is the Mr. Theremin web site! This site is about a home-built
tube theremin similar in design to theremins of the early past.
site is merely a detailed account of what I had to do to build
my theremin, and this author does NOT in any way recommend building
a tube project because of the real risk of fatal electric shock.
This unit produces 330 volts DC! Be warned!
all, what fun is getting killed building something you picked up
Internet when you did not understand how dangerous the technology
tube circuitry (the only silicone is diodes).
four NOS 12AU7s and one 6X4 rectifier.
four P-C70-OSC standard oscillator internal tunable coils.
copper plates as the volume and pitch antennas.
volume, volume sensitivity, and pitch beat frequency controls.
to a common guitar jack to be played in a standard amplifier
(and effect petals).
design is from the Forbes
A/C and B+ have fuses.
and standby switches.
wood case (from an old Canadian Whiskey gift box).
theremin dates back to around the very beginning of the birth of the
thermoionic valve (tube) triode in 1906. In 1917, Lev Sergeivitch Terman
(name commonly referred to as Leon Theremin) created the theremin while
trying to create a better radio.
basic principle of the theremin is to take two harmonic radio signals,
mix them, ignore the sum of the two frequencies and use the difference
between the two to generate a tone. The pitch of the tone is altered
by moving your hand back and forth close to an antenna (or a metal plate).
radio wave disruption caused by the hand in the air moving back and
fourth is harnessed to produce a musical tone.
first theremin had two antennas: one controlled pitch and the other
controlled the volume. Traditionally, the pitch antenna is sticking
straight up on the players right hand, and the volume antenna is horizontally
located at the players left. The volume antenna is bent into a oval
coil. Most theremins now use a rod antenna (like the original), but
use a metal plate mounted horizontally as the volume antenna.
theremin designs use only pitch antennas (the volume is fixed). All
of the best modern commercial theremins are transistor-based. The hands-down
best theremin you can get (the holy grail for theremin enthusiasts)
was the RCA theremin. They were made around the 1930's and have a unique
sound that has yet to be reproduced. The
RCA theremin was the first large-scale model for the general public.
best theremins ever produced were from Lev himself for such great players
as Clara Rockmore ('The Art of the Theremin' was her album). The cello-like
tone was unique to the theremins Lev produced. Theremins are compared
to either the Clara Rockmore theremin or the RCA for their tone and
quality. Modern theremins can produce excellent sounds, and high-end
theremins can emulate classic theremin sounds (such as the Big Briar
by Ethervox). There is no modern theremin that is considered as good
as the RCA (depending on what you consider good).
theremin uses oscillators that are capacitor and coil based. There is
momentum to remove the dependency of coils to produce the same effect.
The RCA and Lev theremins used big hand-wound coils. The use of these
inductors (coils) seems to not be necessary to produce the effect, but
rather the sensitivity and tone quality. Coils seem to be the biggest
problem with the theremin general design (in designs that use coils).
oscillators (volume and pitch) are configured to operate at different
frequencies. The operational range of pitch and volume is different.
Each oscillator (pitch and volume) is comprised of two oscillators (capacitor
and coil) that are harnessed to do different tasks.
pitch section has one oscillator wired to the antenna, and the other
oscillator is not. The frequency produced by the antenna-wired oscillator
varies with the hand changing the capacitance of the antenna minutely
(enough to matter). The frequency of the non-antenna wired oscillator
does not change with the movement of the hand. It is the difference
of these two oscillators within the pitch section that produces the
musical note. The technical name for what it is doing is called a beat
volume section works the same way as the pitch section, but its two
oscillators are harnessed into a voltage controlled amplifier for the
output. Traditionally, closer is loudest and diminishes to completely
off farther away.
first theremins were tube-based. There is a movement (small but present)
within the theremin world for tube-only theremins. This is no doubt
a subtle quest for a theremin that sounds like the original (or RCA).
