Posted on Feb 7, 2014

This was a quick experimental prototype I put together based on an elegantly simple design by Doug Forbes, which uses common vacuum tubes. (Since Mr. Forbes`s original site is no longer available I have taken the liberty of reproducing his schematic and text above. ) In its simplest form a theremin works by mixing two high-frequency oscillators, on

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e fixed and one tuned by the capacitance of the player`s body, and extracting the audio-frequency difference between the two signals. Typically a second capacitively tuned oscillator is used to control the amplitude of the audio output. I had the requisite 12AU7 tubes and most of the other components already on hand, but had some difficulty finding information about the coils shown in the schematic. The solution is best summarized by this Usenet post from 2001: I`ve been working on an experimental model of this same circuit, and I was unable to find a value for the coil in question. I believe it`s available from Antique Electronic Supply as part no. P-C70-OSC, rather than PC-70-OS as listed in the accompanying text, but didn`t want to blow $7x3 to find out otherwise. So I read through the archive of the Levnet theremin enthusiast`s mailing list and found this message pertaining to the Forbes theremin schematic: ". Take a plastic film can-this will become your coil form. Use number 30 enamelled magnet wire (it winds very easily) and wind, in a single layer with the turns of wire laying neatly side by side, 66 turns of wire. Pull out some wire from the end of the winding (about five or six inches is fine), then with this "center tap" pulled out, continue winding an additional 33 turns of wire. Secure the ends of the coil, and the center taps, with silicon caulk. "The smaller coil (between the center tap and the end of the 33...

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