Variable capacitors?

[ethrbunny]

I see lots of mods involving capacitor swapping but never any mention of using  a variable capacitor. So I decided to look into this. It seems that variable caps are either trimmers or these bulky monsters (3-6" across).. whats up with that? How come I can't find any in a convenient size? Am I looking in all the wrong places?

[sta63bmx]

I replied to this in the DIY guitarist forum earlier.  Weird.   I just salvaged some rotary switches and soldered in a range of cap values I wanted to see.  I mounted them in a small metal project box with binding posts for each "variable capacitor" so I could just run leads out from the board.  This is really nice because you can turn the switches while you're playing and listen to the changes.  If you're interested in seeing a picture, I can dig one up.  I also soldered in a bunch of clipping diode arrangements, same deal.  I used DP12T switches for one set of caps and then the diodes, so both sides of the component (or group of components, in the case of the diodes) would be switched.  For caps, I also just used a SP12T rotary, soldered a cap to each throw, and then soldered all the caps together on the other side and took off a "pole" there.  Really handy and cheap if you can salvage the switches.  Old lab equipment, maybe the rotary switches on old TVs?  Good luck.

[Connoisseur of Distortion]

the capacitance is directly proportional to the area of the plates and inversely proportional to their distance.

the distance between the plates of the variable capacitors i have seen are minute, perhaps thousandths of an inch. But, that's not actually very practical, as it just isn't close enough / wide enough to make a lot of capacitance. so, to make it more effective, you just keep adding on plates.

the plates are restricted to certain distance as well. Remember, with air as a dielectric, you're risking some pretty severe arc problems, thereby turning your capacitor into a reeeeally crude resistor.

rotary switch with caps is a good idea. BUT, there is a better one. Use a DIP switch and put in several assorted values. then, adjust as needed!

[Skreddy]

This isn't strictly a variable capacitor, but if you place a pot in series with a bigger cap, and that combination in parallel with a smaller cap, it's the same practical effect.

[A.S.P.]

http://www.elixant.com/~stompbox/smfforum/index.php?topic=31312.0
http://www.elixant.com/~stompbox/smfforum/index.php?topic=29898.0
http://www.elixant.com/~stompbox/smfforum/index.php?topic=25504.0
http://www.elixant.com/~stompbox/smfforum/index.php?topic=27289.0
http://www.elixant.com/~stompbox/smfforum/index.php?topic=24307.0

[bwanasonic]

This isn't strictly a variable capacitor...

But it's a nice illustration! 

Kerry M

[Paul Perry (Frostwave)]

Old AM transistor radios have a small variable cap about 30 to 300pf (usually twin gang, so you could parallel to 60 - 600pf). About 1 inch cube or so, typically.

[ethrbunny]

Okies. Thanks.

Didn't realize is was a FAQ. Im tinkering with the CE-2 from Tonepad. The 'depth' is adjustable via a small cap value (< 100pf) so I thought this would be a great place for a knob. I'd rather not have to line up a trimmer with a hole though.

[Paul Marossy]

Hey that two cap with one pot thing is pretty cool. I'm going to try that sometime.

[DDD]

I would like to confirm that combination of the bigger cap - pot - small cap is actually very effective thing.

[puretube]

IIRC, Joe Gagan brought that up, here... 

[ethrbunny]

Skreddy - where does one tap that pair of capicitors to get the 'variable' effect?

[Skreddy]

Input cap is the most dramatic place for it.  The uses are limited only by your control-panel real estate. 
See the Big Muff Pi tone stack for more variable tone ideas.

[ethrbunny]

Im ignorant. When you've used this - what size pot works best? And what if I want to vary between 0 and some higher amount?

For some reason (see ignorance) Im having trouble wrapping my small brain around this.