I made a sim with the values published by peter J.. 50k pots is too low for vacuum tubes.ruffrecords said:abbey road d enfer said:In most cases, the biggest load will not be the EQ components, it will be the actual load to which the circuit is connected. That's what really motivates the existence of the output CF.
I am not 100% sure of this. Referring to the last schematic I posted with a 50K pot and 5K resistor in series at the cut side; when maximum cut is applied, the feedback impedance tends to 5K. Assume the reactive part of the feedback is also 5K. If the virtual earth is working then the load on the output is 10K. At maximum boost it drops to 55K or so. A CF will be fairly happy with these changes in load. A CC 12AX7 will not. I know I have simplified the issue a lot but I think it is clear the load the stage sees varies a lot. Hence my comment about scaling component values. I'll do some sims to check this out.
Cheers
Ian
I simulated some variants of the original.
The center-tapped HF pot does not seem to be nice. In fact it makes the variation faster around neutral, which may counter the idea of making the neutral position flat.
See attached: graph with center-tapped pot