Building the E. J. Schultz tube theremin

You know the one. 1949 radio & television news, here: http://www.theremin.us/Schultz/Schultztheremin.html

I'll start by saying this will be my 2nd theremin attempt. The first one was over 10 years ago, and was a 4069 design that i probably got very wrong. It didn't work. 

Won't be the first circuit I've built from scratch by a long shot, nor the first tube circuit or first high voltage circuit. I know my way around a soldering iron but I'm no EE. 

I first saw this design around that time, and picked up some 6c5-gt tubes but didn't get much further than that. Already had the 6sa7 in the junk bins. 

I was inspired a few weeks ago to try and actually build it. I picked up a 6sn7-gt and some other parts. 

I recall about 10 years ago i couldn't find any mention of anyone having built one, successfully or otherwise. I recall that there was a site on the theremin webring that existed back in the day where someone had got as far as winding the inductors on I think PVC pipe, which at .810 inch OD might be too far from 3/4". I know there are two threads here, that went nowhere, with people asking if anyone had built it. I came across a forum post somewhere - I forget where - where someone said they'd built one and couldn't get it to work. 

After I spent money on parts, and a chassis that I'm now pretty sure isn't going to be the best way to do this, I finally found a Bob Moog interview from an old magazine. It looks like, at the moment, he's the only person I'm sure has built and played one. And he said it didn't work very well. In fact, it could be what started his whole career, since he built it when he was 15, kept trying to improve on it, and by the time he was 19 he was selling theremins that he and his father were building in their garage. 

So i already feel like it's a mistake. Especially since i discovered Art Harrison's model 126 the other day, and reviews and audio recordings. half a dozen 12au7 types? Got 'em. Got lots of 'em, even. I could probably work up an instrument amp using another three of them for a nice square 9 little identical tubes. But I'm already committed. It'll have to be my next attempt. 

I've got a power transformer from an old rca console radio that should do the job according to PSUD2. Got sockets and tubes, almost all of the caps. I went for air variable caps for the variable cap positions, even though it looks in the picture like he used a PC mount part for at least one of them. have a 100pf screwdriver adjust, a pair of 30pf screwdriver adjust that share a ceramic body, and I'm waiting for a 1/4" shaft 2-17pf for the 15pf position - zero beat adjustment - since it looks from the picture like that one should have a knob. 

I'll probably end up buying some 3/4" hardwood dowel for the coils, bakelite rods being somewhat expensive these days. I will probably visit my mother this weekend and see if she has some empty thread spools that are close enough to 3/4". Might visit a craft store too.  

I asked an EE friend about the "double silk & enamel" wire specification. He said "At those frequencies anything from apple sauce to sawdust would work" so i am waiting on some 28ga wire with probably PVC insulation. 

I do have a few questions. 

I am concerned about Mr. Moog's "didn't work very well" attitude about it. Is there a considerable risk that this design may be nonlinear or in some other way just a pain in the butt to use as an instrument? 

About the coils - The 3/4" diameter 5/8" length seems odd to me. He must mean that the "jumble wound" coil occupies 5/8" of the length of the rod? It does look a bit from the photo of the guts of the prototype that it's just a mass of wire and wax on a dowel. Rather than what we'd recognize as a coil former. 

I've seen someone say that tube theremins are often unstable and need a long warmup time? but, I don't see people complaining about their RCA clones, or Art's model 126. I can build a regulated power supply if that is really recommended. 

Anyway, thanks for any input and support. I'll try and keep you all informed as the project moves forward. 

Hello Tim,

That is one heck of a writing! You are a rare individual, since facebook few have interest in a deep discussion of analog theremin design. I've never been to facebook so not sure. Not many analog design types left, maybe two or three.

We are almost neighbors on the world scale, off Interstate 15. I spent five days in a canoe down the San Juan River south of you when I was younger. It was my reenactment of Deliverance.

* I always say do not build a theremin that has no sound sample, so what are you after?

The challenge of finding old parts, using tubes, the look, the classic sound, soldering experience, madness, etc.

In my own tube design I used 100 volt zener diodes to control the voltage on my RF oscillators. Without them you could hear the pitch rise and fall slightly as the AC wall line level fluctuated. I have since (15 years later) abandon tubes for the practical and the classic Clara sound I like.

Edit: I also found great stability in tube design, no thermal drift issues.

Christopher

Hello Tim,

Nice to hear that just another one is trying to built a tube theremin. Adrian Bontenbal and myself managed to built a Clara Rockmore theremin clone and we both like the sound of this theremin very much. Tube theremins are not unstable if you use a well regulated PSU, stable wound coils and variable air condensers.Also try to use silvered mica or styroflex condensors for frequency depended parts. When I was building my tube theremin I first used ECC 82 (12AU7) tubes instead of 27 tubes that Lev Thermen used. With the anodes and the cathodes tied together and not more then 90 Volts on the anodes everything worked fine. For the 24 modulator tube I used a 6V6 tube which also gave good results. After I was certain that everything was OK I changed the tubes for the originals and after some tweaking I am pleased with the result. So I hope that your efforts also will succeed and that we have yet another good sounding tube theremin. If you have more questions feel free to ask.In the Photoalbums under media you can see some pictures of my theremin. (Clara Rockmore replica, page 2)

Henk

Thanks for the encouragement and info. I guess i will look into a regulated supply. 

I have some high voltage switching supplies that i might try, or i may build a zener referenced mosfet regulator of the style that pete millett and others have been using for years. 

So far the small capacitors are a mix of silver-mica and Z5U ceramics, which i've heard are supposed to be OK. I guess if i run into problems I'll have to resolve them. 

