Building the E. J. Schultz tube theremin

"Is there a 4069 based design that isn't terrible?"  - TS

The 4069 is pretty slow, so the resonance point won't be ideal, and the delay is temperature dependent so you may get drift.  And static can kill CMOS pretty easily so you will likely have to add protection.

Is there anything wrong with using discrete transistors?  The NPN Colpitts is really simple, no coil taps, draws almost no current, gives pretty good voltage at the antenna.

A note to those interested in my 126 theremin design- I modified the volume circuit a while ago to allow either response. See the link under the first schematic at:

http://www.theremin.us/126/126revised.html#Schematics

Some prefer the "louder for further" response, and this version can provide that with a selector switch.

-Art

Lots of folks who build the 126 ask me if a regulated supply is required. In my experience, it is not for typical mains conditions, since the oscillators have ample stability without regulation (or "supply rejection," in the parlance of circuit design). Also, as with many heterodyne theremins, there is direct cancellation of common-mode effects because the pitch reference and pitch variable oscillators change in unison. If you use the parts I recommend with a 120V 60Hz mains source, the 12AU7s never get too hot to touch, and the B+ stays around 50V. I selected these parameters for safety, so that the only severe shock hazard is on the transformer mains side. I encourage builders to put the power supply, therefore, in its own sealed metal box (details provided). Under these very-relaxed bias conditions, the 12AU7s should last for decades.

"Lots of folks who build the 126 ask me if a regulated supply is required. In my experience, it is not for typical mains conditions, since the oscillators have ample stability without regulation (or "supply rejection," in the parlance of circuit design). Also, as with many heterodyne theremins, there is direct cancellation of common-mode effects because the pitch reference and pitch variable oscillators change in unison." - Art Harrison

I'm curious, have you experimented much with voltage regulation?  Even with identical fixed and variable oscillators (I haven't looked at your schematics - usually there are topological differences between these oscillators in most Theremins) I've found voltage regulation to be the first line of defense against drift.  It's trivial and inexpensive to do this with semiconductor based circuits - so why not? - but I imagine it is more challenging with tube circuits, and I imagine this is why tube Theremins generally don't have voltage regulators.

Art, did you discover some kind of new vitamin? If this is really you then let me say I always viewed you as one of the giants in theremin research. I developed my own ideas but you most definitely were one of the kindest souls. Many people in the world of theremin I found over the years seemed to have no positive input, just sharing the gritty stick up their butt.

Dew I have mentioned before that my vacuum tube designs did not have any drift. The audio remained on the same musical note over a 10-degree F room temperature change, from night to morning. I used a 100 volt zener to keep the plate voltage stable. Without it I could hear the pitch very slightly rise and fall with AC power line fluctuations.

What I learned over the years is there may not be any significant advantage to using tubes today. If someone really wanted the best sound I would still use a tube in the pitch and fixed oscillators but probably not for the reason most people would think.

Christopher

"Dew I have mentioned before that my vacuum tube designs did not have any drift. The audio remained on the same musical note over a 10-degree F room temperature change, from night to morning.  I used a 100 volt zener to keep the plate voltage stable. Without it I could hear the pitch very slightly rise and fall with AC power line fluctuations."  - Christopher

Interesting.  How about drift at start-up?  Sitting around waiting for the thing to stabilize must be a spontaneity killer (one reason I hate long boot times in digital musical instruments - warmup is analog boot time).  Theremin's own designs had a "standby" position on the power switch, so even the master's stuff must have taken a while to stabilize at turn-on.  I want my own personal Theremin to snap to attention and be ready at a moment's notice without being a vampire or me having to somehow anticipate my own spontaneity (need a time machine).

"What I learned over the years is there may not be any significant advantage to using tubes today."

There are clearly obvious and definite disadvantages to the use of tubes today, which is why - except for a few tiny niches - they're basically gone from electronics.  The downsides are so severe that even if they were the ONLY way to get the BEST Theremin tone <insert other highly subjective criteria here> I'd continue to eschew the use of them in my own designs.  I'd conjecture that the current interest in tube Theremins is mostly quaintly historical (not that there's anything wrong with that).

Are there any modern Theremin virtuosos who prefer tube Theremins, particularly for public performance?  With the exception of a Peter Pringle performance here and there, and those from the distant past, virtually all the web video performances I've seen feature transistorized Theremins (though absence of evidence...).

"Art, did you discover some kind of new vitamin? If this is really you then let me say I always viewed you as one of the giants in theremin research. I developed my own ideas but you most definitely were one of the kindest souls. Many people in the world of theremin I found over the years seemed to have no positive input, just sharing the gritty stick up their butt. Dew I have mentioned before that my vacuum tube designs did not have any drift. The audio remained on the same musical note over a 10-degree F room temperature change, from night to morning. I used a 100 volt zener to keep the plate voltage stable. Without it I could hear the pitch very slightly rise and fall with AC power line fluctuations. What I learned over the years is there may not be any significant advantage to using tubes today. If someone really wanted the best sound I would still use a tube in the pitch and fixed oscillators but probably not for the reason most people would think. Christopher"

Hi Christopher! Thank you for your kind words, which are very-much appreciated. I concur that correctly-designed vacuum tube oscillators are not nearly as prone to temperature and bias induced variations as are transistor oscillators. Tube-based theremins may not exhibit much stability improvement with either heater/filament nor B+ regulation, although there is nothing inherently wrong with such features aside from the added complexity. In addressing Dew's comment, my 126 theremin design, for one, has no noticeable warm-up drift, and is ready to play in about 15 seconds.

As an aside, please excuse my scarcity in these forums! Regrettably, I haven't had enough time to converse here. if any of you good folk happen to try the latest 126 design variant I listed previously in this thread, please do give me your thoughts on the design's performance.