RCA Theremin Infomation and Pictures

The connections for making a 120 to 3Q4 tube adapter are:

120 tube 3Q4 tube
Pin 1 to Pin 1, filament to filament
Pin 2 to Pins 2 or 6, plate to plate (these are internally connected so it makes no difference which one)
Pin 3 No connection
Pin 4 to Pin 7, filament to filament

The rest of the pins on the 3Q4 are not connected to anything. Pins 3 and 4 are two of the grids which are allowed to float. Pin 5 is the center tap on the filament. This is so you can use the tube on 1-1/2 volts which we aren't doing. You could try connecting these grids to pins 2 or 6 to see if it makes any difference. Note that one grid (pin 5) is already internally connected to the filament. Don't worry about this grid. The tubes are both connected as diodes so it probably makes no difference. Leaving the grids unconnected worked for me. I used an old 4-pin tube base as the base and stuck the 7-pin miniature socket inside. Wire the socket with long bare wires and stick them in the appropriate base pins and pull them down and solder.

Mark

Mike

In your estimation, how did the sound of your repro compare with the original?

Philip

The sound of my reproduction is identical to the sound of an original. When I first built it I used two modern interstage audio transformers but all the rest was electronically and as close as possible physically identical to the original. I subsequently replaced the modern transformers with the same transformers that RCA used in the original and the sound was unchanged. The interesting thing is that a significant amount of "tuning" is possible by swapping the oscillator and mixer tubes to get a sound one liked (as suggested by Andy Baron). Some tubes sounded horrible and the old "balloon" tubes sounded better than the more modern "ST" tubes. By swapping tubes one could make the Theremin sound really bad or really good and this is typical of original RCA's as well.

Mark

I would say that my sound is now pretty close to an RCA, though I still need to do a lot of tuning still. I had discovered a soldering error that corrected that sound that can be heard on the video that I posted. The range is now reduced, but the tone and volume is better.

All my components are identical, including the 2 GW-42s and power supply. Tube swapping hardly changes the sound of my machine. Though I understand that it's an important factor. All of my tubes test excellent on a Hickok 539C.

I think it's good to point out that even among RCAs the sound can very. Andy Baron has spoken of RCAs, which after some restoration, that have the dreamy, ideal RCA sound, while his own RCA, which is factory original and had the extra resistor on the grid cap of the mixer tube, sounds different.

Philip,

Are you desiring a more faithful RCA sound from Gabby? Or are you looking for another theremin?

Mike

Mike,

What value concentrating coil did you use? This may influence the range.

Mark

No, I'm pretty happy with Gabby. Lydia liked it!She has her own unique sound. My myspace page, panelectronica has a clip I recorded last night.

I'm just another enthusiast who is curious about your work.

I can't afford another theremin right now anyway. I just dropped a year of my time building this one and I have no idea how much cash I have put out, but if I were to venture a guess it would be in the 2K to 3K neighborhood by now. Go to www.flickr.com and look up panelectronica and see the final cabinet pictures.

Also, my machinist friend Rudd Long, who made my exact repro antennas from Jeff's drawings, has offered to manufacture custom RCA hardware for anyone who needs it. All he requires is a drawing and your patience. He does this for a hobby and is always looking for something different.

Good work man.

Philip

So I came across a cat who has a bunch of surplus gear. All radiola items. Forms for coils chassis' and tubes. If anyone is interested this guy is probably someone that can help. He even mentioned another way to power it so as to get more to the speaker. He sold me the RCA-106 that I will be using for my RCA. He has several of those laying around as well as 100, 100-a's and pretty much a plethora of parts from the era. SO let me know if you want to get in touch with him and I'll get you two together.