THEREMIN for TUBE ENTHUSIASTS
Circuits designed by CRAIG KENDRICK SELLEN except the last two which was designed by Leon Theremin
                                                               WHAT YOU CAN DO

   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. 

                                                               USING THE THEREMIN

   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. 
   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. 
   The power supply uses a 5Y3 or a 6X4 tube for the rectifier. To regulate the voltage to supply the oscillators and the control amplifier and mixer stages of the unit a 0A2 tube is used as a regulator. This adding to the over-all stability of the instrument. 
   As a musical instrument, the Theremin should be as compact and portable as possible. However, a distance of at least eighteen inches should separate the two antennas. The unit is assembled on a chassis about 17 x 7 x 3 inch. The antennas are mounted on the wooden covering, on top of the chassis. The pitch control antenna is made of whip, and the volume control antenna is a piece of a 6 x 9 inch plate. C1 and C24 are 15pF variable capacitors which are used to ￿tune up￿ the instrument and should be placed on a convenient part of the front panel. The unit must be adequately ventilated to prevent an extreme temperature rise that might otherwise will result in frequency drift from the tubes. A small fan should work nicely. Transformers T2, T3, are both conventional AM broadcast i.f. transformers and may be either of the shielded or unshielded type. If unshielded transformers are used, they should be mounted under the chassis and holes drilled for adjusting the trimmer capacitors. T1 is hand wound as well as T4, which is a volume control type. Placement of parts is not critical, since the oscillators work at low frequencies, and all circuits are of fairly low impedance. The oscillator coils should be carefully constructed however, since these are the ￿heart￿ of the instrument. In constructing the pitch control coil assembly (L1, L2, L3) a ring with an outside diameter of 1-1/2 inches and inside diameter of 13/16 inch is cut from ￿ inch wood. On this form, L3 is wound. A layer of insulation is placed over L3 and L2 is wound over the insulation. L1 is then placed in the hole in the coil form. With L1 in place, the entire assembly is coated with cement or coil dope. 
   The same procedure is done for the volume control coil assembly (L6, L7, L8), but the dimensions are different, L8 is a coil of the same type as L1, except that enough wire is removed from L8 so that its outside diameter is just under 9/16 inch and its resistance is 30 ohms. The wooden ring for this assembly has an outside diameter of 1-14 inches and an inside diameter of 9/16 inch. 
   The fixed oscillator coil (L4, L5) is wound directly on a 3/8 inch slug tuned coil form. L5 is wound and covered with insulation. L4 is then wound over the insulation. 
   T1 is wound on a 7/8 inch wooden form. Both primary and secondary consist of 250 turns of No 30 wire. The two windings are wound next to each other. 
   The two pitch oscillator coils should be placed at least six inches from each other and at right angles. This will prevent interaction between the magnetic fields surrounding the coils. To insure mechanical stability, all leads going to these coils should be solid rather than stranded wire the antenna connections are brought out to connectors on the top of the chassis. 
   To aligned first test to see if the amplifier tubes are working. Touch with a screwdriver grid No 1 of V3, you should hear a hum or buzz in the speaker. This means the amp. Tubes are working. It is now ready to be aligned as follows:
1.	Turn R9 to zero volume. Place a DMM across R1 and set C1 to minimum capacity. Make sure that the pitch control antenna is connected properly. Start with the slug out of L1 as far as possible and advance the slug until the DMM begins to dip. Continue advancing the slug until the DMM reaches its lowest point and begins to climb. At this point L1-C1 is at the same resonant frequency as L3-C4, and the oscillator is most sensitive to hand capacitance. 
2.	Now set C1 at half capacity and turn R9 up half way. Adjust the slug in L4-L5 until a loud beat note is heard in the speaker. Continue adjusting until the two oscillators are zero-beating. 
3.	Set C1 for an audible note. Close S2 and open S1. Tighten the trimmers on T2 until a marked change in tone quality is heard. T2 is now tuned to the second harmonic (octave) of the fundamental and may be shorted out by simply closing S1. 
4.	Close S1 and open S2. Loosen the trimmers on T3 until a change in tone quality is heard. T3 is now tuned to the third harmonic of the fundamental and may be shorted out by closing S2. With both S1 and S2 open the resultant tone will contain both second and third harmonics.
5.	Place V6 in its socket and allow it to warm up. Set C24 at minimum capacity and connect the volume control antenna. Place the DMM across R20 and adjust L8 until the DMM shows the greatest dip. V6 is now oscillating at about 500 kHz. 
6.	Now set C24 at maximum capacity and place the DMM across R12. Adjust T4 trimmers for maximum voltage.
T1-TV horizontal osc. Coil Stanwyck LHO-2 (formally L1)
T2-see text
L2-40 turns No 30 enamel magnet wire wound over L3 (see text)
L3-60 turns No 30 enamel magnet wire on 1-1/2￿ form (see text)
L4-30 turns No 30 enamel magnet wire wound over L5
L5-85 turns No 30 enamel magnet wire wound on 3/8￿ slug tuned coil form
L6-20 turns No 30 enamel magnet wire wound over L7 (see text)
L7-40 turns No 30 enamel magnet wire on 1-1/4￿ form (see text)
T6-TV horizontal osc. Coil Stanwyck LHO-2 (modified per text) (formally L8)
T7-r.f. transformer (P-C70-RF) from (Antique Electronic Supply, 6221 S. Maple Ave. Tempe AZ 85283 USA)phone 480-820-5411)
T3, T4, -standard AM 455 kHz broadcast i.f. transformers with trimmers (see text)
T5-5K to10K to 8 ohm output transformer, 5K for a 6AQ5, 10K for a 6AK6
T8-power transformer primary 120VAC/secondary 500VAC, C.T. @ 90mA, 6.3VAC 3amp.

