Clara's exacting standards

If this has been answered before, please provide a link! I couldnt find one I'm afraid.

Much has been made of Clara Rockmore's exact specifications for the instrument (and how she was even reluctant to let Bob Moog fix it), but I was curious as to exactly what she was specific about? Tone linearity? Tone? Volume sensitivity? I know it's not great logic, but part of me thinks that if Clara needed even Theremin-made instruments to be modified, then maybe we should be thinking along the same lines.

Thanks,
Chris

Chris,

Parts degrade over time. Tubes lose their exotic gasses, capacitors lose their capacitance (especially the old plate capacitors), etc. etc. Clara did not want her theremin modified. She wanted it restored to what it was. I think the linearity was very similar to the RCA, with a slightly snappier volume response.

As for the tone, I'm sure you know that Clara's instrument (like Lucie Rosen's custom theremin) was provided with many timbres, but Clara used only one of them.

I'm not certain of this, but I think the timbre controls varied the voltage by placing resistors in the cathode lead of the variable pitch oscillator, the fixed pitch oscillator, and the screen grid detector.

The ideal theremin sound, the GOLD STANDARD, is whatever you think it is.

I have posted schematics for both Clara Rockmore and Lucie Rosen's theremins here on this Element-14 Discussion (http://www.element-14.com/community/thread/12071)

Some discussion on the tone control in Lucie's theremin can be found here (TW) ( http://www.thereminworld.com/forum.asp?cmd=p&T=4645&F=715 ).

Tone control on Lucie's theremin consists of capacitors which are switched across the audio output transformer's primary.. And is, in fact, a function of the power amplifier.

Fred.

...capacitors. I think you meant the paper ones? I have often contemplated building an RCA replica. My concern is the supply of the rare tubes drying up.

FEELEEP

[i]"...capacitors. I think you meant the paper ones? I have often contemplated building an RCA replica. My concern is the supply of the rare tubes drying up." -FEELEEP[/i]

Lucie's theremin is not a RCA.. It is the only schematic I have seen where this method of 'tone' control is employed.

My understanding is that Bob Moog replaced these particular capacitors with modern ones - but I may be wrong about this.. Whatever - these capacitors are of varying values, and the tone switch is a single pole 12 (?) way which switches the selected capacitor across the primary (all the capacitors have a common connection which goes to one end of the primary - the switch simply selects which capacitor is connected to the other end).

I do not think that replicating this circuit will be easy - The resonance charactaristics will (I think) be quite a complex function of the transformer's charactaristics and probably the speakers charactaristics.. It is not likely (IMHO) to be a simple function based on the primary inductance || the selected capacitor - all sorts of other factors, such as the speakers frequency dependent loading, will affect the resonance on the primary side.

Once again, I suspect that the apparent simplicity of the scheme obscures its genius - I am sure that this was crafted - probably numerous transformers were tried and capacitor values selected 'on test' to get the most pleasing results.. And, to my ears, they are the most pleasing results I have ever heard from any theremin.

What makes it sound so good? I suspect that the way the harmonic content of the waveform changes as a result of the complex resonances induced by the transformer+speaker, which will be a function of the primary audio frequency from the theremin, will produce a far more 'organic' sound than one could obtain from 'pure' electronics - It is (I guess) the fact that the speaker, acting as an electro-acoustic 'loading' on the transformer, which does this. I suspect that the vintage of the speaker and (by modern standards) its "poor" performance, which actually make this work... Replace the speaker with a resistor, and take the output to a Hi-Fi amplifier, and I doubt that it would sound good.

I suspect that obtaining 'suitable' capacitors will be the least difficult part of 'cloning' this theremin.

Fred.

Fred,

My impression that the tone controls on Lucie & Clara's custom theremins might be related to voltage differences in the cathode circuits came from the well known letter from RCA engineer, M. W. Wills, written in 1930. I had posted a link to it here on the TW Forum, in the discussion you mention above.

It was pure speculation on my part and, as you know, I don't know my ass from my elbow when it comes to these things. Wills' letter is a fascinating piece of history but I haven't any idea if his theories make any sense. I did some experiments many years ago with theremin builder Chuck Collins using Wills' ideas, but the RCA we used was Julius Goldberg's instrument which had already had a number of modifications made to it. Our results were inconclusive. We would have to have worked with a mint condition RCA and the only one of those I have is the Hoffman theremin. There was NO WAY I was going to experiment with that!

Wills Letter (http://www.theremin.info/-/viewpub/tid/10/pid/46)

Hi Peter,

The letter you refer to is extremely interesting (the TW link I posted above has your posting with the link in it) but it is not (as far as I can see) in any way related to either Clara or Lucie's theremins - It appeared to be related to pre-heterodyne wave-shaping, and certainly in Lucie's theremin, tone shaping is absolutely as far from the (heterodyning) mixer as one can get.

Clara's technical notes (edited by Bob Moog) suggest that tone 'control' on this theremin is performed by altering the bias to a tube - Now, I am not 'natural' with tubes, but it looks to me like this is shifting the operating point within the mixer (something similar to how the bias is shifted on some modern theremins at the mixer, so that assymetrical signals are heterodyned, resulting in a more complex waveform) - This would be more along the lines of Wills letter -

Alas, I cannot be sure of what Wills was trying to convey - I am not sure if this is because the quality of communication, schematics etc is poor - or if it is my lack of understanding on the employed technology - or both.. But, bottom line, I dont really understand the contents of this letter well enough to be sure about anything related to it.. One day, when I have time... ;-)

Fred.

Yeah, old tube gear needs periodic re-building to keep working properly. Particularly the large electrolytic capacitors in the powersupply. Typcilal supplies are a step-up transformer, a rectifier, and a string of pi-filters made of chokes or power resistors, and big hairy electrolytics. Newer electrolytics are pretty good, but older ones really were never trustable after 10 years or so. They tend to dry up and the dielecric layer fails and a short blows through. This often results in the cardboard body failing and blowing acidic goo all throughout the chassis. The power supply caps need to be replaced after a decade or so to keep the ripple (noise) low and keep them from blowing up.

Also these old units used carbon composition resistors, which even when new had very wide precision tolerances, like 10% or 20%. Over time these carbon comps can drift and have some resistance value other than what the original designer had intended.

Carbon comps have some neat characteristics though. They are good at swallowing large inrush currents or spikes without failing. They can add some harmonic content to an audio signal because they vary their resistance slightly with the frequency of the signal going through them. But in many cases this "feature" just adds noise.

Yeah, it is totally reasonable that she had the unit "modified" just to bring it back into it's original spec. This happens to vintage guitar amps all the time.

Thanks 'coalport'. I know the tone is 90% in the fingers (having heard several people play Etherwave Standards with various outcomes), but was curious to know why an RCA needed to be modified.

In the past few days I've re-watched the film and re-read the "Lost" album liner notes - the outcome was a snappier left [volume] hand; 5 octaves instead of 3; and Moog's mention of the tone.


I won't pretend to understand what you electrical geniuses are talking about, but remain delighted that there are people keeping the theremin knowledge alive, for when I will inevitably break my instrument :)