Vintage tube theremin project

New member here, pleased to have found this terrific resource.

I am interested in building a Theremin using vintage tubes. I have just enough 27's, a 24A, and a 53 to get close to the circuit shown in the schematic of Clara Rockmore's instrument.

I am currently prototyping using less scarce (and 6.3v) tubes: 76's for the oscillator, 36A for the mixer.

A problem that I'm having at the moment: the pitch oscillators lock to each other. This happens when I connect the two oscillators to the mixer - if I leave them disconnected, I can move the frequency of the variable oscillator; but once I connect to the grids of the mixer tube, bing! the oscillators lock.

I would like to look at Mark's measurement data (but it's no longer accessible) and Pierre's schematic (but I see a blank Word doc).

Appreciate any pointers from people who have done something like this.

Thank you very much,

--ian

I have made some good progress. Wound a couple of transfofmers similar to those used in the RCA Theremin. Rewired the oscillators to use these (instead of the Hartley configuration that I'd been using).

I can now pull one of the oscillators over a good range. The two oscillators lock together at zero beat (which is desired behavior).

--ian

Ian,

Sounds like you are making progress. Any updates on your progress? I presume you found my article (http://www.thereminworld.com/article.asp?id=27) on this site with all the RCA measurements?

Mark

Hi Mark,
Sorry for the slow reponse, I am away from home.
I have built the pitch section (as far as the mixer). I would be very interested to know whether you made any inductance measurements on the RCA inductors. I think that the value will be rather smaller than the published value, because of the need to use the 'short solenoid' formula.
I plan to build the gain controlled stage (using the Rockmore circuit) and an output stage. The effect on the pitch of the interestage transformers, etc, is very marked. I will probably be resuming this at the end of August and will write up any interesting finds.
Regards,
--ian

Did you mount one of your coils perpendicular to the other?

ba

Sorry for the long delay, back home now.

The two inductors are perpendicular to each other, and are about 6" apart. I plan to add the variable capacitor that's in the Rockmore circuit to allow the coupling between oscillators to be varied (there is already some coupling between them due to capacitances around the mixer tube).
--ian

I tried installing that capacitor at some point to little effect. I hope you have more luck than I did. What I found to make the biggest difference were the variable resistors between the oscillator tubes and the mixing tube. They made a pretty dramatic difference in tone and intervals between notes. I replaced the variable oscillator pot. with a 20K resistor or so. I just tuned the variable until I liked the sound and took a measurement. The reference pot. I left on, however, it seemed to do the most as far as changing tone and evening out the intervals between notes.

Let us know how that variable capacitor works out. Also, did you put in the two rf chokes between B+ and the oscillator circuits? If so, what did you find? I figure, just a precaution that became standard by the time Theremin was working on Clara’s instrument.

ba