With grid current limiting, it is the grid signal that is being clipped, and the plate continues to amplify the grid's already-clipped signal. This type of clipping can have rounder edges* where the signal enters and exits clipping, which gives a bluesy flavour to the tone.

* affected by the source impedance of whatever is prior to the stage in the circuit.

Biasing cooler causes cutoff at the plate which is a different type of clipping. In cutoff the grid isn't being clipped, but the bias is so cold that when the grid voltage swings down to its bottom 'peak' there is no more voltage for the plate to go 'up to' because the plate is effectively at HT voltage. So the signal at the plate becomes clipped. This type of clipping typically has 'squarer' edges where the signal enters and exits clipping, and produces a more 'bitey' rock tone.

In many hi-gain amps they set up the bias of successive pre-amp gain stages in order to alternately have both types of clipping. 1st hot, 2nd cold, 3rd hot, 4th cold etc to get a 'sustainy' tone.

This is all explained in great detail in Merlin Belcowe's 1st book BTW. http://shop.dobermanamps.com/product-p/book-dvpgb.htm

[jojokeo]

In many hi-gain amps they set up the bias of successive pre-amp gain stages in order to alternately have both types of clipping. 1st hot, 2nd cold, 3rd hot, 4th cold etc to get a 'sustainy' tone.

This is all explained in great detail in Merlin Belcowe's 1st book BTW.

That's exactly what I did for my version of the Dumble HRM amps a while back. Great sounding, milkshake thick n rich tones w/ a lot that "vowel" blooming type of thing that I think tubenit likes and tries to describe.

[Voxbox]

Thanks Pysch!

I'm a Man on a Mission.....it's a tonal "quality" change with bias, not radical, but it's there. Fatness is probably the best way to describe it subjectively.

Regarding the liquid soap -
 
William Shepphard first patented liquid soap on August 22, 1865. In 1980, the Minnetonka Corporation introduced the first modern liquid soap called SOFT SOAP brand liquid soap. Minnetonka cornered the liquid soap market by buying up the entire supply of the plastic pumps needed for the liquid soap dispensers. In 1987, the Colgate Company acquired the liquid soap business from Minnetonka.

But you knew that anyway didntcha? The question is... WHY?

Thanks Tubeswell for the link to the Martin Belcowe book. Is there info in there that isnt covered by KO'C's TUT series?

I'm really appreciative of everyone's contribution, it's brilliant! 

[jjasilli]

Here's some stuff from Bill Machrone. He focuses on the Blues Jr., but is generally applicable. 

Power tube bias & tone:  http://home.comcast.net./~machrone/bjr/bjbiastone.htm

Preamp & PI stuff:  http://home.comcast.net/~machrone/bjr/bjtubes.htm

[thermion]

'who invented liquid soap and why"
Corpses turning to adipocere in potassium-rich graves.
WRT bias, I hear more differences in clarity/ detail (and of course apparent gain) than a tonal change.
For fixed-bias, I make sure the bias tap voltage is reasonable and adjust by ear.
Scope, shmope.