whats happening with a Vox cut control

[pjd3]

Heyo, sitting here at Neros sucking some coffee, doodling some RC math.

I get that a Vox cut control will allow a particular frequency on up to be cancelled due the 180 degree out-of-phaseness of the signals on their way to the power tube grids in a typical long tailed pair push pull topology. However, I'm having a blank spot in my thinking about what is happening as the potentiometer (reostat) is varied with regards to cutoff frequency.
One dynamic I can see is that perhaps the decreasing of the resistance of the pot allows more of the 2 signals to meet up and cancel each other. What I'm wresting with is what is happening with cutoff frequency as the pot is being varied.

Does the cut off frequency vary also as the pot is varied? I'm so used to thinking of the formula "Fc = 1 divided by 2 pye times R times C" that its difficult to not assuming the cut off frequency would also vary with the pot but, this is not an R/C to ground. Its 2 out of phase signals somewhat close in amplitude but out of phase 180 degrees, that there may be some other dynamic taking place that doesn't make the cutoff frequency vary as the potentiometer is varied.

I hope I was OK at explaining my brain fart on this.

Thanks for your time and effort!

Best,
Phil D.

[martin manning]

Does the cut off frequency vary also as the pot is varied? I'm so used to thinking of the formula "Fc = 1 divided by 2 pye times R times C" that its difficult to not assuming the cut off frequency would also vary with the pot but, this is not an R/C to ground. Its 2 out of phase signals somewhat close in amplitude but out of phase 180 degrees, that there may be some other dynamic taking place that doesn't make the cutoff frequency vary as the potentiometer is varied.

Often you can reduce a circuit to an equivalent R, and use 1/(2πRC) to get a corner frequency. This thing is a bit more complicated since with the addition of the cut control each PI output has additional paths to AC ground, which are shared with the other output. Here's a simulation of the AC30 PI with Cut control (250k pot and 4n7 cap). You can see the result is a shelf filter where the upper frequency and the depth of the treble cut is determined by the pot setting. Note the effect is different on the two outputs, and more so as the pot's resistance is reduced. A smaller Cut cap will shift the whole effect to the right.

[roberto]

Hi Martin,

isn't the difference due to the fact that the PI, as in most guitar amps, is not perfectly balanced?

[pdf64]

When I’ve tinkered with a type3 master volume, I found that when the control resistance went much below 10k, the signal from the common grid side of the LTP flipped and became the same polarity as the signal from the common cathode side.
So the mute wasn’t due to zero signal, the balanced signals don’t cancel each other out per se, but rather become a (very small) common mode signal.
I assume that at high frequencies, the cut control has a similar effect.

Thanks for taking the time to run a sim Martin, I had visualised it working very differently to that, with much more variation of the turnover frequency.

[martin manning]

If I short the cap and look in time domain, the two signals stay 180 out of phase all the way down to 1 ohm, so no issues with the simulation, anyway. With the cap in-circuit, there is a progressive phase shift relative to the baseline (no cut control), but again the two outputs shift together and remain opposite in polarity.

[pjd3]

Thank you Martin and all for this info.

And for the very nice and telling graph. This all helps put a couple of things in perspective for this.

Next, I will be wondering if the Vox cut control can be opto-switched in and out for channel switching, reduce some highs in a very high gain Marshall type channel

Thank you for the great feedback,

Best,

Phil D.