The obvious way to do this is with tubes, but also copying the original
construction techniques. It is possible to create a not-fantastic sounding
theremin with tubes (as the famous Uncle Howie calls 'an electronic
I am not hear to say that tube sound is better than transistor sound,
but I am a vacuum tube enthusiast. I also believe that the best sounding/playing
tones come from tubes. This may or not be true with theremins, but is
true with guitar amplifiers.
original theremins used all tubes and large hand-wound coils. The volume
and pitch operational frequencies were less than 500 kHz. They had low
sensitivity (meaning more hand distance was required). Most of the components
that were used in the original theremins are not available today.
is not clear why a modern tube copy of the original has not been created:
It is probably due to the fact that there are many transistor electronic
engineers today and few tube electronic engineers. Simpler tube design
versions can be found, but none of them is in the same league as the
original or RCA theremin.
understanding links (in no particular order).
Therminvox site technical description of the theremin
were links I used to provide information needed to build the theremin:
Doug Forbes theremin site
is the site put up by Doug Forbes. The only complaints are no finished
product pictures and no way to contact him for questions.
is perhaps one of the best theremin sites in the world! It is with
Arts help that I was able to build my theremin. He is a theremin
electronics expert and also has
kits available for the digital builder.
does not condone building tube theremin projects (as I do not) because
of the real danger of electrocution if something is wired incorrectly.
site is a must have, as it contains some good technical information
about theremins. The author DID build a Forbes Tube Theremin, but
was not able to get the volume to function as he thought it should
(he thought the volume response lacked some sensitivity). I have
noted in my build that the volume DOES have far less sensitivity
than the pitch (so I can confirm what Giorigio is reporting). Some
elements of the design may need to be rethought to change this.
author is a theremin expert credited with designing a first class
sounding and performing analogic (not digital) solid state theremin.
theremin web ring
found some good sites and information here.
non-Forbes tube theremin description
is a site is a description of a tube theremin (not a Forbes theremin).
found some good information here.
found some good sites and information here.
tube amp project
is the world famous tube amplifier project web site. In this site,
Chris Hurley offers the tube DIY'er a selection of tube amplifier
projects (guitar). I gained tube technology with this site. He also
has a very active forum to support these projects (as well as other
is the famous Doug Hoffman amplifier web site. Doug is a guitar
tube amplifier expert, and his discussion forum is second to none
when it comes to solving your tube amplifier issues. He also is
a great vendor of tube project parts.
site is the tube projects site that is know for its popularity with
tube enthusiasts. This site is a must!
site is a collection of users and builders. It is here I first met
the (theremin) world famous Uncle Howie. He is an expert is design
and playing theremins, and is one of many helpful individuals on
LEVNET site is the only real meeting place for the worlds
thermin-people. It is a mistake to not be involved with this site if
you are serious about the theremin!
AND TUNING CAPACITORS
variable capacitors I am using are NOT 150pf.
I could not find any in that range, so I bought 365pf, as this allowed
me to bump my fixed pitch oscillator tank capacitance up to around
360 or so if I wanted to.
did not know anything about these air capacitors, so I wired them
wrong. They work by the case being grounded (GND) and you can pick
one of the 4 places to connect the other connection (typically to
the grid of your valve).
can see that the darn thing is all but impossible to mount, so
I used a strap retainer out of some 1/2" wide brass stock
and used two nuts and bolts through the chassis to hold each one.
used a fender washer to eliminate the wobble and you only need
to tighten just enough to hold to be effective.
coils were quiet unexpected from what I expected:
I thought a tube theremin needed big coils, and I was prepared to
wind my own. I decided against this, as I don't know anything about
that, the design was around the Antique Electronics p-C70-OSC adjustable
oscillator coil available and inexpensive. I even made my own coil
adjuster (I carved it out of some real firm plastic).
included the literature that came with the coil (in case anyone
find it interesting that these are actually transformers (they have
a primary and a secondary).
you read these coils looking at the bottom (opposite of the little mounting
'crown' they have).
The use of the center tap may not be correct (but is working in this design).
used an aluminum chassis, as I knew I would put it into a cabinet.
