Variable capacitors in stompboxes?

[vfr800fiman]

Hi all-
After building quite a few pedals, I started thinking  :idea:
I see a lot of schematics where you can tune the circuit for your particular tastes in frequency by substituting different capacitor values. Has anyone tried using a varable capacitor for this purpose (read another knob to twiddle)?

Since I'm a tube radio freak, the only thing that springs to mind are the air core varable tuning capacitors on old radios. These tend to be very large, but maybe there is something more compact? Does anyone manufacture a small variable capacitor?

I'm kind of a knobaholic, the more the better. Must have been all those old science fiction movies I watched as a kid!
Glenn

[StephenGiles]

Yes, I have a variable capacitor in my ADA Flanger.
Stephen

[vfr800fiman]

Where would I look to find the manufacturers info?
Is it a small device?
Glenn

[captntasty]

vfr800fiman,
Hello fellow Connecticutite...  an interesting question.  Check Mouser.com for some options.  I don't know if what I see would give much flexibility.  The values are all pretty small (the largest range I found was 12-180pF).  Perhaps in a few situations where small values are called for.  It is, however, possible to do this with a regular trim pot and 2 caps of your choice.  I'm sure there's info on this site as well as out there on other sites.
Peace

[vfr800fiman]

Thanks for the information!
I'm intrigued with the 2 caps & trim pot solution.
I'll have to investigate this further.
Glenn

[captntasty]

hmmm...  upon further reflection, I'm not positive a trim pot (or any other pot) wouldn't have some effect - maybe good, maybe bad.  It CAN be done with various types of switches - no resistance.  Guess it's time to fire up the soldering iron and get to experimenting!

[vfr800fiman]

I assume something like this would work:

..........________
..........|
..........|
......^^^^....trimpot
......|.......|
.....__.....__
.....__.....__..capacitors
......|.......|
....gnd...gnd

Sorry for the periods, but this system won't recognize spaces.
Glenn

[D Wagner]

I assume something like this would work:

..........________
..........|
..........|
......^^^^....trimpot
......|.......|
.....__.....__
.....__.....__..capacitors
......|.......|
....gnd...gnd

Sorry for the periods, but this system won't recognize spaces.
Glenn

Glenn,

That is pretty close.  You will need to connect the small value capacitor to the center and outer lug.  The other lug gets the large value capacitor.  Watch your polarity if you are using a polarized cap.

______________
..........|
..........|
.....^^^^....trimpot
......|..|__|
...+__.....__
.....__.....__..capacitors
......|.......|
....gnd...gnd

I hope that makes sense.  You can look for Joe Gagan's easy face schematic.  (Watch out for the posititive ground version.  You will have to switch the cap polarity for use with a negative ground effect.)  

Forget the trim pot!  This is the type of modificatiion that you would want to put on the outside of the box.  

Derek

[bigjonny]

I would do something where you always have one capacitor in series with your path, and then in parallel (around the aforementioned cap) put a pot/cap combo.  You'd just use the pot as a variable resistor.  The less resistance you dial in, the more the two caps operate in parallel (i.e more capacitance = lower frequency response).

It would look pretty much like Aron's lo-cut mod from his simple mods/tips, except you'd have a cap in parallel with the pot.

[vfr800fiman]

Forget the trim pot!  This is the type of modificatiion that you would want to put on the outside of the box.  

Derek

Definitely on the outside of the box! Like I said, I'm a knoboholic
Things like this look REALLY cool to me!
http://www.obsolete.com/120_years/machines/moog/mmog_modular.jpg

Thanks for the information.
Glenn

[Duff]

how many farads do the radio air-cores handle?, cause the variable caps are too small for what I need.

[puretube]

max ~500pF...