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Theremin for Tube Enthusiasts, page 4

 

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.

L1-TV horizontal osc. Coil

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)

L8-TV horizontal osc. Coil (modified per text)

T1-r.f. transformer (see text)

T2, T3, -standard AM 455 kHz broadcast i.f. transformers with trimmers (see text)

T4-volume control transformer (see text)

 
INSTALLATION

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).

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Schematics

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


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