Clara Rockmore's Theremin
A uniquely-issued theremin designed and built personally by Leon Theremin for Clara Rockmore, a noted thereminist of the 20th century, is shown in the schematic. Theremin greatly respected Rockmore's skills and opinions, and her suggestions for improvements to the instrument led to refinements. In 1989, after several decades of use, and during a period of time when Theremin himself was sequestered in the Soviet Union, the instrument required repairs that were performed by Robert Moog. I used Moog's hand-drawn schematic as my reference.
The instrument was constructed with two major assemblies, the power supply chassis (V5, 6, 7), and the pitch and volume chassis (V1, 2, 3, 4, 8 and 9). To enhance clarity, I eliminated the two inter-chassis connectors in the Moog drawings. I also arranged the three oscillator transformers' windings to show their relationships.
Moog's drawings provided reference designations and values for some of the
components. I re-designated each component (e.g., B1, C1, etc.) for the purpose
of discussion, and provided the component values from the Moog schematic
in tabular form.
Transformer T4 converts the AC mains voltage to high voltage for dual rectifier tube V5. A 5V winding provides filament current for V5, and a 2.5V winding for the heaters of audio power amplifier pentode tubes V6 and V7. DC high-voltage filtering is provided by capacitors C15, 16, 17 and inductors L2, 3, and 4. They are followed by two 90V in-series voltage regulator tubes, VR1 and VR2. R5 serves as a current limiting ballast for the regulators, and series-connected R6 and R7 bypass VR1 to supplement current for VR2. Capacitors C18 and C19 are post-regulation filters. T5 provides a total of 10.75A of heater current for the pitch-section tubes (V1, 2, 3, 4) and volume section tubes (V8, V9). Capacitors C13 and 14 suppress noise across the line input. There is no indication of a safety fuse in the source document.
Pentode tubes V6 and V7 comprise a push-pull (class "B") power amplifier with input transformer T6 and output transformer T7. Paralleled resistors R9 and R10 provide DC cathode bias for the tubes, with C21 providing AC bypass. +340 V is provided to the tube plates via T7's center-tapped primary.
Triode tube V2 and associated components form a resonant Armstrong oscillator that provides a pitch reference frequency. Air-core transformer T1 provides plate-to-grid feedback coupling, and capacitors C3, 4 and 5 provide frequency adjustment. Variable capacitor C3, presumably located on the operating panel, allows the player to adjust zero-beat when the pitch hand at the furthest distance. Variable trimmer capacitor C4 adjusts C3's operating limits. Resistor R2 provides DC grid bias for V2, and capacitor C2 provides AC bypass.
Triode tube V4 and associated components form a second Armstrong oscillator that provides a variable frequency dependent on pitch hand position. Air-core transformer T3 provides plate-to-grid feedback coupling. Capacitors C10 and 11 and variable trimmer capacitor C9 provide frequency adjustment. Resistor R3 provides DC grid bias for V4, and capacitor C8 provides AC bypass. Coil L1 has a large inductance, permitting a reactance that is easily affected by small hand-capacitance variations, effecting a greater change on oscillator frequency that would otherwise be obtained if the pitch antenna were directly connected to V4's plate. In addition, the composite response of the two tuned circuits (T3-C9, 10, 11 and L1-Chand) somewhat improve the linearity of hand distance and pitch. Blocking capacitor C12, not a significant part of the tuned circuit, ensures there is no high-voltage DC potential present on the antenna.
Tetrode tube V3 provides the heterodyning function that mixes the waveforms from the two pitch oscillators. Rheostats RV1, in series with the control grid, and RV3, in series with the screen grid, are surmised to provide a degree of tone control by both altering the degree of mutual coupling (through V3) of the two pitch oscillators, and by changing the DC operating point of V3, thereby affecting the heterodyne waveshape. +180 V is provided to V3's plate via T2's primary.
Variable trimmer capacitor C6 provides a small amount of coupling between the two oscillators, which affects their waveform outputs and thus the tone characteristics.
Radio-frequency chokes RFC-1 and 2, capacitors C1 and C7, and resistors R1 and R4 provide power supply decoupling for the oscillator sections.
T2's secondary drives triode V1's grid via rheostat RV2. RV2 is presumably another variable to affect tone. V1 provides amplification of the tone and drives the power amplifier input via T6's primary. The amplitude of V1's plate waveform is controlled by the amplitude of a DC voltage provided by the V8, 9 volume control section.
Triode tube V9 and associated components form a third Armstrong oscillator that provides a variable frequency dependent on volume hand position. Air-core transformer T8 provides plate-to-grid feedback coupling between its center and right winding. Capacitor C28 variable trimmer capacitor C29 provide frequency adjustment. Resistor R12 provides DC grid bias for V9, and capacitor C27 provides AC bypass. Coil L5 has a large inductance, permitting a reactance that is easily affected by small hand-capacitance variations, effecting a greater change on oscillator frequency that would otherwise be obtained if the volume antenna were directly connected to V9's plate. Blocking capacitor C31, not a significant part of the tuned circuit, ensures there is no high-voltage DC potential present on the antenna.
T8's third (left) winding couples the oscillator's output into a set of tuning capacitors, C22 through 26. Variable capacitor C25, presumably located on the operating panel, allows the player to adjust the volume for the softest level at the closest distance. The winding inductance and net capacitance of C22 through 26 form a resonant circuit that provides a frequency-dependent waveform amplitude corresponding to volume hand position. This waveform is applied between the cathodes and grids of dual triode tube V8, which modulates its conductance. (Both sections of V8 are paralleled, so it essentially acts as a single triode.) As V8's conductance varies, so does its cathode voltage, which feeds the plate supply of V1.
Batteries B1 and B2 are inserted in V8's cathode-to-grid circuit to offset the operating range of the tube, presumably allowing its complete cut-off when the volume hand is closest to the antenna. C20 presumably bypasses high-frequency AC voltage imposed on V8's cathode to ground. R8's function is not obvious, although it may serve to damp the inductance of T6's primary. A tap between the two batteries is presumed to further ensure volume cut-off, perhaps by offsetting the operating range of V1. Beyond these general assumptions, however, the exact function of the batteries is difficult to define without making direct measurements.
Text and drawing ©2013 by Arthur Harrison
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