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Theremin for Tube Enthusiasts, page 2

 
In the volume control circuit, the addition of hand capacity to the volume control antenna causes a change in the frequency of a third oscillator, V6. The output of this oscillator is fed through a series of tuned circuits, T4, and then rectified. The rectified voltage is used as grid bias to control the gain of a variable mu Amplifier tube. A change in the oscillator frequency will vary the grid bias and therefore, the output of the Amplifier. The output of this Amplifier is fed directly into the power Amplifier and from there into a loudspeaker.
The variable pitch oscillator uses the triode section of V1 and its frequency is determined by two tuned circuits, L1-C1, and L3-C4. L1 is a variable coil of high inductance, and has only its distributed capacity and a small variable capacitor C1 across it. Therefore, the resonant frequency of this tuned circuit will be appreciably lowered by a slight addition of hand capacity. On the other hand, L3 has much less inductance and is tuned to resonance by a large fixed capacitor C4. The two coils are coupled so that the tube oscillates at the average of their resonant frequencies. This pitch control oscillator is extremely sensitive to changes in hand capacity, but because of the low impedance of L3-C4, changes in input capacitance of the tube have no effect on it. Naturally, this stability is an extremely desirable feature in an electronic musical instrument.
The pentode part of V1 acts as a buffer Amplifier to further isolate the oscillator from extraneous disturbances. The output frequency is about two hundred kHz.
Except for the absence of an antenna coil, the operation of the fixed oscillator V2 is identical with that of the variable oscillator. The outputs of the two oscillators are fed through isolating capacitors into a buffer/mixer V4a stage. This is passed on to the three RF transformers, which pass or reject the harmonics produced by the oscillators. These high frequency harmonics produce corresponding overtones in the beat note and by controlling these harmonics; distinctive qualities of tone ay are produced by the instrument.
The volume control oscillator is identical to the pitch control oscillator, except for the tuned circuit values. The output is passed through transformer T4, and then rectified by the tube diode V4b. When the transformers are tuned to the same frequency as the oscillator, a maximum voltage difference of about eighty volts exists across R12. The oscillator frequency is then raised to a point where the voltage across R12 is about one half maximum, or forty volts. This provides operation on that portion of the sensitivity curve those results in the greatest change in voltage across R12 for a given slight variation in the oscillator frequency. To take advantage of this maximum sensitivity, the positive end of R12 is placed at a point on the voltage divider R10-R11 which is about forty volts above ground. Therefore, the negative end of R12 is at zero volts in relation to ground. With a slight addition of hand capacity to the volume control antenna, the junction of R16 and R12 will become negative in relation to ground. This negative voltage is used as grid bias to control the gain of the pentode section of V3. The Amplifier will be completely cut off when the grid bias exceeds minus ten volts. R16 and C14 make up a time constant which eliminates clicks and pops resulting from sudden hand movements. The diode connected across C14 prevents the grid bias from accidentally going positive. The sensitivity of this volume control is such that the Amplifier is completely cut off when the hand is a few inches from the antenna, and full volume is obtained with the hand about eighteen inches from the antenna. R9 is used to set the maximum volume.

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Schematics

 
Designed by:
     CRAIG KENDRICK SELLEN
     HIGHLAND MANOR
     164 SOUTH MAIN ST.
     CARBONDALE PA, 18407-2655
 


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