The tuning method is similar to the previous R.C.A. model. Frequency is slightly higher and capacity of the resonating coil is smaller. That increases the possibility of making the playable range bigger (near five octaves). To obtain correct tuning, the antenna coil must be tuned on a slightly lower frequency than the variable oscillator. When the hand reaches the greatest distance from the antenna, the system comes close to resonance. If on these conditions the circuit comes too close to resonance, breaking out in high frequency oscillations can occur. To obtain maximum musical range, this resonance must be close as possible to critical, but still far enough to prevent sudden periodic breakout of high frequencies. The correct tuning can be obtained by means of the variable trimming condenser in the antenna oscillatory system. Increase of that condenser increases antenna sensitivity (making the resonance lower)

CONSTANT OSCILLATOR TUNING

The constant oscillator must be tuned in such a way that when the position of the pitch tuning condenser on the front panel is approximately in the middle of the dial, both oscillators (constant and variable) will be in resonance (zero beat). This can be obtained by means of a trimming condenser in the constant oscillator circuit. To obtain the best possible stability of both antenna and constant frequency oscillators, grid resistances have a certain critical value corresponding to a maximum negative potential on the grids. Further increasing these resistances will create an interruption in high frequencies related to acoustical pitch.

INTER-COUPLING BETWEEN OSCILLATING CIRCUITS (ANTENNA AND CONSTANT)

A double-grid tube detects and mixes the two high frequencies for further amplification and volume control. ...(sic) which creates coupling of both oscillators. Correct value of this coupling is very important to obtain a desirable tone-color and correct distribution of the musical intervals in the space around the antenna. By means of the phenomenon of pulling into synchronization of both high frequency oscillators (antenna and constant) whenever they get near the resonant point, certain distortions of the sinusoidal wave form occur, as evidenced when differential (beat) frequencies appear. These distortions create desirable acoustical harmonics. Besides this effect, when the antenna oscillator, by playing low acoustical frequencies, approaches the resonant point with the antenna coil system, the tendency of synchronism decreases, which tends to increase the distance between the intervals in the space around the antenna.

Therefore, the correct value of resistance in grids of mixer tube controls tone color, and distribution of intervals in space and the optimum value to obtain enough output from mixer tube.

TUNING OF VOLUME CONTROL SYSTEM

The left antenna of the instrument is attached to a high inductance/low capacity coil to the oscillating circuit. Resonating circuitry is placed in the grid circuit of the detector tube. Near the resonance point the current in its plate circuit increases. This increases drop of the potential in the cathode resistance. The cathode potential is used to supply plate or screen current for the first amplifier tube. To obtain more sharpness of tuning, a grid of the detector tube (24A) is supplied with the negative potential from dry cell battery. The other tap of the same battery serves the purpose of supplying the plate (or screen) of the amplifier tube with a negative potential when no drop of voltage occurs in the detector tube cathode resistance. To obtain correct tuning, the resonating circuit must be adjusted by means of the trimming condenser on the front panel of the instrument, which is connected in series with the other trimming condenser for spreading its range. Auto transformer coupling of the volume antenna is tuned with the oscillator circuit, similar to the pitch system

NEON LIGHT

To indicate correct pitch of the instrument before any sound is produced, a little neon lamp is used. Two stages of amplification are attached to the plate of the mixer tube. The second tube has in its grid circuit a choke and condenser in parallel, resonating on the frequency corresponding to the musical pitch "A". Exact tuning can be obtained by adjusting the value of this condenser to proper resonance with the frequency of "A"

TONE COLOR CONTROL

On the front of the instrument is located a ten position switch which changes the value of capacity in the plate circuits of the output tubes. The first (left) position corresponds to zero additional capacity, which produces sound with the maximum of high frequency harmonics. Rotation of the switch to the right connects larger capacitances, and in this way limits the high frequency harmonics. If it is necessary to make other changes in the tone color of the instrument, it is possible to use some of the last positions, which are at present left free, so that some other combinations of capacitance, or capacitance with resistance may be used

VOLTAGE REGULATION

Automatic voltage regulation is important to eliminate changing of the pitch or volume of the sound due to unstable line voltage, or to compensate for the changing load on the power unit with audio volume alteration.

The stability of high frequency circuits independent of voltage drop is very good, but still, in the low range of the instrument, pulling into synchronism is considerably affected by slight voltage variation. This causes changes in pitch related to volume. To eliminate this effect, a series pair of voltage regulating tubes (874) are used, or an additional loading tube (43) is applied, in which its grid is connected to the fluctuating line through a condenser and plate, producing a load on the power supply through a resistance leading to the high frequency circuits. Change of this condenser in the grid of the tube compensates for fluctuating rail voltages

RESIDUAL TONE - THE HUM

Shown in the diagram of the latest instrument of Sept. 1938, a shunt condenser is installed across the 27 to shut off (or allow) leakage from plate at the will of the player. The object is to have sufficient leakage tone during concert hall work so that the artist can hear the pitch before the note is played, but the audience cannot.

 

QUESTIONS answered by Professor Theremin regarding care of his instruments:

• What tube, or other things, control the neon pilot light?
  
  On the AC Theremin instrument of January 1938, two tubes, type 57 on the diagram; on the September 1938 instrument, two tubes- one 57 and one 58.
  
  The neon light should be tuned to accuracy by the maker of the instrument (see Professor's own notes on tuning) and the modern instrument should only require changing the bulb when it burns out.
  
  On the Jan. '38 Theremin, the .05 variable condenser parallel to the choke regulates the resonant pitch determining when the lamp lights: for higher resonant pitch, use smaller capacity.
  
  The .001 condenser in the input, and the .002 in the grid circuit control the sharpness of the neon light tuning. Increase capacity for more brilliance, but less sharpness. Decrease the capacity for sharper tuning (i.e. accuracy), but you give up some brilliancy for it.
  
  Hardness of the ageing tube will in time cause diminished brilliance, but increased sharpness, till the light is entirely lost and useless for a guide. You can change the tube before this; but if both light and sharpness are defective with a new tube, the trouble is proved not with the tube, but with the circuit.

• How are the tone colors made?
  
  By a group of condensers beside the battery, which add overtones according to their arrangement. A knob in front of the player is connected with each one of these, and can be turned to call forth any one of them. Or in technical language, the switch is connected with different capacities paralleled to plates in the output transformer, through cables nos. 10 and 11.
  
  Note: Number 1 position on switch has no capacity in parallel, and No. 7 has .1mF.
  
  In the cabinet of the September '38 instrument, the capacities are in order of magnitude from front to back.
  
  The 1,000 Ohm and 15,000 Ohm resistances in the grid of the mixer tube 24A control the harmonics on the original, or "natural" sound of the instrument. Therefore, the tube affecting tone color is the 24A. To get a more brassy sound increase the number of high frequency harmonics in the output circuitry removing shunting resistance and capacity out, or by diminishing them.
  
  Other changes in tone-color are affected by the player's right hand vibrato, or the left hand volume control: or again, by the condition of the tubes. If the tubes or condensers deteriorate, the weak ones must be found and changed.
  
  Removing the 100,000 Ohms resistance in the 1938 instrument will give a trumpet tone.