variable gain stage bias

[Andy Le Blanc]

I've been playing with variable bias in an interesting way.

The power side of the tweaker on my bench is cathode biased, so I had the
fixed bias adjustment free to play with, so I hooked it up to vary the grid
bias of the pre-amp stages, I can swing the grid bias from around 3 to 9 v.
Zero bias or grid leak only made around .7 v, and depending on the range
you set up, it can be a critical tone or touch, symmetry adjustment, it can be applied to
more than one stage a time.

Imagine a transfer curve for a gain stage, your moving the applied sine wave
up and down between the knees...

[es345]

Adjustable bias as you show at triode is used often for compression or tremolo,
see e.g trace elliot V8. They use the lower knee with small signal applied. The result is voltage controlled gain.

[Andy Le Blanc]

I dressed it as a user control, it is effective as a vol./ gain, although
I was more interested in tone change as the bias point moves.

it is a direct parallel to a tube limiting amp, it could be used in place of a
"clipper" tube in a high gain pre, you could swing between marshal and soldano

crunch adjuster