Is this a Resonance circuit?

[Cartman]

And is the 470K/680K voltage divider critical? I have put a pot between the 470K and 0.003uF cap so that the 470K can be varied as high as 2.25M and it gives a wide variety to the tone. I’m not sure if varying the resistance that much will cause any problems for the phase inverter tube.

[xtian]

This is just NFB, not resonance. The 680K is the grid-to-ground reference for the PI.

[Cartman]

Hmm, OK. The NFB loops that I have seen don’t have the cap, so I thought it might be something different. Thanks.

[maxkracht]

The cap is cutting bass in the nfb loop, which boosts the bass you hear. Just a fixed version of a normal variable resonance control. Your pot is changing the amount of feedback, not the resonance.