Variable capacitor question

[idlechatterbox]

Ok, I have a strong feeling that this is one of those questions that will mark me as the novice that I am. But here goes...

If I remember right from the little formal training that I've had in electronics, most radio tuners use a variable capacitor with the plates etc that mesh. If turning the knob changes the capacitance, and if the tone that leaves the guitar is partly a function of the capacitor on the Tone pot, why couldn't you use a variable capacitor there instead, to give the sound one final tweak? I ask partly because, as I surf the discussions here for advice, I see that very often a simple mod that is recommended is to change out a capacitor. Why not just put in a variable one and get a larger range of new sounds? Or am I missing something really simple? 

[The Tone God]

Welcome to the Forum!

The function is your friend.

Search criteria: variable capacitor

http://www.diystompboxes.com/smfforum/index.php?topic=36402.0

http://www.diystompboxes.com/smfforum/index.php?topic=43037.0

And there is more where that came from.

To sum up the major reason for their lack of use is that they come in small values generally too low to be useful in many applications around here.

Andrew

[R.G.]

It's a good idea. It is a novice idea, but then it's also the mark of a curious, thinking novice, which is a good thing to be.

The only problem is that variable capacitors are physically quite large compared to fixed capacitors of the same value, and the adjuster mechanism makes it difficult to mount them on circuit boards easily. They are also quite expensive compared to fixed capacitors of the same capacitance, and the available ones are quite limited in the range of capacitances.

So it's difficult and expensive to do with the available parts.

Good question. Keep thinking and asking questions of yourself and others.

[tiges_ tendres]

I remember seeing a drawing someone had done where they used 2 capacitors and a potentiometer to dial in treble/bass. maybe this might help?

I couldnt turn up a link.  I think it was either jack orman or joe davisson who drew it up.

[sta63bmx]

You mean the "variable capacitor" like you find in a wah pedal?  Somehow the potentiometer makes the second cap look like a variable cap to the first.  I think that was discussed in R.G.'s technology of wah pedals article.  I have a few boards etching, so I'll look...

http://www.geofex.com/Article_Folders/wahpedl/wahped.htm

"It took me a while, but the trick is - the wah pot, the second transistor and the fixed capacitor implement an electronically variable capacitor. The inductor is and remains fixed, and the capacitor is electronically varied and so the circuit has a variable tuning LC filter to cause the effect."

I think it's the part with the two 0.22uF caps and the pot between them.  So it makes useful use of the Miller effect rather than the circuit being limited by the Miller effect?  I have to watch my boards.  I gotta go.

[idlechatterbox]

Thanks everyone, and apologies for not doing a Search first. Funny as this might sound, the only reason I didn't was because I said to myself, "yeah, right, like someone's asked THAT before...."   Just goes to show you! Thanks again 

[Peter Snowberg]

Welcome to the forum. 

Great question.

There is one place where a radio-style variable cap can be very useful which is in the tone stack of a tube amp. By adding an additional larger-value fixed cap to isolate the tone-stack from the high voltage of the previous tube stage, you can use a variable cap to tune the frequency response of the treble control. Depending on the stack design, the treble cap is usually somewhere between 50pF and 500pF. I've bought several multi-section variable caps off eBay that were scrapped from old Soviet radios that vary between about 30pF and 1000pF. They're bulky, but they give you fine adjustability you just can't easily get otherwise.

[birt]

this wah: http://smg.photobucket.com/albums/v324/caseyseffectpage/Wahs/?action=view&current=svetlanafuzzwah.jpg uses three paralleled variable camps so it becomes a bigger value.