While a steel chassis would have been better, I was able to cut
all of the needed holes in one day by hand with the aluminum.
put some little feet on it, because I thought it might work without
a case (it turns out that it needs the wooden case).
If you want to have something not hidden in a box (like mine), use
a steel case. My aluminum case would flex with the weight of the
mounted components (stressing the chassis).
big drawback to steel is that it does not nearly cut as easily as
aluminum. I spent three days cutting holes for my guitar amplifier
(with its heavy steel case).
layout mentality was to keep distance between the A/C and the coils,
and keep the coils as far apart as possible (from each other).
used point-to-point wiring, and only used PCB for the power supply
and star ground isolation points.
broke the tube plane (a no-no for guitar amps) to bring the tubes
close to where they were needed to eliminate inevitable wire crossing.
I kept them 3 inches (about 8 cm) on center. The heater wires were
did not use the specified power supply (it did NOT work for me
at all). I am using an unregulated power supply and trim the voltage
with a 50 watt variable resistor (sounds hillbilly, but it works).
A regulated power supply is the way to go if you can. I put 2
extra stages of RC filtration AFTER my B+ fuse, and grounded the
filters with the ground point used by V2 and V3.
operational voltage is 130 volts currently, and this does not
change with different hand positions (except by +/-15 volts in
did burn up a 5 watt 1K resistor testing the power supply (with
no load) as to not risk my 12AU7s. Later, I installed the specified
power regulation system (The Forbes zener-shunt system), and weird
things happened (nothing worked). I noticed that this damaged
one end of my 50 watt resistor!
had to use the other end and the wiper (it is as if the other
end was burned up). That was it for me, as the big resistor power
supply is serviceable (so my power source is unregulated).
An unregulated power supply with less than half the voltage
specified seems to work optimal with no ill effects.
An unregulated power supply does appear to work with this design. My
power supply is nothing more than a standard guitar amplifier type.
I also do not see any advantage in a very high working voltage for this
design. I am using 130 volts and I get serviceable results.
I first assembled the theremin, it seemed to work, except there was
a buzzing characteristic to the sound. I thought that was normal, until
it was pointed out that this is NOT normal.
would seem that there are small design flaws in the Forbes theremin,
but it appears they are NOT insurmountable. It appears it is up to the
builder to adjust some values of components to compensate on the layout
is a few suggestions to make the cicada sound go away:
- Clean B+. Use lots of RC filtration.
- Use a plate resistor on V1, V2a, and V4 that is 470K. This worked
for someone else, but not me. I restored the plates to their original
specification (no resistor on V1, V2a, and V4).
- Change the 220pf condenser on each coil to 47pf or 56pf. This worked
for me. I used 47pf.
- Use a high voltage ceramic disk capacitor (0.01uf or 0.1uf) on each
plate as a high frequency by-pass filter. I did not try this (it might
work for you).
- Try grounding the bottom of the 1M grid resistor INSTEAD of connecting
it to center tap with the 220pf capacitor value (there is a possibility
that using the tap like this is a misuse of the coil). If this cures
the problem, that means there is unwanted feedback through the 1M
resistor (let me know if this is the case! I did not try this).
is the sound if it is NOT working properly. Art Harrison calls
this "the cicada syndrome"
used a total of 5 ground points.
- A/C has its own ground point, right as it enters the chassis.
- The power and rectifier has its own point. The stock filter capacitors
are grounded here.
- The volume has its own. Both oscillators are grounded at this point.
I also ran the 1N4148 diode to the volume ground to further isolate
noise (or so I believe).
- The pitch has its own. Both its oscillators are grounded here.
- The mixer, amplifier, and input are grounded at one point.
of these points are grounded to one point.
for a guitar amplifier, this theremin is quiet.
used two copper plates (8 inch long, 4 inch wide, 1/16 inch thick).
could NOT get any form of a rod antenna to work (I almost had to touch
the antenna to get it to work).
could not get the theremin to work when the antennas were close to
the chassis. I instead had to install them about 10 inches (about
23 cm) above the chassis mounted to the wooden case. Even using tested
insulated mounting brackets to the chassis did not work (in case you
were wondering if I tried it).
plates I am using give good sensitivity and I recommended for that
reason using plates. I think it would have looked cooler to have the
antennas I tried to use were standard off the shelf Radio Shack replacements
for a cell phone base. This reason alone may be why I could not get
them to work (perhaps the wrong type of antenna for what I needed).
