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RE: Volume Pot Question

Posted by Tre' (P) on December 3, 2016 at 12:08:50

In Reply to: RE: Volume Pot Question posted by amandarae on December 3, 2016 at 11:44:19:

Edit, I made a mistake. The datasheet I looked up must not be for a ux 201a.

Please disregard the info below WRT the gain and the Miller capacitance of a ux 201a.

____

Depending on operating point, a 201A stage has a gain of 25.

The grid to plate capacitance is 5pF.

5 X 25 is 125pF plus 2.9pF grid to filament for a total Miller capacitance of 127.9pF.

A pot has a worse case output impedance of 1/4 it's resistive value at the -6db point.

A 250k pot would have an output impedance at the half voltage point (-6db) of 62.5k ohms.

62.5k ohms driving 127.9pF of capacitance creates a low pass filter with a -3db point of 19.9kHz

I think if your source was happy driving the 47k grid resistor you should stay with that. Replace the 47k resistor with a 50k pot.

A 50k pot has a worse case output impedance of 12k ohms.

12k ohms driving 127.9pF gives a -3db point of 103kHz.

The amplitude will be down only 1db at 51.3kHz and the phase will be shifted all the way down to 10.3kHz but I doubt you will notice.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

Edits: 12/03/16

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