Help understanding a tone knob value in relation to a volume pot

[midwayfair]

I feel like I should be be able to figure this out on my own, but no dice ...

Really simple high cut tone knob: A cap in series with a pot to ground, coming from the wiper of a voltage divider volume knob.

Does the actual value of the volume pot factor into what value can be used for the tone pot? If, say, the volume is 10K or 100k, will a tone knob of, say, 500K stop doing anything after the base value? How do I calculate when the cap is taken out of the circuit completely?

I've been testing it, and I swear it's all bunching in one side of the sweep regardless of whether it's a linear or audio pot, but I'd also swear that there's at least a slightly audible difference at all points in the sweep.

I'm trying to use a large value to avoid resistance loading on the volume circuit, but it's clearly having an undesirable effect on the function of the tone knob.

If I do need to change the value of the volume pot to get closer to a match, is the solution to put the large vp in parallel across another resistor on the board, so that my volume circuit divides to the proper value, but the tone knob functions the way I want?

[Mark Hammer]

When connected to the wiper of a volume pot, as opposed to its input, the two controls become much more interactive than the other way around.

Think of it this way.  Let's say the tone pot were set to zero ohms.  We would now have a cap between the wiper and ground.  That cap would, in conjunction with the input-to-wiper leg of the volume pot, form a single-pole RC lowpass filter, whose corner frequency would depend on the joint value of the input-to-wiper resistance.  As you turn the volume down, you not only lose more overall level, but you also lose more treble.

Okay, let's say we increase the resistance of the tone pot above zero ohms.  That resistance, plus the input-to-wiper resistance of the volume pot, form a voltage divider for that frequency content dictated by the tone cap value.  Since ANY voltage divider attenuates as a function of both the input-to-wiper (Ra) AND wiper-to-ground (Rb) resistance, the impact of changing Ra, will depend on the value of Rb, and vice versa.  LikeI say, a tone control tied to the wiper is VERY interactive with the volume pot setting.

That is neither good nor bad.  There may well be settings you can get with that arrangement that are unattainable with the traditional arrangement.  If you're VERY familiar with yours, then it maynot be very problematic for gigging, but my gut sense is that reproducability may be low, with a steep learning curve to boot.

[midwayfair]

When connected to the wiper of a volume pot, as opposed to its input, the two controls become much more interactive than the other way around.

Think of it this way.  Let's say the tone pot were set to zero ohms.  We would now have a cap between the wiper and ground.  That cap would, in conjunction with the input-to-wiper leg of the volume pot, form a single-pole RC lowpass filter, whose corner frequency would depend on the joint value of the input-to-wiper resistance.  As you turn the volume down, you not only lose more overall level, but you also lose more treble.

Okay, let's say we increase the resistance of the tone pot above zero ohms.  That resistance, plus the input-to-wiper resistance of the volume pot, form a voltage divider for that frequency content dictated by the tone cap value.  Since ANY voltage divider attenuates as a function of both the input-to-wiper (Ra) AND wiper-to-ground (Rb) resistance, the impact of changing Ra, will depend on the value of Rb, and vice versa.  LikeI say, a tone control tied to the wiper is VERY interactive with the volume pot setting.

That is neither good nor bad.  There may well be settings you can get with that arrangement that are unattainable with the traditional arrangement.  If you're VERY familiar with yours, then it maynot be very problematic for gigging, but my gut sense is that reproducability may be low, with a steep learning curve to boot.

Thank you, Mark. I'll probably just stick with the SWTC in this design, then.