Spring 2011:

A Vacuum Tube Theremin!

Ok, ok. I'm starting to realize i have kind of a sickness that compels me to build theremins. The theremin just really appeals to me as an elegant invention and (of course) an obscure "underdog" instrument.

When i finished my EM Theremin a year and a half ago, i speculated that the day may come when i could see myself wanting to build a tube theremin. And it is this day. When Lev Sergeivich Termen was doing his work, vacuum tubes were of course the only game in town, so it seems fitting and proper to do a tube one. Also, though i'm far from being a tube-head audiophile, i do think there's a difference between the voice of a good tube theremin vs.a solid-state one. Plus, it's just plain cool to build stuff, and i've not worked with tubes before. So that's it for excuses and justifications. On to the build!

* * *

There are several tube theremin designs out there. As a kind of "intro to tube building," i've chosen the "126 theremin" designed by Art Harrison. It uses only one kind of tube, and runs on about 50vdc, both of which make it a less daunting project than some of the more involved designs. Plus Art Harrison has been really good about answering questions via email, and he can also conveniently supply some of the more esoteric parts used in his designs.


How big is a theremin? Well, this one will be exactly the size of a breadbox. Just like my first theremin kit, i got a neat old breadbox from eBay, which will make a nice cabinet. This one has the lid opening upwards, which will be great for accessing the tubes. I'm planning to sand the box down and refinish it, but it's in great structural condition.

Here's my new and improved jig for bending volume antennas. The round part is a wooden wheel i made but never finished during the early stages of my hurdy-gurdy project, and it was just about the perfect size for the volume antenna. The moral of the story is: never throw anything away!

The antennas, after annealing and bending the volume loop. I wanted to use regular 3/8" brass tube instead of the chromed copper water supply tube i used last time around. I'm used to bending this kind of tube into much tighter curves, so this was no problem at all. I didn't even bother filling it to keep it from kinking.

The top of the pitch antenna has a small delrin cap, which makes it look nice and finished.

Art Harrison specified plate-style antennas in his original design, but i'm kind of a traditionalist, so i'm going with the more standard rod and loop. Others have apparently had success using the old-school antennas with this design, so i'm not too worried.

I'll be using these 3/8" tube compression fittings to attach the antennas: the straight ones for the volume loop, and the right-angle one for the pitch rod. I got this idea from the construction of the Moog EM/Etherwave theremin, and it works well. Especially on this breadbox, where the whole top is the lid, i have to attach the pitch rod to the side of the box., as opposed to straight up out the top.

Two of the fittings have solder lugs attached, so i can connect wires from the theremin circuitry. I did this by soldering some thin walled brass tube into the fittings, leaving the small tags hanging out.

This theremin design uses only one type of tube (12AU7). They're plentiful and inexpensive if you're not stuck on the geeky audiophile "sought after" varieties. These came from eBay, and are particularly neat because they're (re)branded for the Baldwin Piano & Organ Co. They were apparently pulled from a tube organ. How cool is that - these tubes have already spent years making music!
The power supply. It provides 50vdc for the tube plates, and (regulated) 12vdc for the filaments. I've chosen to run the filaments on DC for a couple of reasons: first, there's less chance of any 60-cycle hum entering the instrument; and second, using DC makes it easy to use LEDs and any other solid-state add-ons i might choose.

Here's the chassis and front panel assembly. The chassis plate is 1/8" thick aluminum; the large blank area was crying out to be engine turned. I'd never done it before, but this was the perfect time to try. It was actually quite straightforward - i used a "unitized scotchbrite" abrasive wheel in my drill press.

The control panel is a piece of thermoforming plastic, bent over and screwed up to the chassis.

The underside of the chassis with the tube sockets and turrets installed, ready for the electronics. I found these insulated turrets for cheap on eBay, but they're male threaded, which means i had to drill and tap all the holes.
Here's the first stage of the electronic assembly - this is the variable pitch oscillator. I've subbed in a mica trimmer capacitor (the big white block on the left) to allow for reasonable ease of calibration, rather than swapping out fixed-value capacitors (which is what Art Harrison suggests in his article).

