I can't find anything in the nF range. It seems like it'd be reasonable to vary the bass-cut cap, but the treble cap would be tricky. The question then also becomes, are variable capacitors up to snuff for tone circuit applications.
I haven't looked for specific values, but AFAIK, most of these things are physically too large to really be practical, and pretty darn expensive to boot. You could probably use a larger value variable C in series with a smaller static C to get some variability. Switching and/or capacitor/variable resistor combinations are usually much easier to implement, and usually work fine.
The problem is, capacitance is a function of the distance between two conductors, as separated by a "dialectric". Air is the most common dialectric material, but in order to achieve the desired numbers, any such capacitor in question will have to be pretty large. See this page for images and more info;
Mouser, Digi-Key, JDR and others all have tiny variable caps, but nothing ranges over a few picofarads (pF), perhaps upto a 100 or so. That's only a couple of percentage points of what we need in a guitar - most tone control caps run between 22 and 43 nF, though some folks like them somewhere on the order of 1 to 1.2 nF, for that "woman" tone. Also, this lower value is often preferred for treble bleed circuits. But it's still way above whatever is available at these "fine" electronics parts houses.
There is another method of varying capacitance that isn't discussed in the above Wiki article, that of "switched capacitance". This isn't the digitally controlled stuff, it's a matter of taking a regular cap, and switching it into/out of a circuit with a timer chip. A better overview than I could provide can be found here:
I was hoping for something voltage-regulated, so that you'd have electric potential forcing the two plates apart powered by some battery. A varactor would do the trick, but I'd want separate leads to control the capacitance and for my tone signal to pass through. (i.e. two low-current voltage leads to define capacitance, two low-voltage current leads to pass your signal through)
Maybe I could plug a 9v into the varactor and use a pot to tune voltage, thereby changing capacitance? I get basic principles, but I know nothing about electrical engineering, so I'm worried that might overvolt something like the Clapton Midboost kit I've got installed, or that I'll sketch up a circuit that winds up using a 9v battery as a high-current voltage source.
A varactor is two-pole device, and is in fact a diode. Right there, you are clipping off half of the signal presented to it, and that's not gonna sound so good now, is it? Check with ash or John, they've experimented more deeply with various types of diodes for different distortion effects.
Besides which, there are two voltages running through it - a micro-volt sized signal (usually radio frequency, or possibly microwave (radar)) and a bias voltage, which is DC. It's that DC component that ends up on the signal path that needs to be dealt with, and at that point, you're adding component complexity.
Not to mention that final nail-in-the-coffin, these things are well below the nominal pico-farad range - not large enough for use in any audio circuits, let alone a standard guitar.
Sorry about that.
To the best of my knowledge, there are no 3-pole or 4-pole varactors out there.... but I could be wrong, who knows?
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OK, back to searching for exotic variable capacitors!
I'm floating this out here, because I have not seen it anywhere before, nor tested it in real life:
I believe the following buffer circuit amounts to a variable capacitor within a tone control, controlled by a pot, with a continuously variable range of 0 up to to the value of C2:
Schematic and frequencies plot shown, which are familiar from looking at the result of changing a tone capacitor.
It works by having a high-impedance buffer sample the pickup output, at one end of the capacitor. the other end of the cap is not grounded like a normal tone cap, but is connected to the buffer output. If the buffer receives no input signal, due to R5 being at minimum, the cap works just as if it was grounded to a zero reference, at its full value.
But if the R5 is at maximum, the buffer is giving the same voltage to the lower end of the cap as received by the upper end. Nothing flows through the cap, it is like no cap at all. In between, the signal flowing through the cap changes, as voltage across it changes. We change the voltage across the cap, to control the current flow through the cap, as if it was the cap value was being changed. You can see on the traces how this works out, like a variable cap.
Note that this is not a buffered output, nor an active tone control stage. The signal path from pickup to output is all passive.
This version is taking a max cap value and reducing down to zero. If the buffer is inverting, we can also multiply a cap value. And if it has gain, we can actually create a negative capacitance. That last one can be used to do things like reduce unwanted capacitance in a long cable.