Signal Envelope dependent clipping characteristics

[Vivek]

Everyone says Sag and Variable Harmonics (a la Ampeg) are an important part of " that tube sound"

Which Amp in a box / distortion pedals implemented Signal Envelope dependent clipping characteristics ?

Or two single sided clippers for variable duty cycle ?

[Vivek]

https://youtu.be/3oez0Hcfufc

[Steben]

Regarding preamp distortion, which is probably the thing captured into the pedal in the video, one must look at duty cycle and bias shift between two preamp stage. Not sag.
What happens there is that the mean "center line" of the signal (when the DC bias gets cut through the decoupling cap) get shifted once the signal gets assymetric. The cap does not load off the center line if the signal is perfectly symmetric (what gets built up above the line gets loaded down beneath it).
The result is that not only the signal is clipped, but the balance between upper and lower signal halves gets distorted.
This is in theory NOT strictly a tube thing, every preamp with similar class A stages with assymetry (be it any FET or BJT ...). Yet it is perfected in high gain tube circuit.
Once you have a cascade (yup there it is again) of stages, every step to the next stage can have this effect, which means with the building up of a signal, the amount of steps having shift rises, which means the character of the clipping gets shifted. It feels dynamic, because the combination of harmonics is a variable controlled by low or hard signal.

A perfect non tube example is the Fuzz Face, although it does not take decoupling caps. At lower signals (think of volume clean up), the signal is so small the assymetry (aka nonlinear response) is very small yet with large signal it is huge.
at electrosmash.com you can find a nice sim of the max output of a FF
The output is very assymetric, yet ... note the surface between signal and center line is equal above and below which means there is an equilibrum.
This exhibits huge amount of even order harmonics combined with hard clipping. In other words: there is no universal strict rule even order and soft clipping or odd order and hard clipping are tied to each other.



This happens in preamp tube stages as well, some more than others according to what the designer wants.
Do not forget preamp tube stages can clip very hard as well especially compared to some sorts power amp clipping. And mostly they clip hard on one side and less on the other.
If you look at the cascade of inverting stages you see a complex of switching sides of the signal which gets clipped hard or soft. This all adds to a character of the sound which changes at different pick strength...



I know this is a vague introduction, but the longer posts will follow.

[Steben]

[PRR]

https://www.diystompboxes.com/smfforum/index.php?topic=125497.msg1195466#msg1195466