I've plugged in one tube to run the single oscillator, and it seems to work fine! A beautiful sine wave, and the frequency decreases as i approach the pitch antenna. Right now it's running at about 305 kHz, which is right in the ballpark of where it's supposed to be.

So far, so good!

That's two sections down out of six. (I'm kind of working on each tube socket and associated cluster as a unit.) So i've got the variable and reference pitch oscillators made, and they're both working fine!

The rest of the wires running around are all for the ground (green), high voltage (red) and tube heaters (yellow/orange).

Three sections done. The centre bottom tube acts as the mixer, which extracts the difference frequency between the two oscillators. This is called "heterodyning," and is how a theremin generates the pitch you hear.
That's four down! The lower right section is the volume oscillator - it works like a charm. It's fun to play with it hooked up to the oscilloscope and watch my volume hand "push" the waveform up and down. Pretty soon this thing is going to be making sound!
This is the last stage of the electronics. All the tube sockets and components are wired up (more or less) as per Harrison's schematic. I'm planning a few add-ons once the basic instrument is working as designed.

Here we are in my dining theremin testing room. The bunch of wires hanging out the front of the instrument are there so i can connect the scope/counter/multimeter to various key test points inside, to check that everything's working as planned.

...And it does work! There's some tweaking and fine-tuning to do, but it is playing and sounds quite nice. The tone control does give a lot of useful variation in timbre.

The instrument as designed by Art Harrison has the volume behaving "backwards" to the original theremins: as the player's hand gets closer to the antenna, the volume increases. That's his preference, but i'm used to the closer=quieter behaviour of my other theremins, so this one feels really unnatural to me.

So i designed this little module to reverse the volume response. It goes after the volume circuitry, before it's fed into the amplifier tube, and flips the control voltage upside down. It uses an LM337 and a trimpot to generate a negative reference voltage based on the normal behaviour of the volume tubes, and an opamp (NE5532 which is what i had handy) as a differential amplifier to flip the control voltage relative to the reference. There's a switch to select the amount of gain given by the opamp, which also allows it to widen the dynamic range somewhat. Here's another reason to use +/-12vdc for the tube heaters: i can power this module!

The whole thing fits in a 35mm film canister tucked away inside the instrument; it's switched in or out of the circuit with a toggle switch on top of the chassis. It's kind of a shame to stick a solid-state kludge like this in the midst of an elegant tube theremin, but i'm not expert enough to do it with tubes.

Here's the final control panel. It's just printed on heavyish paper stuck to the plastic panel. I've added a power light (i'm a sucker for those red jewel lamps!) and a master volume control. Also, i've included a mute switch; the LED changes from red to green to indicate "stop" and "go."

It's finished! I sanded down the breadbox, stained it and varnished it. And it looks great, though i do say so myself - like it was meant to be a theremin since the day it was made!

You might notice the pitch antenna has gotten thicker. I ended up replacing it with a 3/4" diameter tube. It more closely approximates the surface area of the originally-specified plate antenna. The skinnier tube didn't present as much capacitance to the instrument, so the available pitch range wasn't as large as i wanted. The new thicker antenna does give a greater pitch range.

A picture with the lid opened to reveal the chassis and tubes. The antennas connect with banana plugs, so the guts of the instrument can be lifted right out of the cabinet.

(And no, i did not bother staining or finishing the inside...)

Click the notes for a sound sample. I am blown away by the loveliness of this instrument's voice. I was playing the EM theremin while the finish was drying on this cabinet, and it just sounds like a kid's toy to me now. I have seen the light, and it is the warm glow of a vacuum tube! (Uh, heard the light?)

Many many thanks to Art Harrison for a great project and instrument!

(Today also happens to be my sister's birthday. Happy birthday Jenny!!!)

Last updated on 10 June 2011, by eric.