What do you do when you really want a vintage theremin but just can't ever find one for sale? If you're like Dayfan, you build your own! He's just posted an amazing article detailing his construction of a semi-replica RCA theremin. Enjoy, and thanks Dayfan!
Link: Constructing a Semi-Replica RCA Theremin
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30 Comments
Thanks
Philip
KA4KOE
I would like to work up the cojones to actually build one myself but I am not much of an electonics whiz so don't hold your breath.
If anyone decides to make these for hire I wil be first in line...
Does the solid brass have a tendancy to wiggle or move around?
I have heard that that can be a problem with solid rods instead of tubing.
What an accomplishment!
That you have the knowledge and ingenuity for such an undertaking is very impressive.
Well, count me among those who are clamoring for audio samples.
very nice construction.
thanx for building it
If you would like to discuss tone/timbre mods send me your email address.
Mark
Best regards,
Andy
Thanks for your efforts.
Philip
I am preparing a short article on the measurements I made. Some need explanation.
Mark
Anyway, Kudos for you efforts.
Philip
What an amazing accomplishment!
I am trying to better understand the operation of V5, the volume tube (originally UX-120).
You mention 2 volts on the filament of the V5 (31, replacing the ux-120). Is that the maximum voltage on that filament from T3?
Did you set the plate current through V5 to 2 ma per the rca service guide? I am curious about this current because the rca document also says this plate current is max 4.1 ma. They seem in conflict, to me.
Sounds like you made a good choice in substituting the 3Q4 for the 120 volume tube.
bret
These are the measurements I took on a real RCA Theremin. Filament Voltage for the 120 Tube Measured Across T3, terminals 3 and 4. All have 1 volt minimum to maximum change:
No load 2.5 volts maximum
120 tube 1 volt maximum
1 megohm 2 volts maximum
T3 is essentially just a filament transformer that supplies power to the 120 or substitute. As more current flows in the primary the voltage in the secondary or filament winding can increase thereby increasing the heating of the filament in the tube. The hotter the filament the more current can flow through the tube from B+ to the plate of the audio amplifier tube and the louder the volume. I wouldn't worry about the current that the RCA manual says should flow, just tune the oscillator so that only the amount of current necessary to get the volume you need can flow. As you can see from my measurements you can't get more than a volt anyway. I added a switch to my Theremin so that I could theoretically drop the filament voltage by one volt at the published filament current and it makes no difference whether a resistor is in series with the filament or not. This is in reality a rather crude way to control volume and seems to be the legacy of how volumes were controlled in the early twenty's radios.
Mark
No load 2.5 volts maximum
120 tube 1 volt maximum
1 megohm 2 volts maximum
--------
Mark,
Thanks. Are these peak to peak or rms? Since this is a 430khz signal, Did you use a scope or an rms meter?
bret
I used a RMS meter. My guess is that that is what RCA would have used as well as the average serviceman. An interesting question for sure.
Mark
I think it is interesting to think about the meter used, especially in light of the comment regarding the UX-120 filament voltage in the RCA service notes:
http://www.thereminvox.com/article/articleview/26/12/2/
Tube No. Filament or Heater Volts
5* UX-120 unreadable
bret
I checked the reference and believe that the meters used back then having very low internal resistance would not be able to read anything. I used a digital meter with a high internal resistance with little load on the circuit. Notice how sensitive to load the readings are.
Mark
Great theremin. I've been collecting the parts to do a full replica for about two years now, but the project hasn't really taken off. Now that I have more time and supplies, it's a different story...
I have a few questions, but what I really would like to know is where you found a 14x8x3 project box. Hammond MFG. doesn't make these, and they are the only big supplier I know of. Any help from you (or anyone!) would be greatly thanked!
A cabinet made to the same dimensions as an original RCA (if you're planning to use Mark and Jeff's marvelous RCA cabinet plans found elsewhere on this sight) will accommodate a chassis measuring up to 17 inches long and 6.5 wide, maybe 7 although that would leave minimal room for the tall coils and might interfere electronically with them. If you're not planning to recreate a cabinet to RCA dimensions, the 8" width of the semi-replica RCA chassis shouldn't be a problem. A sheet metal shop likely to be found in any decent sized town, can make a chassis to your specs, type and thickness of metal, etc. You'd need to make a cardboard or paper pattern to serve as a guide. These shops typically make air conditioning and heating ducts, so they're used to custom work at reasonable prices.
Thanks for the advice. I wasn't sure how wide I could push a to-spec original cabinet. The idea for my project is to make the finished item as close as possible in outward appearance to the original RCA version.
I have already gathered the required tubes to make it possible, including a few UX-120 tubes, however I am considering adapting a 3Q4 tube to stand in for the 120 during initial power-up and typical use.
That being said, I just want to verify that wireing an adapter for a 3Q4 to replace a 120 only requires connecting the suppressor and screen grids to the plate.
One last question for anyone who might know: are there any sources of 5" paperboard tubes anywhere? I see Dayfan couldn't obtain any, but I have been on the lookout for something with an O.D. to match the original spec.
Thanks again!
Another question: I've looked through the documentation and perhaps I am missing this, but nowhere can I find wattage values for the resistors used in the circuit or the values of L1, L2, and L3 (inductors) used in the power supply not to mention their individual resistances (Art Harrison's page states a 1000 ohm series resistance as per the RCA Manual).
Hammond Manufacturing still makes "classic" inductors so I am trying to figure out what combination of items I will need to get the circuit built.
Thanks again for everyone's help!
http://tinyurl.com/y...
Nominal 3 inch mailing tubes are about 3-1/8 inch o.d. and work fine. That is what I used. I shortened up the coil a bit to compensate using an inductance calculator (google one)to back calculate the winding length. Calculate the inductance using a 3 inch form and then substitute the larger diameter to get the coil length for the same inductance. The chokes in the power supply are not critical, I used a couple from my junk box. A rule of thumb is that heavier has more inductance. The Theremin doesn't draw much dc current so it is not critical. You can used larger filter capacitors than they used back then because they are easier to make now. The reason for the pair of chokes is because big caps weren't practical back then. Now days chokes are not used and only big capacitors (e.g. 40 mfd) work fine. The only thing critical to the Theremin is to have nice smooth direct current.
Mark
The knobs are exactly like the originals. Unfortunately they came from my junk box. This does indicate that they were probably a commercial item or were used on other RCA equipment so they could be found at ham or antique radio swap meets. I also have several knobs the same style only bigger.
Mark
Thank you for the pointers. My project is moving slowly and right now I'm waiting for one of my friends, who has access to a transformer winding 'lathe,' to wind the transformers with me when he gets some time. I've seen a repaired theremin mentioned here, I believe, where they were able to get the original white cloth-covered wire. Now that would be a trip to use something like that, but I'll be using magnet wire like you did. Easier to work with, and it's everywhere.
Thanks again,
Jacob