I started building theremins in 1997 on the basis of my amateur experience in the field of electronics.
My first theremin was made using transistors and only had a pitch antenna: the design involved the use of a transistorized oscillator connected to the antenna and an integrated circuit (NE602) used in the radio apparatuses, which served as a second oscillator and mixer. The result was a high stability but decidedly poor sound.
I subsequently decided to begin using electronic valves by building the Tube Theremin proposed on the Net by Doug Forbes. This instrument uses easily found 12AU7 triodes, and the mixer stage is made of a triode that acts as a preamplifier; once again, the sound is not the best and I also had some difficulty in obtaining an adequately incisive volume control. Nevertheless, it is a good starting point for anybody who wants to become familiar with building tube theremins.
As I was not completely satisfied with the results I had achieved, I decided to buy a commercial theremin to use as an initial reference. I chose Big Briar’s Etherwave mainly for economic reasons, but I consider it a good instrument, well made and fun to play. It also has the enormous advantage of being easily transportable.
My current experiments are following two parallel routes: the final assembly of a tube theremin made using the same design logic as that built by Leon Theremin for Clara Rockmore, and the design of an easy-to-build transistor circuit for novices that possess at least some of of the characteristics mentioned above.
The idea of building a new tube theremin came to me after I had heard Clara Rockmore’s disc The Art of the Theremin. The instrument used by this virtuoso performer has such an excellent sound that, when you first hear it, you have the impression of being in the presence of an opera singer or, depending on the piece, a ‘cello.
On searching the Internet, I found the circuit diagram of Ms. Rockmore’s theremin drawn by Robert Moog at the time of his electronic restoration of the instrument some twenty years ago.
The circuit was very similar to that of the RCA theremins, but there were some differences that I felt must be the key to its incredible sound.
After spending months studying the diagram, the breakthrough came as a result of correspondence with Reid Welch, who has built a tube theremin that has an exceptional sound. Reid made me understand that the secret of producing an excellent sound was the use of a tetrode for the mixer circuit. Like the RCA theremins, the original circuit made use of a 24A tetrode, a 1020’s valve that has been out of production for more than half a century and so, for my own experiments, I used a 36 tetrode (the direct successor of the 24A) which, although also being out of production is much easier to find and less expensive.
If we look at a detail of the diagram drawn by Moog, we can see that the signals coming from the oscillators are connected to the mixer (a 24A tetrode) by means of variable resistors, thus allowing them to be mixed in such a way as to obtain a satisfactory audio beat. Another interesting aspect of the circuit is the variable capacitor known as the coupling trim: this forces the connection between the oscillators with the aim of controlling the amount of beat distortion. Finally, we can see that the anode of the tetrode has an interstage transformer, the characteristics of which influence the beat.
Let us now look at the original diagram of an RCA theremin. It also used a 24A tetrode (the RCA equivalent was called UX224), but there is no control of the intensity of the signals coming from the oscillators nor the variable capacitor for forcing the binding; furthermore, the interstage transformer has a different transformation ratio. It is these particulars that lead to the substantial difference between the sound of the RCA theremins and the better sound of Clara Rockmore’s theremin.