RCA THEREMIN

"In simulations at least, this oscillator loads down at antenna resonance a lot less than any other I've simed, including uncle Bob's EW." - Rob

Yes - The Lev oscillator is (IMO) a superb design in every way - the series configuration means that loading only occurs across one inductor, one half of the total tank inductance.. As far as I can see, this means that stalling the oscillator, or going close to "the edge" is unlikely if not impossible.

I think that this does mean, however, that the playable span is reduced - the "virtual inductance" imposed by the resonant antenna circuit is only seen across 1/2 of the tank inductance, rather than the whole tank inductance as is the case with other theremins, so the change in tank resonance frequency applied through this mechanism is self limiting.

but are more than 3.5 octaves really needed? - I think probably not. If one is looking to construct a RCA "clone" (or more ideally, a ClaraMin Clone) then I think we stick with the Lev oscillator.. I think that this is by far the easiest way to implement a theremin front-end that promises good linearity. I would suggest that even if one doesnt end up with a RCA sound-alike, this simple oscillator / antenna circuit is an ideal basis for whatever theremin one develops - Probably ideal for home constructors and "newbees".

Fred.

" Why not just attempt to build a clone and see what does or doesn't work?Surely a clone of an RCA will have to use valves (tubes)?" - MarkT

There are people who have done the above - brave souls, IMO! ;-)

The word "clone" is perhaps the problem.. A true "clone" probably requires tubes and transformers with the same specification (genetics if you like ;-) as its "parent".

This is not what we are (or certainly not what I am) after.

My personal objectives are:

1.) To actually understand whats going on in the original Lev designs.

The fact that there have been such wrong ideas about the design perpetuated for decades, is IMO astounding, for example, the idea that the oscillator was an "Armstrong" oscillator, with the implication that the tank was parrallel resonant, has probably led to a whole generation of theremins being designed and built with parallel resonant tanks.

I suspect that this is the major disclosure to have come from our investigations - But there may be more.

2.) If nothing else, determine the primary mechanisms which give the Lev designs their distinctive sound.

3.) Develop circuitry which allows theremins giving at least good aproximation to the sounds from Levs designs to be built, and for this circuitry to be constructable using readily available low cost components which do not present a hazard to inexpierienced constructors.

Fred.

ps.. If I had the time, money,and a functioning lab, I may well attempt to build a RCA from tubes etc - with the object of having an instrument to probe.. I agree that this would be the best way to really see and understand the operation, and that from this it would be a lot easier to design and build a solid state "replication" -

It would also be useful to have a RCA / Lev theremin to compare any "replica" against.

But as it stands, I can only use what I have access to - a few usable sample sounds taken by microphone from the loudspeaker, almost no samples / waveforms taken from inside the instrument (I have 3 photos of waveforms taken from the loudspeaker connection of an RCA) -

And a simulator without adequate models even of the triodes, let alone the tetrode..

I agree that this would be the best way to really see and understand the operation, and that from this it would be a lot easier to design and build a solid state "replication"

That's really what I was saying, I know where there is a box of valves, a chassis and some other stuff Fred......

Mark,

It would undoubtably be the best way - but I do not have the time or facilities right now to even think about undertaking this task.. the implications of your suggestion are exciting, and I would certainly jump at any offer which allowed me to examine a true RCA or clone anywhere in the UK (and possibly even Europe) - and / or to assist in debugging any construction - But I could not engage in construction.

Also, for me, there are the practical / economic aspects to weigh up.. I have almost no money - I have family in London I could stay with, but I could not afford any lengthy stay anywhere else.

This "RCA Clone" and any dislosure / understanding of Lev's original theremin designs are, as I see it, a public endevour - I would want all information / designs etc to be placed in the public domain freely for everyone to use.. I personally have no real interest in the "clone" other than the challenge of designing it (or being one of those involved with designing it) - My theremin direction is quite different - I believe that my latest design can produce a RCA / ClaraMin / Other theremin emulation now, using analogue additive heterodyning synthesis - but it is a complex expensive instrument, and has only been tested with 3 harmonics generators, which does not allow full evaluation (I believe at least 9 generators are needed for realistic emulation) .. And I have a commercial interest in not disclosing it even if it was simple ;-)

So if you were to build a Lev theremin with tubes, audio transformers etc, I would certainly be happy to spend a week or so in London and pay you visits with my 'scope and audio recording equipment..

I hope it happens!

;-)

Fred. 

"And a simulator without adequate models even of the triodes, let alone the tetrode.." Fred.

Dream on my friend....

I've lost count of how many 12AX7 models I've tried, and every one of them is useless.

I can sim a class A stage just fine, but start to take it out of its linear area, or overdrive it, and all semblance of reality disappears.

"Dream on my friend...." - w0ttm

So much to do - so little time - LOL ;-)

Creating good models isnt really that big a deal with spice - one can draw a schematic using spice primitives, embed maths and tables, define non-linear relationships, then "compile" this to a model..

