Build Report: Forbes Tube Theremin

[charbot]

Doug Forbes Tube Theremin

ok, I know that this isnt a stomp box, but who here isnt interested in theremins?
I actually built this a year ago.  I had it apart a hundred times but never took any pics.   I had to open her up today to reattach a coil that had come off of its support. 




Here is a link to the schematic:http://www.thereminworld.com/files/Projects/Forbes/theremin.gif
and the build notes: http://www.thereminworld.com/files/Projects/Forbes/theremin.txt


BUILD NOTES:
   All of the component parts were given to be by a guyI work with.  He had tried building it years ago and had given up.  He had given it away but somehow made its way back to him, unfinished,  and was  it stored at work. When I told him that I was into building effects he offered it to me under the condition that I wouldnt bring it back! 
    Everything else I made.  The original chassis was a mess so I bent up a new one from trash picked heating duct scraps (my favorite for enclosures), and de-soldered everything.
I work in a really nice wood shop with enough scrap mahogany to build the enclosure/cabinet and I turned the knobs. The antennae are 7/8 brass tube and are the approximate dimensions of the RCA theremins.  The pitch antenna was pretty straight forward, but the curved volume antenna was something else. Bending thin walled tubes sucks....and brass is expensive.  Every trick I tried failed.  After several tries  and $40 pile  of  kinked up tube I finally gave up on bending an antenna out of a single tube. I ended up just cutting out decent sections salvaged from my previous attempts, and soldering them together. 

Overall, this is a pretty simple design and went together fairly easily ...with one exception: THE POWER SUPPLY!   The power supply drawn on the schematic was very problematic. Its uses a zener shunt regulator and seems to be precariously designed.  Every time I tried it the diodes blew and the resistor burned. (btw this was the problem the the previous builder experienced)   
  I was in over my head. I had only built a few effects before and had never messed with vacuum tubes or high voltage power supplies.  I can follow a schematic, sure, but troubleshooting and redesigning a circuit is  something else. The guy who gave it to me thought that   And this is where I have to bow down and thank this forum, and one member in particular...

  .... MR Keen saved the day! Here's a link to the thread where he helped me figure it out (told me what to do) and a drawing of the power supply that I eventually used.  :http://www.diystompboxes.com/smfforum/index.php?topic=45182.0

The new power supply is built on the perfboard section and uses a mosfet.  R.G. gave me a couple options  , but this seemed the simplest.
After giving the power issues the "Keen treatment", the sucker fired right up!

SOUND: Sounds great! Unbelievable!   There is no unwanted noise or hum from the AC lines.   The tone is rich and warm... sounds nearly identical to the recordings Ive heard of RCA theremins.    if thats not enough,   messing with  the variable  capacitor can reveal improper tunings that sound completely different.  .  These are modes are quieter  and usually have less of an octave range then the proper tuning; but they add some range to the unit and make up for the lack of any kind of proper timbre or wave form control. (sort of)  I have found one that sounds  "buzzy", more like an early transistor theremin, and others that have more of a sine sounding whistle.
   Tuning is simple, once you get the hang of it;  usually only needed when its first turned on with minor pitch adjustments needs while playing to accommodate drift. 

CONCLUSION:  This is an elegant, beautiful sounding circuit.  This was my first venture into the world of tubes and with R.G. Keen's advice it turned out well. I cant imagine that this is anymore difficult than a tube preamp project so I would recommend this for a first time tube builder.  Dont bother with the power supply on the schematic, though.  Find a new one or used the one that I used and it should be fine.
All of the parts, including the adjustable coils can be found at Antique Electronic Supplyhttp://www.tubesandmore.com/     
Thanks stompbox forum, and than you R.G., without you this thing would just be a box of junk in storage!



By the way, the other pedals in the photo are a tube driver, a fuzz face, a big muff and a meatball/mcmeat: a few of my more presentable builds.

[Ptron]

That is wayyy too cool. I've wanted to build a theremin for years but have never taken the dive. #1 question... how did you know what to make the antennae out of, what shape they should be, etc.?

[Paul Perry (Frostwave)]

My hat is off to anyone who makes a working theremin! especially tube!!

