LC Tank / Linearizing Coil Excel Simulation

"If that EQ coil had a transformer winding near the base I'd get it a little more (where the leakage inductance could be doing EQ duty)." - Dewster

Correct me if im wrong - but isnt that almost exactly what the "concentrator" coil is? (a seperate coil fitted at the base [inside] of the antenna coil, series connected [perhaps even via a capacitor] with loose coupling to the antenna coil..)

- I cannot remember, will check.. But I am reasonably sure that I saw this on a Lev theremin, probably the RCA.. And the schematic shows a two-coil EQ inductance if I remember correctly - these drawn like they should be coupled ..

Fred.

Yep - the RCA schematic does show two inductors constituting the pitch control resonant coil - they are connected with a capacitor, and the coil closest to the oscillator is not visible because it sits inside the lower section of the tall coil.. Its called the concentrator coil - and, as you say, its ideally positioned to make use of the leakage.

So heres yet another area of blindly following without understanding.. Copying the tall thin coils without copying the concentrator coil.. and in so doing losing the advantage of following Levs brilliant engineering, but paying the high price required to do so!

Slowly, layer by layer, I think we are at last starting to understand.. Every little piece, when it clicks into place, is so damn obvious.. Thinking about it, why has so little attention been paid to this concentrator coil? Back in those days a coil like that was no simple off-the-shelf part like a 6300 series inductor - Lev would not have included such a part in the crudest theremin he ever developed if he didnt know it was vital! ... For me, its time to look at every component, and be sure I really understand what every coil and capacitor and resistor and (most difficult) tube REALLY does.

One interesting thing I notice when looking at Art Harrisons RCA Theremin data (which shows the concentrator coil, on page 4) is that..

this is "a small phenolic-core "concentrated coil" ...... But right in the centre of this coil, through the phenolic, bolting the coil to what looks like a phenolic mounting "shelf" inside the main air coil, is a.... METAL SCREW!

"Yep - the RCA schematic does show two inductors constituting the pitch control resonant coil - they are connected with a capacitor, and the coil closest to the oscillator is not visible because it sits inside the lower section of the tall coil.. Its called the concentrator coil - and, as you say, its ideally positioned to make use of the leakage."  - FredM

Oh, thanks very much for that Fred, it actually makes quite a bit more sense now and restores my faith in (and displays my ignorance of) these early instruments.  It would seem perhaps easier to put this "concentrator" stimulus coil on the outside rather than the inside, but whatever. 

Does anyone here know how tight the inner winding is physically to the outer winding?  My sims show something like this working pretty well with middling to good coupling (k>~0.7) but to obtain even OK coupling I believe the inner coil has to be pretty close to the outer winding in order to share significant flux, and the windings of the inner coil might have to be somewhat spread out if it is constructed of thin wire in order to share more common area.  A gap of two mm between coils might be OK, but much more than that would give poor coupling and perhaps poor results.

Here is a photo: (from Art Harrisons report)

So I would imagine coupling is loose..

But - Its sure not easy to visualize what (if anything) is really going on with regard to the effect of this coil and its position / construction.. To me, it brings more puzzlement than answers..

We have an antenna-side air coil with its low capacitance.. Feeding that, via a big capacitor, we have this little (probably quite high inductance - I do not trust any of the figures given) multi-layer coil with a screw of some sort in its core.. if this screw is iron, this will have a big effect on the coils inductance and probably on its coupling.. But at this point I am way out of my depth.

You see, to me, a screw (of any metal composition) looks like folly - it will have some effect on the inductance - and this effect will be temperature dependent - Its got to be worse than the temperature issues one has from a good ferrite I would think...

But this was designed by Lev, the RCA's dont (AFAIK) drift.. But a lot of care was taken by Lev (and those constructing his theremins) to compensate temperature variations by doing select-on-test capacitor swapping...

Fred.

the screw is nickel plated brass- like all the other RCA hardware.   This screw is just holding the coil to the wood plate inside the large coil.  it should be about 20mH.  Mine (from the radiola 60 is 18mH ) -

Also FYI-  brass screw vs steel screw vs no screw  makes no measurable difference here-   I recently changed this coil on mine after meeting with Mike Buffington.   I had previously been using something else for the concentrating coil and he recommended using  the one left over from the from the #60 carcass- which as far as i understand -is the same coils as the #1264 (only with an added center tap)....-  Anyway -I could not find any brass hardware,  only.... "gulp" - stainless.   I 'knew' this was a no-no  but out of desperation i tried it anyway, and hooked up my inductance  meter to quantize the damage.   Steel screw or no steel screw, the measured inductance didnt budge! **    I recon that this is because it is wound on a 1/2" form and the #6 machine screw has about 1/4" of air between it and the windings. 

**ill bet that it probably did increase slightly- but was under the range of my scope and well within the tolerance of the component.

