The 125 Vacuum Tube Pitch Theremin

The circuit uses AC mains voltage and otherhigh voltages that can cause injury or death. The 125 is a basic heterodyne configuration, in which the outputs of two similar radio-frequency Colpitts oscillators are fed to a mixer, producing their difference frequency as an audible tone. A popular vacuum tube, the dual-triode 12AU7 (or its European equal type ECC82), is specified.

Exceptional frequency stability for temperature variation is achieved, thanks to the low temperature coefficient of tube interelectrode capacitance, first-order cancellation between the two similar oscillators, and the use of stable frequency-determining inductors and capacitors. In fact, the temperature stability of the 125 theremin rivals that of many, much more complex, solid-state instruments. While a second resonant circuit is used in some theremin pitch variable oscillators to increase hand-sensing distance, the 125 theremin exclu

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