So yeah, my interest in building a tube theremin is largely for tube distortion and visual aesthetics. Why this old design? apparent simplicity perhaps. 

Initially i thought it would be a good idea to use an all-aluminum fully enclosed chassis, but I'm getting the idea that it might be more stray capacitance than it's worth. So now I'm thinking it should just be an aluminum plate on top of a wood box. 

I've never used bakelite for a coil form, but historically that was one of the early uses.  Wood is slightly capacitive but works fine.  PVC works really well.  I don't think I'd jumble wind anything as this increases self-capacitance - though I suppose if you're slapping 250pF across it nothing really matters!  250pF seems like a lot of capacitive padding to me (like too much) but I haven't calculated the invariant. Too much padding could easily make it a dog.

Not a tube designer, but the combined oscillator / mixer seems a bit odd.

The coils mounted inside of a metal chassis with ceramic feedthroughs seems like kind of a mistake, the physical design here isn't well thought out.

And, yeah, regulated oscillator supply is rule #1 for stability, returns via other means are generally of the diminishing sort.

I don't know a lot about coil winding or inductor design but it did seem odd to me that mr. schultz took sort of a DGAF attitude to the coil construction. If it were 1954 i would absolutely be building the design Moog published in the same magazine that used common TV parts. Common at the time anyway. 

I do have access to a 3D printer, so if i wanted to go all out i could design and print my own ABS (or PLA) coil formers Maybe with a standoff for mounting further away from the metal. Would be a learning experience, since I've never used solidworks before. 

With the coils being tapped and all I'd think he'd give a bit more info as to the best way to wind them.  Kind of hard to tell if significant coupling is going on, if coupling is necessary for oscillation, etc.  This is why I prefer non-tapped single layer coils, it's pretty much impossible to screw them up and the Q can be quite high.

Tim,

You have begun a theremin journey of discovery, that is plowing through the 10,000 reasons it will not work before you have great success. Today we have many more tools to work with and aid us than Lev Sergeyevich.

A tube oscillator can add a character to the theremin vocal sound (magic). Not every vacuum tube in an identical circuit will demonstrate it. My best description is a fatter sound is developed that seems to me to be a slower roll-over of the sine wave. Thierry might be the only one on the planet today that could give this effect a name.

Modern day components are much better to use than searching for too much for old stuff.  I never found anything special about the RF coil design, after winding for years on pvc now use the tiny IF cans in my tube oscillators. As long as the oscillator oscillates that is what is most important to me. Silver mica caps are nice but not necessary, use NPO caps.

I could leave my tube hybrid theremin on over night and the next morning my counter showed it was still on the same musical note.

A single 1N914 diode can do the entire signal mixing you need. Pick-up the RF signals from the two oscillators by proximity, no direct connection.

Why not build on a PCB in a modular form, that way it is easy to scrap out the sections that do not work.

This sound is dirty but on the other side of 10,000 failures you might find it.  Henk and I both found it but we had to suffer to get there while having the patience of a saint. Tube Sound.mp3   <= No Puny Whistle

I wish you the best of luck,

Christopher

 Edit: The most critical area of theremin design is to not let the pitch oscillators know the other one exists. I learned from dewster the importance of having a buffer between them.

"Tim, You have begun a theremin journey of discovery, that is plowing through the 10,000 reasons it will not work before you have great success. Today we have many more tools to work with and aid us than Lev Sergeyevich. A tube oscillator can add a character to the theremin vocal sound (magic). Not every vacuum tube in an identical circuit will demonstrate it. My best description is a fatter sound is developed that seems to me to be a slower roll-over of the sine wave. Thierry might be the only one on the planet today that could give this effect a name. Modern day components are much better to use than searching for too much for old stuff."

Yeah. Digging up air variable caps was not fun or particularly cheap but it's done already. other than that, the old stuff I'm using is mostly just tubes. And the transformer, but I've had that for years. 

"A single 1N914 diode can do the entire signal mixing you need. Pick-up the RF signals from the two oscillators by proximity, no direct connection."

Right over my head. well, mostly over. 

"Why not build on a PCB in a modular form, that way it is easy to scrap out the sections that do not work."

The idea has merit. Makes me wonder if i have four of those octal relay sockets with barrier terminals. I know i have at least a couple. 

Or i can build each section on perfboard with phoenix type screw terminals, and just have wires dangling from four octal tube sockets. 

The high voltage regulator would be on a pcb anyhow. 

"A single 1N914 diode can do the entire signal mixing you need. Pick-up the RF signals from the two oscillators by proximity, no direct connection."

Tim said:  "Right over my head. well, mostly over."

The theremin is simple in concept with attitude, that is why many modern engineers are baffled by her.

I am not an engineer so I can talk like a normal human. The theremin principle is to use two very low powered RF transmitters and a crystal radio detector. Heterodyning occurs when both oscillator signals meet in the silicon diode together. 1N914.  Just a short length of wire near each oscillator can pick up enough energy to do the trick connected to the diode. On the other side of this detector diode shows up the audio signal because of the mixing. The RF is easily attenuated and not needed beyond this point.

At the diode you want to have an ideal sine wave which determines if your sound is going to be thin or fat, the follow up circuitry is what shapes the final audio wave shape.

On an EtherWave Standard the audio p-p signal at the detector diode is around 1.5v, I found that to be interestingly high and easy to work with. 

After the detector diode is where my tube generated audio hybrid feeds into a modern IC LM358. Everyone has his or her own trick in charming the magic into the sound. A good Thereminist also helps.

Christopher