Eighty-five years ago, the April 1941 issue of QST included this hint (or perhaps it was a kink) sent in to the magazine by C. Ray Wagner, W2FEN, for a “powerless” (other than filament) keying monitor. It uses a 117L7GT tube, half of which is a simple audio oscillator.  The rectifier half of the tube is powered by a pickup from the transmitter, and this RF is rectified to supply the B+ for the oscillator.  The result is that when the circuit is powered up (in other words, when there’s RF coming from the transmitter), the oscillator puts out a side tone.