How to get more capacitance from old radio variable capacitor

[mac]

I have an old radio tuning capacitor like this one,



and I want to use it in a single transistor middle booster as the input capacitor, instead of a spXt switch,



I hear you say, why bothering if a sp6t switch can get you there, and is not that big?
Curiosity? 

To get the desired quasi-gaussian bell curves with peaks between 300hz and 1000kz I need a variable capacitor going from 3.3nf and 15nf.
The problem is this cap is up to 470pf or so.

Is there any gyrator or other circuit to make it look like a bigger cap?
If any, it has to preserve the delicate interaction between the guitar mic coil, the input cap and the transistor input impedance that makes the middle bandpass filter.

mac

[brett]

Hi
You can increase apparent capacitance with a simple transistor circuit (look up Miller capacitance).
But if it's a tuned RC for a particular frequency, what's stopping you changing R?
Your C is abour 30x too small, so make R 30x larger?
A jfet input stage lets you go to several meg of input impedance if necessary.
You might want to allow for cable capacitance - an average quality cable has > 100pF and sometimes as high as 500pF.
cheers

[amptramp]

You could submerge the plates in non-conductive oil to increase the dielectric constant.

[J0K3RX]

I know this isn't using your radio variable cap but it's a useful little circuit! fake variable capacitors...

[mac]

You could submerge the plates in non-conductive oil to increase the dielectric constant.

Yes, I tried vaseline gel but still too low.

I know this isn't using your radio variable cap but it's a useful little circuit! fake variable capacitors...

Also tried this but the resistance of the pot flattens the desired bell curve.

And capacitors multipliers, like those used in power supplies have a gnd connection.

mac

[brett]

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

[Lurco]

Isn`t a Wah circuit effectively a variable capacitance-multiplier with an added inductor?

[Mac Walker]

You might try glycerin or isopropyl alcohol, both have high dielectric constants relative to air:

http://www.engineeringtoolbox.com/liquid-dielectric-constants-d_1263.html

I don't know much about how old variable air caps are constructed (I think they would be considered a parallel plate capacitor), no guarantee this would work but worth a shot.....

[mac]

You might try glycerin or isopropyl alcohol, both have high dielectric constants relative to air:

The problem I had with vaseline is that it was difficult to keep it between the plates as I rotated them...
Que idiota! it has to be INTO a pot full of vaseline. Glycerin looks nice, alcohol more problematic due to evaporation.
I thought of water at first because it has a bif constant, but corrosion is a problem. Anyway I´ll give it a try.
470pf *80x = 37,6nf which is enough for this circuit.

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

The first picture looks like a floating multiplier, I'll try it. Thanks Brett.

Isn`t a Wah circuit effectively a variable capacitance-multiplier with an added inductor?

PS capacitor multiplier looks like the wah buffer.

mac