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The RCA Theremin Theory of Operation

THE PRINCIPLE of operation of the RCA Theremin is that of the beat frequency oscillator, The frequency of one oscillator may be varied by the capacity change caused in an associated circuit by the movement of the hand to or from a pitch control rod, Also an additional oscillator provides radio frequency current for heating the filament of a UX-120 arranged so as to control the volume of output, this control being due to the movement of the hand in relation to the volume control loop. A detailed description of the functioning of the various circuits follows:


As stated in the foregoing, the musical note of the RCA Theremin is produced by two oscillators of slightly different frequency beating together. This beat note is then amplified by two audio stages. The change of note caused by the movement of the hand is due to the change of capacity across what is known as the pitch coil, this slight change having sufficient effect on an adjacent oscillator circuit to change the frequency and thus the beat note, the amount of change depending on the position of the hand in relation to the pitch control rod.

Referring to the schematic circuit and starting from the pitch control rod we find the circuits functioning as follows;-

The pitch control rod is connected to a coil having a very high inductance. Connected to this coil is a small condenser and a small concentrated coil. This entire circuit is tuned by the distributed capacity of its coils and resonates at approximately 172 K.C. Not having any fixed capacitor connected across it for tuning the ratio of inductance to capacitance is very high. Thus the small increase of capacity caused by the hand close to the pitch rod will cause the circuit to change its natural period considerably a great deal more than if a large capacity and small inductance were used.

This pitch control circuit is connected to the grid side of the variable pitch control oscillator, the frequency of which is slightly greater than that of the pitch control circuit. Bringing the hand close to the pitch rod will increase the parallel capacity in that circuit and thus reduce its frequency. As this capacity is reflected in the oscillator circuit a similar decrease in frequency will result in that circuits. The amount of decrease depending. an the closeness of the frequency of the two circuits. Thus a greater decrease in frequency of the oscillator circuit is obtained when the pitch control circuit is close to the oscillator circuit in frequency than when it is a greater frequency difference.

The next circuit to examine is the fixed pitch oscillator. This circuit oscillates at a frequency, when correctly adjusted, at a maximum of 1400 cycles greater than the variable pitch oscillator.  The amount of the difference is dependent on the frequency of the variable pitch oscillator the frequency of which is determined by the position of the hand in relation to the pitch control rods The frequency of the fixed pitch oscillator does not change while playing.

The hand approaches the pitch control road and increases the capacity across the pitch control circuit. This capacity is reflected across the variable pitch-control oscillator and thus reduces its frequency. This causes an audible frequency difference between this oscillator and the fixed pitch oscillator, the frequency of this note depending on the position of the hand. Bringing the hand close to the rod will increase the capacity in the pitch control circuit, reduce the frequency of the variable pitch oscillator and increase the difference between the frequency of this oscillator and the fixed pitch oscillator. Thus an audible note is obtained, the note increasing in frequency as the hand approaches the pitch control rod.

Examining the circuit diagram we find that each oscillator gird is connected to the control and screen grid respectively of a Radiotron UY- 224. As the screen grid has the largest area, a 10,000-ohm resistance is connected in series with it to balance the input to this tube and have each oscillator have the same effect on the detector action. This tube is a detector or combining tube that functions much in the same manner as the first detector in a super-heterodyne circuit. The output of the detector is then amplified by a two-stage audio frequency amplifier, the output of which goes to the loudspeaker.

The remaining two tubes, Radiotron UX-120 and UX-171A together with the first audio frequency amplifier constitute the volume control system. Examining the UX-171A we find that it is in an oscillating circuit that oscillates at about 420 K.C. Connected to the grid side of the oscillator is the volume control loop circuit. This circuit resonates at a frequency below the oscillator frequency when the hand is entirely removed from the volume control loop. This is done for two reasons. If the two circuits were in exact resonance, the load on the oscillator would be too great and operation would be unstable. Also the pickup current would be high and might damage the tube. The ratio of inductance to capacity in this circuit, is also quite high. A small pick-up coil is wound around the inductance coil of the volume control loop circuit and when both circuits are nearly in resonance (hand entirely removed) sufficient radio frequency current flows in this pick-up coil to light the filament of the Radiotron UX-120 to which it is connected.