DIY: Construction of a Semi-Replica RCA Theremin

Andrew,

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!

I'm pretty sure that 5" is a typo. it should be 3" diameter.

Ok, thanks. Yes, I've checked the component values sheet and it specifies 3" so now I'm looking for 3" O.D. coil forms and have contacted a paperboard place.

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!

Regarding 3" OD tubing, I see this at ebay
http://tinyurl.com/yyw82c

Those would be sweet coils. I could imagine them in an all plexiglass theremin.

Thanks for that link! The tube sockets and anode cap came in a few days ago, so the parts are piling up. Still, does anyone have an idea on values for those inductors in the power supply? It's probably not super-critical (for power filtering only) but I'd like to get it right. I guess the proper question should be: how many mA must the power supply unit supply to the theremin?

Londo,

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

Mark, looking back at your article now, I see that your pitch and volume adjustment knobs are pretty close to the original RCA knobs. Were they in your junk box already? If you bought them new, can you provide details?

PK,

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

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