This is how I calibrate it:
- Set the coil slugs at about half way
don't have to do this, however).
- Set the pitch beat frequency and volume sensitivity knobs around
the center of their travel (half way).
- Set the voltage (ala 50 watt variable resistor).
- Take all of the tubes out (except the rectifier tube).
- Set the fixed oscillators base frequency:
- PITCH: With just V1 in, power on, warm up, and then turn theremin
on. I set this to 575
kHz on the AM radio, and find where the oscillator
is at by tuning the coil (with a non metallic tuner). I have found
this sound is the sound of the coil being tuned at its loudest point
(it sounds like rubbing violin strings with bow rosin on your fingers).
I then repeat this process with V2 as the only valve in.
- VOLUME: With just V4 in, power on, warm up, and turn on. Notice
this is the same procedure as 5.1 (that's because it is, except
with V4 and V3). I tune it to about 700
think its best to start with the fixed oscillator coils first, because
the range is more limited and is thereby the limiting factor in
frequency determination for the pitch (or volume) oscillators.
that with an unregulated power supply, the current draw is different
with just 1 oscillator tube in verses all of them. I started out
at 250 volts nominal with all tubes in, and at the end of tuning,
my operational (plate) voltage is just 130 volts! (The Art
Harrison 125 theremin uses a just 50 volts DC).
means I have to adjust the variable capacitors with the new settings
of the variable capacitors (the volume one can change the voltage
from 190 volts to 310 volts in its entire range). And after I set
the voltage, the oscillators need retuning. Fortunately, this cat
and mouse system seems to be quite forgiving, and I am getting great
- Now comes the fine tuning.
thought that was it, and it appears that there is a degree of fine
tuning by ear to set the volume sensitivity and correct operation
of the pitch. This is done by tuning the fixed volume coil to get
the working range dialed in, and testing different settings of the
volume sensitivity control. The pitch is so sensitive, I fine tune
it by bumping it lightly until I hear it making a low tone so low
I do not hear it (you have to activate the volume when you do this).
turns out that it is possible for this theremin to be slightly out
of tune and it will not function as you would expect. In fact, it
also seems possible to get different types of theremin response
with different tunings.
I find that I don't have to fine tune it so much as when I had
originally written this. All I have to do is to adjust the variable
volume control to activate with hand proximity, and adjust the pitch
the same way (why I made it so complicated in the beginning, I don't
have discovered it is possible to have two operational modes:
is where I only have a pitch control (volume becomes nonfunctional).
The sound is great, but REAL quiet. The working range starts about
10 inches to almost 1/4 to the plate. The sound is like an electronic
This is NOT a correct operational mode of this theremin. It
happens to be one way it operates, and is not correct.
is where the volume and pitch both work, and the sound is more
like a voice than an electronic tone. Also, the volume can get
REAL loud. I do get little strange sounds with different volumes,
and I can find a working volume with minimal effects (for a clean
I find that the volume is best set at 50% and left there and never
moved. I play it through a 100 Watt Marshall amplifier through its
clean channel and get great sound and power with little (or none)
internal noise from wither amp or theremin.
that these two modes were discovered early, and I am sure that many
more interesting combinations exist with factors such as correct
working voltage, different antennas, and setting pitch and volume
at other working frequencies.
set my volume frequencies HIGH (around 735-775
kHz) and my pitch
frequencies LOW (around 550-600
kHz). This way, a lot of tuning
will not cause heterodyning between the volume and pitch sections
because of overlaps.
do find that the volume and pitch mode is the way the theremin wants
to be used in. Finding the spot with the pitch adjustment control,
as the point where it works is the loudest spot I can find activity
on (many places on the dial seem to work, but are not as loud as
the 'one main' spot on the dial). My only drawback is that fine
tuning is difficult (the gradients are very small fractions of a
is the CICADA SOUND.
is the sound if it is NOT working properly. Art Harrison calls this
"the cicada syndrome". The theremin appeared to be working
normally (volume and pitch control), but the sound is not right.
my case, this was caused by the 220pf condenser capacitors were too
high of a value (possibly causing coupling and unwanted oscillation).