I have created dozens of Spice models for power components (Isolated MOSFET drivers for motor control, and things like that) - And quite a few models for devices like optocouplers which have non-linear behaviour..

But for every model one creates, one must understand the component fully - and its behaviour when driven outside its normal operating parameters etc.. Even with modern components, this data is usually not available - so one must build a test jig, and measure these behaviours across as many devices as one can manage, and use this "real" data in your model.

And this is where I am stumped. My understanding of tubes was elementary hobbyist level - from recent study this may have moved up a few notches, but is still miles from the level required to even start modelling.. Then there is a lack of data regarding the behaviour of these tubes - even within their specified operating conditions, let alone outside these..

A test jig and a few tubes would be a way forward - but the high voltages would make life difficult.. I have an ancient automated circuit analysis rig (ABI System 8) which I have used in the past to get curves and analysis from components for modelling - But I dont actually own this kit, it is on semi-permanent loan ( I have it in exchange for occasionally debugging a board for its owner ) - I could not afford to replace it if I blew it up while testing tubes.. And I really am not into tubes, so the effort required to build a test rig just aint worth it.

So thats it.. I have tried - I have downloaded software and simulations and gif to parameter converters, have read about tubes and studied volumes of circuit ideas from 1930's magazines, and I could probably occupy the next 6 months on this task, and be no better placed to develop tube models than I am now - and would still have no way to check such models against reality.

I will just have to wait and hope that someone can provide the waveforms seen at SC and CG and the Anode (Plate) of the 224 mixer tube, and on grid and Anode of the 227 Audio Preamplifier Triode, and at the Grid and Anode of the 171A Power amplifier triode.. And good quality recordings from a microphone recording the loudspeaker output.

The loudspeaker recordings over the whole range is probably the most important data I lack - but from a perspective of really understanding what happens in the RCA, the waveforms from inside are needed.

Fred.

And I continue insisting on the fact that the oscillators, the mixer and their waveforms are only of secondary relevance, the "magic" is in the impulse response of the audio transformers.

Fred, don't call me Christopher, please! ;-)

"Fred, don't call me Christopher, please! ;-)" - Thierry

I wouldnt take the chance of insulting either of you by doing that!

Thierry,

I think you may well be right! - I also think that even if you are not 100% right, what you say is entirely relevant - the transformers certainly will have a huge impact on the waveforms and sound.

To be honest, I am completely and utterly out of my depth on this - I do not have any real idea about the behaviour of the tetrode, have never had a 'scope inside any tube theremin, and its been >30 years since I had a scope inside any tube equipment..

I have no usable simulator models .. And, when it comes to models - well, transformer models are about as reliable as tube models when it comes to real-world applications..

The spice circuit you have simulated, IMO, proves nothing - not saying its not "right" - Just saying that it proves nothing ;-) .. How close is your fet mixer to the RCA 224 mixer? - it may be spot on - it may be an abstraction so far removed from reality as to be completely meaningless - Whichever it is, its what it is purely by luck - good or bad - because neither you nor I nor the man on the moon have enough data to put even a vaguely  reliable  model of the mixer together!

As I see it, understanding of the RCA can now only progress by examination of its innards - with pictures of actual waveforms, we can argue (oops - I meant to say "discuss" ;-) with something to base our arguments on.. But right now, well, whatever names we may call each other, we are almost in metaphysical (as in, pseudo science / majik) territory..

Time to hit the spirits I think - not sure what I fancy.. Something Russian I think - Smirnof should do nicely..

;-)

Fred.

ps.. I have just looked more closely at your simulation  - As far as I can see, most (if not all) of the charactaristics of the waveform shown will be the result of the mixer and the transformers contribution in the simulation will be almost nill.. But I will run it with a resistive load (or CCVS) and check..

With a resistive load you will only get an RF burst... The transformer's impulse response makes the waveform.

But I'll stop discussing that for now. No need to pollute this thread with personal animosities. I made the circuit in LTspice today only to show the result of what I had modeled in my mind for a long time (my imagination tells me a lot when I stare at schematics), as a kind of "proof of concept" which is not perfect since there is another triode stage and a second transformer missing. All that may be subjective and/or false. But for me it will continue to be the working model until the proof of the contrary.

Thierry - I Have run some sims:

The above is a fet mixer as your simulation, with transformer replaced by current controlled voltage source (CCVS).

Below is the output waveform..

I then ran several sims using different transformers - The waveshapes (as expected) are radically altered by a transformer - from slightly slanted as with your simulation, to completely distorted..

But - Whatever the transformer - its output will be directly related to its excitation.. The mixer you simulated does not give a sine output - the fets go into cutoff / saturation with the signal levels applied..

This may well be what occurs with the RCA tetrode mixer - But we dont know that - do we?

Fred.