RCA replica- Volume circuit feedback

Hey all. I've built this theremin and is sounds pretty good, but has a major issue: the volume circuit feeds back into the pitch circuit and changes the pitch when I change the volume. This is very annoying, this thing is hard enough to play, no? I turned the volume resonance coil perpendicular to the PRC to no effect. I added a 10mH choke to the B+ of the volume circuit to no effect. I thought of shielding the PRC or the Volume oscillator coil (already under the chassis)next. Any thoughts? P.S. I have built the Clara Rockmore Theremin, but it's still in R and D--tricky volume circuit. This has the same problem.

Not a lot to go on there. Can you post some pictures of the layout? That might allow some of us to suggest something.

The layout for Clara's theremin was very unique ... there's a picture of it somewhere on TW, I think. It doesn't look the same as the RCAs.

Your schematic would also be helpful.

Don

I'm building a replica of the RCA and it's a real dog to find parts... getting most from old antique electric supply houses. It's not the parts or knowledge involved, it's the "TIME" it takes to do it! Ross.. Bellingham, Wa., Radio Museum.

Organ Man, I would really like to see your schematics and info on replicating Clara's Theremin?
ROSS 1-360-421-7195

[i]"...the volume circuit feeds back into the pitch circuit and changes the pitch when I change the volume."[/i]

This can happen with any theremin.

The volume's variable oscillator is heterodyning with the pitch's fixed oscillator. Even if the fundamental frequency of your oscillators are different, an overtone of one oscillator can heterodyne with an overtone or fundamental of another.

I would suggest that you tweak the frequency range of the volume's variable oscillator such that movement of your hand to and from the loop does not impact the pitch oscillator. Then, adjust the frequency of the volume's fixed oscillator to achieve the desired response from the volume side. [i](Sorry, I'm not familiar with the volume side of your theremin -- I am aware the some designs use the same fixed oscillator for both sides. In such a case you may have to sacrifice some dynamic range to find a frequency range that doesn't interfere with the pitch side.)[/i]

Hypergolic (aka Philip N) may have some comments for you on this. He spent untold hours adjusting his Keppinger (vacuum tube) theremin to find the frequencies that worked for him.

I have believed for some time that Clara's theremin was designed so that the volume side distorted (coupled) the waveform on the pitch side -- not enough to change the pitch but enough to change the timbre slightly. The "trick" may be to find the exact frequency range for the volume's variable oscillator.

Feel free to share more info on the design of your theremin.

To turn your coils may help a little -- bear in mind that hetrodyning can occur at great distances. An issue with multi-theremin concerts has to do with the theremins interference with each other at relatively long distances.

hey all, thanks for your input. I haven't checked in a while or been able to work on my theremin at all. Here's some information for you all.

My schematics were found at this website. I have put in a push pull amp using 2 45's and used a 45 for the volume oscillator. I also added the pot on the cap grid of the 24A in order to adjust the range/tamber, as is done in clara's instrument.

The coil layout is the same in my chassis as the RCA 1264. I've also used this layout in the clara instrument.

Tonight is amp/theremin night at my neighbors so I will be putting a grounded shield over the volume coil to see if that does anything. If successful, I will add a shield to my finished RCA copy.

Parts: I got all my parts at Antique Electronic Supply. I made the coils and antenae, and the case myself.

I have an album on facebook of my rca project: http://www.facebook.com/
album.php?aid=2023540&id=1154072847&l=bdaa2cd3f6

Let me know if you want to see more or if I can provide any more information.

Thank you all for your suggestions and help. I won't wait so long to look at this post next time. Off to look for that photo of Clara's chassis.

ba

Wow, that is amazing work - would love to hear how it sounds.

Wanna build one for me? :)

Thanks, It took a long time. Are you sure you want one like this or one with the fast Clara Rockmore volume circuit?

Well, the Clara volume circuit certainly would be nice, although I doubt I will be playing the Fuilehan concerto anytime soon!


Kevin - didn't Bob Moog publish his service notes to Clara's theremin? I thought I remembered seeing them on the WWW years ago. Maybe they would have the information about the tuning?

Hey, I am trying to build one, but, man, I'll die of old age first I think???
Can you make me one?
I'll pay installments each month till you finish. I want the fast Clara volume added.
How much? I want the old big UX tubes too.
ROSS MARSHALL
( seriously )