I corrected this by changing the value to a 400 volt 47pf ceramic
I move the calibrated pitch control LEFT about 3 mm, the volume control
does not operate, and overall volume is quieter. The working range
is about 8 inches total (about 15 cm). No setting on the volume knob
does anything. The sound is like a transistor theremin I once heard
(cool SCI FI sounding).
the pitch control knob is set where it was calibrated at, both volume
and pitch operate. This is one quick example I did. Notice it can
get louder. The working range of the pitch with this tuning is about
2 feet (about 66 cm). The volume is about 8 inches (about 15 cm).
note that this is one of the first times I ever made any sound with
this theremin and recorded most of the shrill end of what this
theremin can play (in error). In fact, this theremin sounds great at
LOW lows and has an excellent operational range. I can make mine sound
voice-like, and not whistle-like. In the beginning, I liked to make
the shrill whistle scream sound with it, and it shows on this
I do not notice any problems with crosstalk: the grid wires do NOT
be sensitive to plate and A/C wiring (unlike a guitar amplifier).
The grids on each oscillator tube MUST have a negative voltage (this
means the oscillator is working). My variable volume oscillator
grid was shorted to ground for a long time and the problem was that
it was not floating ground with its 1 Meg resistor.
No matter how I insulated the plates from the chassis, the theremin
stopped working when the plates were too close to the chassis. That
is why they are mounted about 10 inches (about 25 cm) and mounted
to the wooden case.
My copper plates were about $5.00 each at Menards (an American DIY
home improvement chain of stores). Plates offer the BEST response
(I have been told).
DC filtration seems to be the number one problem with this design.
I do not seem to have this problem (as I have added 2 extra filter
stages within the B+ line). I am using 3 20uF/500 volt filter capacitors,
and have added 2 22uf/400 volt filters in the B+.
The coils were real easy to obtain (I have heard from many people
they could not find them).
The 10nf value on the by-pass filters is 0.01uF (this is a real
Notice that the schematic shows the variable capacitors rotor wired
to the ground of the coil. I did not run a wire from the variable
capacitor to the coil ground. Rather, I relied on the chassis grounding
of the capacitor and the separate ground of the coil to somehow
meet. They must, as it works. I did isolate them and run the wiring
as it showed, but there was no difference (so I changed it back
to chassis mounting ground).
The cathodes of V3a and V3b are coupled with a 10K resistor. This
one changes the voice of the theremin. I thought it might be interesting
to replace this with a 25K variable resistor and call it the 'voice
I added a B+ fuse (1/2 amp fast blow) after the stock filtration
and before my two extra stages I added. This saved me when I accidentally
touched the cathode and grid of a tube together. A new fuse later
, I was back in business.
I used 400-600 volt rated capacitors, and originally used 1 watt
or better resistors. I did use some 1/2 watt values (the 10K cathode
resistor for V3a and V3b). I also have one for my V3a plate I plan
to replace as I only ordered 1 470K/1watt resistor.
Uncle Howie recommends using a 1000-1500 Ohm resistor for the V3a
and V3b cathodes (instead of 10K). Then use a 20 mfd/150 volt capacitor
at the junction of pins 3 and 8 and put the negative end to ground.
This is supposed to richen up the tone and help flatten the response
Art Harrison has
(well) designed a pitch and volume theremin. It is called the 126
theremin. This may be the theremin to build unless you want
to build the Forbes theremin. His theremins are based upon
the Colpitts oscillator (different from the Hartley of the Forbes).
I do have an example of this theremin being used in a recording at
my MP3.COM site. See "The Succubus" at: