Pot style variable capacitors

[Electric_Death]

I'm wondering if anyone knows of a supplier with variable capacitors in a 1/4 knurled shaft configuration?

[Aleksi]

Hi,

I think you will have a hard time finding those, not even sure if they exist? I've only seen trimmer type variable capacitors and those have been <100pf.

BR,
Aleksi

[Electric_Death]

Hi,

I think you will have a hard time finding those, not even sure if they exist? I've only seen trimmer type variable capacitors and those have been <100pf.

BR,
Aleksi

I'm sure someone makes them but, they're probably difficult to hunt down commercially if they're commercially available at all.
You figure there are amps with pots that vary the EQ point of the midrange control so they're out there somewhere.
They would surely make for a much more versatile tone stack. Apply one to the bass, mid and treble and I don't think there's a tone out there that couldn't be found.
Also bet it would be a good way to get some inter modulation.
I could use a multi position rotary switch but the capacitance values needed for a respectable level of variance would require buying a boatload of caps and, it would be a bit much to do for each tone control. I think with that being the option, a series of active, variable notch filters aka multi band EQ would be more logical.
I suppose I could just go with adding a more leisurely rotary switch with say 3 positions rather than the popular half dozen or more.

Maybe I'll just do what all the amp makers seem to do these days. Use a classic 3 tone stack then add an active bandpass filter with a sweepable way point...maybe that's how all those clever manufacturers are doing it.
Maybe there is a way to use a pot in conjunction with a cap or caps to very capacitance without effecting signal strength??

[runmikeyrun]

what about a pot connected to sweep between two cap values?  I've used that in my pedals before, put a large value on one side and a small one on the other, each connected to an outside lug of the pot.  The center lug connects to my circuit.  I grounded both caps and used the center lug to tap into the circuit and make a variable value for how much farad value the circuit "sees" as being grounded.  Might be kind of ghetto but it works great.

[drewl]

The only ones I've seen are trimmer type in small values up to 200 or so pf of which we have hundreds at work...
I use them all the time in RF amps, doesn't help with pedals...DOH!

[earthtonesaudio]

I've seen 1/4" shaft, panel mount air capacitors... but very small capacitance values, unfortunately.  You could construct a capacitance multiplier to increase the effective capacitance, but then that's more circuitry (and probably wouldn't work as well) than just doing a regular "active" tone control.

Plus those air caps are enormous.

[cpm]

not a pot but a good idea anyway:
http://www.diyguitarist.com/DIYStompboxes/FX-17.htm

[MarcoMike]

If your need is changing a capacitance almost continuously and having a knurled shaft... what about using a 1x12 rotary switch?
12 steps are almost continuous for typical applications... and some of the metal ones come with knurled shaft...

[Electric_Death]

what about a pot connected to sweep between two cap values?  I've used that in my pedals before, put a large value on one side and a small one on the other, each connected to an outside lug of the pot.  The center lug connects to my circuit.  I grounded both caps and used the center lug to tap into the circuit and make a variable value for how much farad value the circuit "sees" as being grounded.  Might be kind of ghetto but it works great.

I've done the same thing with some of my muffesque style tone controls. Seems like if I wire it for pass through, it might give the effect I'm looking for.
The more I thought about it, the more I realized that the mid frequency sweep control on a lot of amps and some pedals these days is simply a bandpass filter using a pot to vary the frequency way point.

[sickbend]

what about this simple circuit http://www.thesquarewaveparade.com/images/MIMEO.jpg

100pf to over 100uf wih 1 pot

[Electric_Death]

Thanks sickbend, I may give that a try some time.
I tried what runmikeyrun suggested and I had already suspected might work and it in fact does...quite well in fact.
So far I tried a 100K pot then a 500 Ohm pot-it's looking like something in the 10-50k range will give the most variance and linearity.
I wired a cap to each outside wiper then a lead to the center lug. Put the two cap leads to the hot of my multimeter and the center lug to the negative.
The gradual value change was incredibly accurate, getting as low as 1/100 of increment value escalation/deescalation.
Just need to pick the right value pot and of course, caps that aren't too drastically different in value.
I went with a .01uf and a 1uf-too drastic but with the right value pot and a resistor in series with the 1 uf, I bet even those obscenely different values would work.

I suppose those clever midband way point pots on those amps and pedals just might not be active bandpass filters and even if they are, this is still likely what they're doing to vary the frequency point.

[cpm]

http://en.wikipedia.org/wiki/Varicap

may it be useful for some circuits?

[Electric_Death]

http://en.wikipedia.org/wiki/Varicap

may it be useful for some circuits?

Oh great..thanks man, now I gotta spend more money! 

Looks interesting, I'm sure I'll figure out something to do with them since I'm not your conventional builder..or the most knowledgeable when it comes to fundamentals.
I like to insert parts here and there, wire them in unorthodox places and so far it's really paid off as you can hear through my synth clip I linked in a different post.
As I understand it, the capacitance varies based on voltage which should make for some really interesting uses.....

thanks man!