Bias Excursion for dummies

[guitarmike2107]

I am busy reading through Kuehnel’s power amp book , I made the mistake of jumping straight to the section I wanted to read rather than starting at the beginning and getting into his way of thinking.

Anyway I am looking at the bias excursion section and am trying to figure its effect into simple terms..

Heres what I am thinking

The three main point I have picked up on is:
- the speed at which it goes into bias Excursion
- The amount of Bias Excursion
- The speed at which it recovers from bias excursion

In terms of simple distortion characteristic can these be translated into
- How quickly the power amp will transition into distortion
- How much distortion results
- The sustain of distortion.

Or am I over simplifying it?

[Steve Conner]

I don't have the book, and this is the first time I've come across the term "bias excursion". I suspect it's the same as what I know as "blocking distortion". A heavily overdriven tube goes into grid current, and the resulting net DC current charges up the coupling capacitor that feeds the grid, adding an extra voltage to the bias. The tube biases itself colder, the harder it is driven.

The result is compression, and a PWM-like effect that changes the harmonics generated. Those apply to preamp stages as well. P-P power stages develop crossover distortion that plays with the harmonics in a similar way.

I know that a little of it (defined as the amount you get in an old Fender or Marshall ) is part of "the tube sound". All of those harmonics shifting around add touch sensitivity and "swirl" to overdriven tones. Every tube amp designer nowadays has discovered this and knows how to control it.

But too much "bias excursion", and with too long a time constant, makes the amp sound weak, fizzy and farty, like a transistor radio with a dead battery. Too little, and it sounds like a solid-state circuit made out of op-amps, which have no bias excursion.

If the time constant is far too long, the sound can disappear completely for a good fraction of a second after you stop playing a note. That was what my first high-gain tube preamp did.

[guitarmike2107]

I haven’t thought about where blocking distortion fits into this, I suspect that it would occur if the total time for bias excursion and recovery is greater than the lowest frequency i.e. for 82hz a bias excursion/recovery time greater than 12ms will result in blocking distortion?

I guess I am trying to figure out how to apply the theory he is discussing with out doing the testing. It may come later in the book. ( or earlier)

i.e. for heavy metal chugga sounds you probably want a fast response both ways and for blues you want a slower response both ways or maybe not depending if you are Gary Moore or not. (The singer song writer not the "rocket Scientist")
For touch sensitivity do you want it to excurd more? is excurd even a word?

Steve, I have no idea what a PWM effect is? But I think I want one

[Steve Conner]

The recovery time is roughly the RC time constant of the coupling capacitor and grid leak resistor. So, tube stages designed for a hi-fi response suffer terrible blocking distortion when overdriven heavily.

You are right, to get a good chugga sound, seriously undersized coupling capacitors are used. I've seen as low as 4.7nF in high-gain amps. Ever since Metallica, a graphic EQ in the effects loop is used to boost the missing bass back up again. (The old 2203 type Marshalls do that naturally, by putting the tone stack last.)

PWM is Pulse Width Modulation. Once a tube stage is overdriven hard enough that the output is a complete square wave, further overdrive shifts the duty cycle, which changes the balance of harmonics. So it's still touch sensitive. That is quite hard to model with solid-state distortion circuits, without also making them very temperature sensitive.