I'd assume that the dual coil construction is basically just to 'add them up' and achieve the desired inductance range.   For what ever reason a 50-60 mH air coil would be unpractical(...obviously it wold too large and unruly  but also maybe coil winding mechanisms could only handle up to 3" forms and 12.5" lengths?)

I havent done this on my RCA-  (tried it on the kep   tho)...but   Id be willing to bet that if one would replace the large-original antenna coils with some small bourns 63xx's in series-  equaling the proper inductance value(50-60mH)- there would be little or no noticeable change on linearity or range. 

Hi Charlie,

"the screw is nickel plated brass- like all the other RCA hardware.   This screw is just holding the coil to the wood plate inside the large coil.  it should be about 20mH.  Mine (from the radiola 60 is 18mH ) -"

Actually, Brass should / will change the inductance.. It is a metal commonly used in unductor tuning slugs  .. GOOD stainless steel probably has less effect.. ;-) AFAIK, brass reduces the inductance.

But whatever - I suspect that Brass is a lot more stable than iron when it comes to temperature.

Thanks for the info though - It a bit dissapointing, would love to find that the coil was optimally placed and had a specifically selected screw which did something brilliant like focussing the magnetic fields or compensating for temperature.. But perhaps I need to check my faith impulses, and remind myself that Lev was probably not an ET, LOL ;-)

Fred.

.. I do still have a nagging doubt .. Why go to all the trouble of constructing a shelf inside the antenna coil, if the concentrator coil could have been simply mounted on the chassis beneath it?

Also RCA's are reputed to have a wide range of discrepancies in the antenna coils- The 'experts' at RCAtheremin.com  report that many seem to have had windings removed and would indicate some factory tuning process.  

I cant go into details so this is just a teaser, but ....... Ive had a sneak peak of their (rcatheremin's) report of repairing and documenting Clara's customized RCA- hot-rodded by the master, himself.   Very Interesting!-  

I believe nickel is ferromagnetic.  Reading on-line, brass may contain some nickel and may be paramagnetic or diamagnetic.  (This is what kills me about "magnetic thereapy" - oxygenated blood is actually slightly repelled by magnets, and the "refrigerator magnets" generally employed have essentially zero penetrating power - lose/lose.)

".. I do still have a nagging doubt .. Why go to all the trouble of constructing a shelf inside the antenna coil, if the concentrator coil could have been simply mounted on the chassis beneath it?"  - FredM

I suppose so you get some flux coupling?  It might be physically easier to mount the inner transformer & capacitor there than anywhere else, and the slight magnetic coupling might have some benefit (if the windings are correctly oriented).  The photo you posted above seems to show a weakly magnetically coupled auto transformer with capacitively coupled (?) windings. 

It must have been a bear developing the first Theremins without simulation!  Where do you start?  Where and when do you stop, knowing it is sufficiently optimized?  (I have this second problem, so simulation isn't necessarily a panacea!)

"It must have been a bear developing the first Theremins without simulation!  Where do you start?  Where and when do you stop, knowing it is sufficiently optimized?" - Dewster

This is what drives my respect for Lev (and all the early electronics pioneers) probably more than anything else.. They must have been saturated in the fundamental physics and mathematics, have studied and understood their field in a way we dont see today..

Today, inductors, transformers, capacitors and the related science / physics is only touched on when one does an "engineering" degree - We have such a diverse range of things to cover, that everything is shallow - oh, still demanding (because there is so much to cover) - but I think that the hands-on approach and the requirement to calculate without electronic computing (pen, paper, slide rules and tables being the tools) turned out "real" engineers - and filtered out those who were "sub standard" (as I would have been, LOL ;-)

"The photo you posted above seems to show a weakly magnetically coupled auto transformer with capacitively coupled (?) windings"

Its just an inductor, connected to the antenna coil by a capacitor - effectively this inductor is in series with the antenna coil... Not even sure if the assembly could be classed as an auto-transformer.. perhaps slight coupling which would give a slightly higher total inductance than if they were completely seperate, but it doesnt look like the coupling would be significant.

You are probably right - the placement and shelf is probably mainly for convienience - somewhere to tuck the coil so it didnt interact with anything else, and didnt take any chassis space.

Fred.

I agree, if there is any coupling it is likely negligible.  Though it's on the correct end (or at least the end I would put it on)!

Playing with the inductor design worksheet some more, there appears to be a shallow minimum of self-capacitance for coils with heights around 2x their diameters.  Even at 2x the wire use isn't too inefficient compared to the optimum.  This is based only on skin depth though (with a crude relative permittivity correction) so it's not the most accurate in the world, but still it's interesting and perhaps a clue.  Any capacitance reduction here is likely worth doing, because an EQ coil with too low of a self resonance frequency can cause much more harm than good (in my very limited experience with them).