From an earlier thread about the frequency response of variacs, I realized that a variac could be used as a variable inductor.
As such, it could be used to vary the frequency of an oscillator.
I kluged this together with stuff I've got around the shop just to see what tuning range I could get. (View with a fixed-pitch font)
. +--------------------------------+ . | POWER AMP | . | +---------+ | . +-|IN OUT|---+-10k-+ | . | GND | | | | . +----+----+ 100R 100n | . | | | | . +--------+ +-----+ | . | | | | . +->VARIAC [10k]