mwhouston wrote:
Matt, how do you decide what value pot to use? I see some tube amps have 100K pots and others 250K pots.
It depends on several factors.
First is the Miller capacitance of the input tube. Worst case is the entire pot value appears across the input at full volume. The high frequency cutoff is 1/(2π*Req*Cm). Since Req is the parallel combination of the driver output impedance and the volume pot, If Cm is large then I'll use a smaller control so it doesn't affect the high frequency cutoff as much if the driver has a high output impedance.
In general, the higher the gain stage, the higher Cm. And in general, the larger the tube elements the larger the Cm. High gain 6SL7 drivers have horrible CM values (140µµf is not unusual).
Second is the expected driver impedance. If is is greater than about 25kΩ then I'll try and go to a higher value pot so that I don't unduly load the driver; so long as I don't compress the high frequency response (see above).
Lastly, the lower the quality of the pot, the lower I'll go. Noise voltage is proportional the the square root of the resistor value, so cheap pots that are already noisy get lower values to try an minimize noise figure.
So there are a bunch of factors in play. I also have really come to like the 250kΩ PEC pots. They have a great feel and are very quiet (for hot formed carbon). I think it also kind of depends on my mood.
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