# A new vacuum tube theremin

Posted: 7/20/2015 6:23:55 PM

From: Lopez Island, WA

Joined: 11/25/2014

Hi Dewster,

Thanks, that's a lot of info to absorb!  Here I'll reply just to the one part of it that I've already studied, namely, the self-capacitance.  And let me say up front that I don't actually know this from direct experience; it's only based on articles I've read and calculations I've done.  So, with that caveat, I'll say that I believe the pi-wound coils have lower self-capacitance than air-wound coils.  Some time ago, I found a 1938 article with a formula for the self-capacitance of an air-wound coil, which it describes as a "much more accurate formula, which is very well verified by experiment."  The article is on page 249 of a collection called "Electronics for Engineers".  The entire book (warning: 35 MB pdf) can be found here:  http://www.tubebooks.org/Books/efe.pdfFor comparison, let's say I want to make a 10mH inductor on a 3.5" diameter form using 36 gauge wire (Tech Fixx green, since I have the specs for that).  I'd need about 333 turns to get the right inductance, and according to the article the self-capacitance would be 17.5pF.  In contrast, the Hammond 1535G is also 10mH and has a self-resonant frequency of 710kHz minimum.  That works out to only 5.025pF of self-capacitance.  I think it's so low because in a pi-wound coil the wires cross at right angles with very small regions where they're in close proximity.

Still, I'd prefer to use air-wound coils in the theremin, especially since you've pointed out the thermal problems.  There's room in my cabinet for big coils, and it would look a lot cooler.  Really, the main reason I used the small commercial coils at first was because I knew I was going to have to do a lot of tweaking.  Once a coil has been wound, it's not that easy to change it without making a mess.  Still, I'm sure I'll try it soon.

John

Posted: 7/20/2015 11:06:10 PM

From: Northern NJ, USA

Joined: 2/17/2012

Hi John,

David W. Knight has published what seem to be practical and definitive papers on solenoids.  The field is full of confusion, and self-capacitance in particular is rather ill-defined theoretically:

http://www.g3ynh.info/zdocs/magnetics/appendix/self-res.html

If you have a sensitive C meter that reads way down to sub pF you can roughly measure the self capacitance of a coil by putting a ~10x SRF capacitor in series with the coil and reading it directly.  Another method is to measure the inductance, then drive one end of the coil with a function generator with the other end hanging out in space, looking for a peak with a scope probe placed some inches away from the free end.  Work back from resonance F and measured L to get C.  I think this is the best way for Theremin use.

A single layer solenoid will beat a pi wound ferrite any day of the week in terms of Q, self capacitance, and thermal stability.  The ferrite wins in terms of space, which often has the highest premium associated with it.

I'm plugging your specs into my spreadsheet and it tells me a 3.5" dia PVC form with 36AWG for 10mH requires 333 turns, for a height of 46mm, or an aspect ratio of 0.52.  Self capacitance is ~3.3pF.  I would aim for a taller coil, it would require somewhat more wire but you would get better self capacitance.  For instance, a 2 3/8" OD would give a coil height of 74mm and self C of 2.2pF.

But, you know, prove it to yourself.  Wind a coil and see how it compares to a pi wound.

Posted: 7/22/2015 12:01:33 AM

From: Lopez Island, WA

Joined: 11/25/2014

Thanks for that reference, Dewster.  After working my way through half of it and checking some other references, I can see that you're right.  The article I had based my calculations on was derived from the paper by Palermo, which clearly has been discredited.

I'll play around with your spreadsheets, but it now seems clear that if I wind an air-core coil, the SRF is going to be well above my oscillator frequency -- which is great, because it means I don't have to worry about it so much.  I have vertical space for a coil around 11" tall, so I'll look for a form diameter (hopefully, a standard PVC pipe size) that'll give me the right height.

I guess I'll start out with the assumption that I need about 45mH, the same as the series combination of RF chokes I'm using now.  If I'm wrong, it'll hopefully be something I can correct by removing turns from the coil, rather than adding them.

John

Posted: 7/22/2015 1:20:58 AM

From: Northern NJ, USA

Joined: 2/17/2012

Hi John,

I've been pretty happy with 34AWG, it is fine but not too hard to work with.  You might look into how the wire diameter influences self capacitance.  A thicker wire coating might be advantageous as well.

You might be interested in something I ran across a while back that I call the "invariant".  Due to the way capacitance padding scales both frequency and sensitivity, you can theoretically get the same range from any frequency operating point with a simple LC Theremin.  Though things might get thornier with EQ types like you have built (something I haven't investigated).  FredM also suggested using higher frequencies and dividing them down digitally before mixing, which would allow you to use much smaller inductors.

Posted: 8/13/2015 6:33:15 PM

From: Lopez Island, WA

Joined: 11/25/2014

By lucky chance, I recently met a fellow named Ross Marshall who has been a concert thereminist.  He doesn't have an instrument currently, but he visited yesterday to try out the one I built.  I was impressed to see him walk up to it and play it nicely almost immediately, despite not having played for several years.  He posted some video of his first encounter with the instrument, which you can find here:

https://youtu.be/4ULSCQf0AiE

He gave me some constructive criticism of both the tone and the pitch field characteristics, which I hope will help me make some improvements in the future.

