Keppinger Tube Theremin Build Log

Keppenger Tube Theremin Coil Sets

Available Here!

the PRIAMP is Here!

PRIAMP hand built tube preamplifier with built in dock for the iphone and ipod, by Charles Hobbs

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Besides the obvious "cool" factor, (well designed) valve-based theremins are generally reguarded as having a richer, more
interesting tone than transistor or chip-based theremins, for the same reason tube guitar equipment is favored over solid state.
The tube amplification stage distorts and compresses the sinusoidal wave from the pitch oscillator in a way that is very difficult
to replicate with silicon. The resulting sound is more complex and and pleasing to the ear. The original instruments, made by
RCA in 1929, are pretty much the gold standard in theremin design and tone, but due to low production numbers and the scarcity
of NOS tubes, they are difficult and costly to aquire (or even clone.)
In the mid 1990's Mark Keppinger designed a new instrument- grade tube theremin. It closely resembles the original RCA
circuit but utilizes cheap, commonly available tubes. A few years ago, Mr. Keppinger kindly released his schematics to the public
domain.
Like early theremins by RCA's and those made by Lev Termen himself, the Keppinger design employs two huge, beautiful,
inductor coils to equalize and evenly space out the playing field response.

Here are a few pics of the winding process:

Winding the large pitch coil.

The winder is pretty basic stuff, just pieced together out of scrap wood, threaded rod, and spare parts. I used some old skate bearings to reduce friction. Originally I had it powered by a high torque/ low RPM geared motor, but found that it was totally unnecessary. I have much better control winding it by hand, using a crank, like this one from an old hand drill.
The counter accurately keeps track of the number of coil windings. The tear-shaped cam strikes the the arm of the counter with every rotation. I initially tried a pedometer wired to a reed switch, but i found it really unreliable. It was replaced with this old mechanical counter my dad had found somewhere and given to me when I was a kid. This works perfectly.

Detail view of the counter mechanism.

...and here are the finished coils:

A hand wound set of inductors for the Keppinger tube theremin.

A complete set of inductors.

Close up of one of the pitch antenna oscillator coils.

One of the pitch oscillator coils.

Close up of the large pitch antenna equalization coil.

Detail of the pitch antenna coil. 1200 turns of #32 magnet wire and over one foot tall!

Some images of the coils and the wonderful chassis sets I got on ebay from electrical engineer and theremin enthusiast, Daniel Schreiber. Check it out here.... As of today (9/01/10) there are five left at an amazingly reasonable price.

Keppinger chassis set with oscillator coils.

Coils and cherry insulated standoffs.

Keppinger oscillator chassis with coils mounted. Looks Awesome!

The insulated standoffs were made from 1/4" x 7/8" x 3" strips of cherry hardwood.

Need coils?

To inqure about purchasing hand-wound inductors and antenna sets for your Keppinger, RCA or other theremin project, CLICK HERE
or contact me at: charbot@gmail.com

A few helpful links for for theremin builders and enthusiasts:

Complete Keppinger Theremin Schematics: oscillator circuit (p.1) | amplifier and power supply (p. 2) | parts list

Chassis measurements and layout: oscillator chassis | power / amp chassis

Mark Keppinger made an excellent series of 14 short videos that cover theremin function, construction, history and even how to play.

Theremin World - the best online resource for theremin info.

Red Arrow Hobbies - supplier of phenolic impregnated tube for coil forms

Construction of a Keppinger theremin - Philip Neidlinger did an awesome job building and documenting his Kep. Lots of great images and info .