Theremin for Tube Enthusiasts by Craig Sellen

Theremin for Tube Enthusiasts, page 3

The power supply uses a 6BM8 tube to regulate the voltage to supply the oscillators and the control Amplifier and mixer stages of the unit, there by 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 ¾ 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.

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Schematics