MOTM

>I have seen only two common applications in my very limited experience.
>1.  Radio reciever tuning units.
>2.  Foot volume controls in older Hammond Spinet organs.
>
>I am sure there are others.

The tuning trimmers in old tube theremins used variable capacitors -- hey,
it's a synth application!!

>Variable capacitors tend to be very "physical," therefore very expensive
>>and a general nightmare to work with. So in an RC type circuit where you
>>can vary either one, the resistor ends up being a MUCH simpler
>proposition.

This is true.

>Variable capacitors (at least all the ones I've seen) tend to be rows of
>semi-circular plates mounted on a spindle, and that hangs over another set
>of fixed plates in such a way that the two dovetail alternately. Turning
>>the spindle moves the variable plates in or out of the others, sort of
>like
>moving nuclear control rods up or down, if you'll buy that analogy.

Yes, it changes the surface area of the capacitor.

>The problem is that to get any real capacitance, you need a giant set of
>plates. I had one of these in an ooooold radio and the thing weighed about
>15 pounds for that one part! It was huge! Yet I think it was a relatively
>small capacitance.

These are called gang capacitors -- one knob tunes several circuits, AM,
FM, etc.  I had one in an old Heathkit tuner.  It was about the size of a
beer can, not that anyone with enough taste to appreciate motm would ever
drink canned beer :)

Otoh, I built a crystal radio when I was a kid -- it had variable capacitor
about the size of postage stamp.

Also, some circuits use varactors -- which in my limted understanding is a
semi-conductor variable capacitor.  I believe that's what used in a Manley
de-esser.  Otoh, acceleration limiters use switched caps.

Has anyone ever built a varactor synth filter??