   A guitar or instrument amplifier is an ideal companion unit for the Theremin; either one allows bass or treble boost, as desired, and fuzz (distortion) or reverberation (if these features are incorporated in the amplifier￿s circuit). Simply provide a suitable cable plug and connect the Theremin￿s output cable to the amplifier￿s input jack. 
   It is not necessary to purchase a special amplifier. The Theremin￿s output signal level is sufficient to drive most power amplifiers to full output without additional preamp stages. The instrument can be used, for example, with a monaural version of a stereo amplifier. 
   If the Theremin is used in conjunction with a power amplifier which does not have a built-in gain (or volume) control, a ￿volume level￿ control should be added to its basic circuit to prevent accidental overdrive. Coils L1 and L4, L5 should be mounted on L-brackets. Positions of these coils can be altered to change harmonic content (see ￿tuning￿ instructions in text). 


   The results obtained depend more on the ability of the operator than on built-in limitations within the unit itself. A good ￿ear￿ for music is a must, of course, but, in addition, a moderate amount of skill is required, particularly in finger or hand dexterity and movement. The latter is learned only through practice. For a start, here are the basic ￿playing￿ techniques. 
   To sound an individual note, first move the ￿pitch￿ control hand to the proper position near the pitch antenna (as determined by practice) to sound the desired pitch. Next, move the ￿volume￿ control hand quickly to the proper position near the volume antenna to sound the note at the desired level, then away after the proper interval to sound an eighth, quarter, half or full note. 
   To sustain a note, hold both hands in position. The note volume may be increased slowly by moving the ￿volume control￿ hand slowly nearer the volume antenna, reduced by moving it slowly away. 
   To ￿slide￿ from one note to another, hold the ￿volume hand￿ fixed in position and move the ￿pitch hand￿ nearer (or away from) the pitch antenna. 
   To produce a vibrato effect, hold the ￿volume hand￿ fixed in position and shake￿or tremble￿the ￿pitch hand￿ at the desired rate. 
   To create a tremolo effect, hold the ￿pitch hand￿ fixed in position and vibrate￿ or tremble￿ the ￿volume control￿ hand. 
   Tremolo and vibrato effects can be produced by simultaneously rapidly moving both hands back and forth. 
   If you￿ve used triangularly shaped control plates as the antennas plates in your instrument you￿ll find that a given hand movement has less effect on operation near the narrow (pointed) end of the triangle than near its broad base. 
   Practice is important!