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Theremin for Tube Enthusiasts

Nearly everyone who has ever watched television or attended a motion picture has heard music and background effects produced by a Theremin, yet relatively few could recognize the instrument, and fewer still have had the chance to own or play one. With its astounding tonal and dynamic ranges, it has been used to produce background music and special effects in scores of science-fiction, fantasy, horror, and mystery shows.
For about the price of an inexpensive guitar, plus a few hours assembly time, you can own and enjoy what is perhaps one of the most versatile of all musical instruments: the unique and amazing Theremin. Named after its Russian-born inventor, Leon Theremin, its frequency range exceeds that of all other instruments, including theater pipe organs, while its dynamic range is limited only by the power capabilities of the Amplifier and speaker system with which it is used. Above all, it is a true electronic instrument, not just an “electronic version” of a familiar string, reed, or percussion instrument. Its tone is unlike that of any conventional instrument.
A musician playing a Theremin seems almost like a magician, for he or she can play a musical selection without actually touching the instrument itself! As he or she moves his or her hands back and forth near two metal plates, he or she seems to “conjure up” individual notes at any desired volume; he or she can “slide” from one musical note to another with ease, can produce tremolo and vibrato effects at will, and can even sound notes which fall outside the standard musical scale. He or she can play tunes or melodies, produce unusual sound effects, or can accompany a singer or another instrument- all by means of simple hand movements.
The Theremin is ideal for amateur as well as professional musicians and can be used for “fun” sound effects as well as for serious music. It makes a wonderful addition to the home recreation room, and can be used equally well by rock ‘n’ roll groups or larger bands. Theatrical groups find it just the thing for producing eerie and spine-tingling background effects to accompany mystery or horror plays, and for the budding scientist or engineer, it is an excellent Science Fair project.
The typical Theremin has two r.f. oscillators, one having a fixed, the other a variable, frequency, with their output signals combined in a mixer/oscillator stage. At “tune-up,” the oscillators are preset to “zero beat” at the same frequency. The frequency of the variable oscillator is controlled by an external tuning capacity-the “antenna”- which is a “whip” or a simple metallic plate.
As the musician’s hand is moved near this antenna, the variable oscillator shifts frequency and a beat note is set up between the two oscillators. The pitch is proportional to the difference in frequency between the two oscillators. This beat note, Amplified, is the Theremin’s output signal. The more advanced Theremin designs- such as the version presented here- use a third oscillator to control output volume, and two antennas. This Theremin also uses a unique tube V3 to control the Amplitude of the output.
It should be noted; that in constructing the Theremin–that the coils L1, L2, and L3 are mounted at least 18” apart from L6, L7, and L8, while L4, L5 and at right angles to both pitch and volume coils. As an electronic instrument using tubes, a Theremin may be constructed at a very nominal cost, especially if you use salvaged parts. And give completely satisfactory performance. In the instrument about to be described, the tone is produced by two RF oscillators beating at an audible frequency. The addition of hand capacity to the pitch control antenna lowers the frequency of one of the RF oscillators, and the pitch of the beat frequency is correspondingly changed. The outputs of the two radio frequency oscillators V1, and V2 in the schematic are mixed, and the RF components of the resultant signal removed by means of a diode detector.

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