I built Jake Rothman's "EPE" Theremin ... this circuit is the simplest discrete transistor Theremin circuit I've found on the net ... following the replacement of the output amp and speaker by a typical emitter-follower circuit the circuit can be interface with a guitar amp or echoplex tape echo ...
... installed in a plastic box with a wire instead of an antenna (wire initially cut and tested at different lenghts, trim cap adjustment hole on opposite side as antenna wire) ... with a wire of length of about ten inchs I get a minimum frequency happening when the hands is about a foot away, sometimes more ... a fine tune (1-5pF) on the cap is a good idea aside from a coarse 2-22pF tuner ... I ended up using MPS3565 (ECG108) npn devices with ft=800Mhz except for a 2n5210 as the output buffer ... the circuit is fairly stable and draws a few mAmps, tone is good alright ...