John

Posted: 8/14/2015 4:18:53 AM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Ross Marshal said: “send us any suggestions on how to force more linearity with the pitch circuit.

It is difficult for me to stay completely away from theremin stuff. As an easy experiment you might try changing C2 & C12 - 4700pf to 220pf or less using your nice 72uh coils. Get the pitch LC oscillator operating above 900khz. You won't need any linearity type coils. Also decrease the diameter of the pitch antenna. A thin wire inside a non-conductive rigid tube is ideal. I always found 18” to 21” pitch antenna lengths as ideal. John you understand theremin, as you recognize the importance of allowing the true heterodyned voice to sing. The sound is what got the attention Lev Sergeyevich at first, then everything else evolved from there.

Edit: All the LC coil needs to do is keep oscillating, no magic, the wonderful sound is developed by a characteristic found in the vacuum tube.

You are fortunate to have Ross M. as your neighbor, I know his name well.

Christopher

Posted: 8/14/2015 5:32:27 PM

From: Lopez Island, WA

Joined: 11/25/2014

It is difficult for me to stay completely away from theremin stuff. As an easy experiment you might try changing C2 & C12 - 4700pf to 220pf or less using your nice 72uh coils. Get the pitch LC oscillator operating above 900khz. You won't need any linearity type coils.

Hmm.  I made those capacitors as large as possible in order to dwarf the effects of the tube's inter-electrode capacitances.  It seems like it would be less temperature-stable if I went down to 220pF, though I don't have real-world proof of that.  Also, I've been assuming it would be wise to keep my oscillator frequencies outside of the AM broadcast band.

However, I did breadboard a theremin using a Clapp oscillator with a 2500uH inductor and capacitors of 56pF, 680pF and 2000pF.  It oscillated at around 386 kHz.  The capacitance of the pitch antenna directly shunted the 56pF capacitor, without any antenna coil.  It actually worked quite well.  Maybe I'll revisit that approach.

Also decrease the diameter of the pitch antenna. A thin wire inside a non-conductive rigid tube is ideal. I always found 18” to 21” pitch antenna lengths as ideal.

Thanks.  That is something I'll definitely experiment with.

John you understand theremin, as you recognize the importance of allowing the true heterodyned voice to sing. The sound is what got the attention Lev Sergeyevich at first, then everything else evolved from there.

When it comes to music, I'm definitely an analog kind of guy. :-)

John

Posted: 8/15/2015 2:09:55 AM

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

CRA said: "It seems like it would be less temperature-stable if I went down to 220pF, though I don't have real-world proof of that. Also, I've been assuming it would be wise to keep my oscillator frequencies outside of the AM broadcast band."

So you would give up having a perfectly linear pitch field so your neighbors did not hear you playing theremin over their AM Radio? I can understand that, but we both know that is not why. (-'  In my future experiments I will try 1800 kHz, I see no reason why this would not work excellent. If only I had my own Pro Thereminist.

I have too many projects right now; our conversation is like theremin foreplay, now I want more. LOL

You should see my 2.4 ghz drone zapper in action, when confused drones just land, it is really funny! Illegal yes but not so much on the reservation where drones are increasingly trespassing.

Christopher

Posted: 8/18/2015 12:17:23 PM

From: Northern NJ, USA

Joined: 2/17/2012

"Also decrease the diameter of the pitch antenna. A thin wire inside a non-conductive rigid tube is ideal. I always found 18” to 21” pitch antenna lengths as ideal."  -- oldtemecula

This is good advice.  My hand / antenna mutual capacitance measurements show a long thin antenna is best (in terms of pitch linearity) for analog heterodyning Theremins, and plates are best for digital period measuring Theremins.  Though the former Theremin will never be as linear as the latter (given that both are well designed).

However, I did breadboard a theremin using a Clapp oscillator with a 2500uH inductor and capacitors of 56pF, 680pF and 2000pF.  It oscillated at around 386 kHz.  The capacitance of the pitch antenna directly shunted the 56pF capacitor, without any antenna coil.  It actually worked quite well.  Maybe I'll revisit that approach.  -- John

The FET Clapp is the best oscillator I've found in terms of oscillating with just the antenna capacitance as the C in the LC tank, which is the most sensitive mode possible (you want maximum sensitivity with digital Theremins, not so much with analog).  The gate bias resistor serves double duty by conducting AC noise to ground.  Though I'm uncertain as to whether I'll go this route with my own designs - digital phase locked loops are too compelling to avoid on the volume side, and a unified approach may give the best temperature drift behavior.

Posted: 9/26/2015 11:59:47 PM

From: Lopez Island, WA

Joined: 11/25/2014

This week I made some changes to my theremin with an eye toward getting a more classic theremin sound from it.  I updated my article with the new schematics, a scope photo of the output waveform, and a new recording of the instrument.

My approach was a little bit sleazy, using diodes and such to shape the waveform into what I wanted.  But I'm pretty happy with the tone now.  I think if I build another theremin, I'll change the mixer circuit to achieve the same thing more "organically".  Anyway, I'd appreciate any constructive criticism of the tone.  To make the recording, I ran the theremin's output directly into the recorder.  There were no amps or mics in the signal chain at all.

John