To bend copper tube without flattening, either you put a special metal spiral inside the tube (applying rotational force to the end of the spiral after bending, causes the spiral to shrink slightly & then it comes out) or else, you fill the tube with sand.

The acutal antenna design is rather a lost - or at least very mysterious - art. Note the antenna is very very short compared to a wavelength of RF at the operating frequency. Sometimes there are series coils before the antenna to change its properties. Very difficult to minimise interaction between antennas.

[Pushtone]

Beautiful and Artfull build. 

Great report, I loved the story about the build conditions you were forced to agree to. 

If that chassis is the reclaimed heating duct, then you did an amazing presentation with it and the wood.
The contrast of wood and sheet metal is very appealing. Very good construction technique. Hides the unwelded corners of the chassis, or could.

I certainly would want some hi-resolution images, even if only available for a short time.
Those thumbnail sized images are a teaser.

Thanks for sharing.

[charbot]

#1 question... how did you know what to make the antennae out of, what shape they should be, etc.?

I found the dimensions of the original RCA theremins here: http://www.thereminworld.com/article.asp?id=27
I chose Prof. Theremin's  RCA antennae design for aesthetic reasons more than anything. The reason volume antenna is bent into a loop is because it  curls the electromagnetic field into a compact area , making it more sensitive than one in a straight line.  You could use a sheet of copper for the same effect; in fact that's what the designer recommends. Supposedly both he and this guy, MR Theremin: http://mrr3000gt.mystarband.net/MRT/index.htm had problems using more tradition antenna (am I being pretentious? it is "antennae" and not "antennas", right?) and used plates.   I had no problem with mine tho'; probably because I made them to the the original specs      also, they didn't work as well close to the chassis as they do a few inches away.  Mr theremin reported the same deal with copper plates.
The original Theremin volume antenna is so distinctive and cool.  A secret symbol only Theremin enthusiasts recognize!  Its also a good size for a largish adult hand.  the Forbes theremin is "goofy foot"    The volume was designed to operate  in an opposite fashion then most other theremins.  hand  closer= louder.    putting my hand though the loop cranks it up and amplifier section the distorts the signal.
I agree with mr forbes that this is more intuitive, but I am also "goofy"
Actually my theremin  can operate either way, but works best in reverse. Forbes says in the note that changing component  values in the  volume oscillators can make it "regular", but he doesn't specify what.

   

To bend copper tube without flattening, either you put a special metal spiral inside the tube (applying rotational force to the end of the spiral after bending, causes the spiral to shrink slightly & then it comes out) or else, you fill the tube with sand.

   Yeah, I tried the sand pack, no luck.  I also tried a using spring, but on the outside. I like your suggestion! that sounds like a winner, now where can I find a 48"x 5/8 dia. spring?     Someone else told me to melt solder or some other low melt temp alloy and pour it into the tube.  Viola ....A rod.... they are way easy to bend. no finesse or patience needed, just gorilla strength.    It can be shaped around a jig or whatever and then the solder is melted out.    HMMM sounds like some new MoJO BS to  spread......." 'center tinned' brass tube theremin antennas are waaaay more linear, sensitive, responsive, warm ...whatever"  

Note the antenna is very very short compared to a wavelength of RF at the operating frequency. Sometimes there are series coils before the antenna to change its properties. Very difficult to minimise interaction between antennas.

The small tunable coils that this design employs make pretty easy to tune out interference from the the other antenna.  A neat feature that make it good gor a relative noob like my self is that the oscillator operate at a higher freq than more traditional versions. this mean that radio set on the AM band can be used to tune each oscillator. no scope needed.       I have the vol set around 550-600 MHz and pitch around 700, a la' Mr theremin.   With the coils, there seems to be a very broad, unmistakable sweetspot. These are like trim pots , set-and-forget; the large tuning caps on the front are used for frequent adjustments. The volume is easy enough, but that pitch... its a mother... its sooo sensitive.  Often even a slight finger tap to the knob can send it way out  of pitch. Don't worry, If you build one you will be tappin' and fiddlin' with that stupid knob long enough to get acquainted with its quirks.   I tried to explain this above, but  there seem to be many different  playable pitch settings.  As the pitch knob is spun, the playable "modes" are heard as audible chirps.  If the knob is move really slow though the "chirp" the tone will from a shrill squeal, down low, and back up.  Dialing in the lowest freq. tunes the instrument to the players body ,sets the "null pitch",   Its ready to play.   The proper tunings are obvious. much louder and definitely sounds the best and can go the lowest. Very nice cello- sounding, resonant lows.   The others are quieter and are "cool". more suitable for effects.

Ive read that theremins built with large coils have more linearity in the pitch...meaning the notes are more evenly spaced.
  Paul,  you mentioned adding coils.....    sounds like a good idea...  Where can I learn about this more?   Any suggestions anyone?    Ive been wanting to try hand wound  coils  since I built this thing. 
Huge ones on mailing tubes.

[charbot]

hope these are better. 


try to ignore my ugly workbench...



this is the volume antenna.  I never finished buffing it. I couldnt reach the inside with the bench grinder at work, so like most projects it was never finished.   


four 12AU7a 's and a 6x4 rectifier.
the large black dots on either side are where the tuning caps were first mounted.  A dumb idea.  impossible to tune without my hand effecting the pitch. below, with the chassis in between is best.
like this...


the antennas/antennae are removable and fit into the pieces of larger diameter tube wired up and mounted in the box.



gut shot, my first point to point job

[Paul Perry (Frostwave)]

http://www.oldtemecula.com/theremin/observations.htm
this site has some very strong opinions on theremins.....

as for the bending, the steel spring that I used to bend some copper water pipe, came from a plumbing supply place, but I think it would be possible to hire as well. It's standard technology. http://www.diydata.com/techniques/plumbing/bending.php

Just as an irrelevant aside, here's why I always try to reply to posts, if I can: even when I "think" I know something, it often happens that researching the query leads me to finding something that I definitely DON'T know - but which may wll save my a$$ another time. Looking for copper pipe bending info, I found that for smaller diameter tube they have EXTERNAL springs. In DIY, just knowing a tool exists is sometimes more than half the battle! Oh, they are called "bending springs" FWIW.

[DanielWong]

Nice work...

How did you do those Moog style enclosures, specifically the pedals?

[charbot]

I had the idea to try the external spring because I have a set for small diameter tube.  I tried using storm door spring.  I even did some research for pipe benders but, never came across the internal spring.  what a cool idea.  I wanna  get one as soon as I have an excuse.

Ive seen that guy's site before... kinda weird. He quotes himself. 
   I tried his "lev Antenna" for linear pitch. with limited results. It's, coincidently, a large storm door spring in a length of pvc. It connects to the oscillator via a ferrite core coil and tuning cap from an AM radio.  He claims that it rotates the field out vertically, allowing pitch to change  with movement up and down the spring /coil.  mine didnt work like that.  I could use the cap to fine tune the pitch, but i didnt notice any other difference just an uglier antenna.  His plan is for another theremin though, maybe that why it didnt work.   


The enclosures for the pedals are also made from scrap medium gauge duct work and pieces of mahogany i get from work.  I cut out the sides and the route out a little notch all the way around.  take a look at my big muff  gut shot.. you can see it on the bottom...

(btw, this is just a little larger than a boss pedal.)
Next, I cut out the metal and bend it to fit around the notch on the side blocks.  This is easier if you have access to a brake.  I didnt, I drew a square line and used a cold chisel and hammer to tap in a score along it.   Ive gotten pretty handy with my "poor man's" brake.  a bench vice and two pieces of angle iron. the irons are placed in the vice jaws and the scored metal is lined up to the edge of the angle iron and the vice is tightened. Next I use a board or something hard and flat to bend down the part sticking out...trying to keep the bend nice and tight and the enclosure faces straight.. make sense?
this is a larger box for a McMeat clone...

and here is how I do the bottom:
I make a bottom plate and attach it with screw in feet, the battery door fits in and is held in by a screw... RAT style....
This one still needs a battery holder or something.  the mcmeat has a RS silver bat clip riveted to the battery cover, but the space is tight in the smaller ones

[booboojerkers]

Hi,

Saw this old posting, so don't know if you'll respond, but here goes:  I also have a Forbes theremin project and i wanted to redesign the power supply.  Do you know what kind of current draw (load) and voltage your unit's power supply ended up having?  This will help me design my own.

Thanks!