Problems with EW build

"- Is it worth paying for the Hammond 1535G coils instead of the much cheaper Miller 6306?"

Yes, absolutely! The precision player will immediately feel the difference in the more equal tone spacing (less compressed field in the high register).

"- I guess the mica capacitor you mention is C1, but the PDF says it should be made of polystyrene. Does it mean that in the new EWs C1 (perhaps also C5) is a mica capacitor?"

Yes, C1, C5 and C14 (the principal frequency determining capacitors of each oscillator) should be silver mica for best stability. There are polystyrene capacitors with similar tolerances and quality at the first glance, but my empiric experience is that the silver mica's temperature coefficient compensates better the one of the coils.

"- Is it worth paying for the Hammond 1535G coils instead of the much cheaper Miller 6306?"

Yes, absolutely! The precision player will immediately feel the difference in the more equal tone spacing (less compressed field in the high register).

Really !?

looking at the specs now... they look practically identical.   What factor(s) make the Hammond coils "better"?

hammond 1535G     bourns/miller 6306

"looking at the specs now... they look practically identical.   What factor(s) make the Hammond coils "better"?"

I have no explanation, just made the practical experience on several instruments. The Hammonds give better linearity in the Etherwave Standard/Plus while the Millers work better in the pitch arm of the Etherwave Pro.

The worst result (very incoherent pitch field) can be obtained on both instruments by mixing the inductors from different manufacturers. 

I guess it could have to do with secondary and/or parasitic resonances or with the way the coils will couple slightly magnetically despite of their geometric distant position...

In ever case there must be a reason why Moog himself used the Millers in the Etherwave Pros and the Hammonds in the newer Etherwave Standards and Plusses.

wow, that's interesting.

It certainly is interesting! - If I get my hands on one of these super expensive pieces of LCR kit I am in the process of specifying for a client, it will be interesting to run a full analysis on both inductors..

One thing I notice in the spec is that there is a large difference in the DC current ratings of the parts - I wonder if the difference in performance could be down to the ferrites and their saturations - this could also perhaps account for "The worst result (very incoherent pitch field) can be obtained on both instruments by mixing the inductors from different manufacturers."- Thierry and strenthen Thierry's " to do with secondary and/or parasitic resonances or with the way the coils will couple slightly magnetically despite of their geometric distant position" hypothesis.

Whatever - Really interesting and useful information (thanks Thierry!) - I have not used the Hammond coils (never saw any valid reason to spend the extra money, from looking at the specifications).. I will now order a few just to try them out on my RCA "clone".

Fred.

Ok, I know the subject of this thread is 'Problems with EW build' but the converastion has turned to coils, which is where I'd like to ask a question.

Original theremin from the Lev generation seem to use coils wound over a former like a tube of some kind, like the lovely coils in Chobbs profile pic. Newer theremin, like the coils (L1-L4) in the EM theremin seem to be of the 'Hammond 1535G' kind (4 posts back).
The question is why use the physically smaller type over the older type?
Is it simply that the 'Hammond type' (ok, I know that there are others of this kind around) are newer technology and can do anything that the old style coils can do or is it current/voltage dependant so that on the older valve sets, the voltages and currents are greater so the coils have to be able to handle the bigger values? Thermal stability possibly? Bigger value induction in a smaller package?

Once again, apologies if the answer is staring me in the face and I'm missing it.

Roy

"Is it simply that the 'Hammond type' (ok, I know that there are others of this kind around) are newer technology and can do anything that the old style coils can do or is it current/voltage dependant so that on the older valve sets, the voltages and currents are greater so the coils have to be able to handle the bigger values? Thermal stability possibly? Bigger value induction in a smaller package?" - Roy

As a designer, one is usually looking to: A) Improve the performance B) Lower the cost C) Reduce the size.

With the possible exception of (A), using smaller ferrite coils achieves these primary objectives. Ferrites are more subject to larger changes in inductance due to temperature change, which is one reason they need to be carefully selected and matched to the other frequency determining components - but if this is done, I do not believe that large coils have any advantage.

The reason small coils are used is not just because "newer technology and can do anything that the old style coils can do" - it is because they result in a much lower cost theremin.

If we look at the component cost, and estimate the other costs for producing an EW for example - then compare this to the retail price - we are probably looking at something like a *6 multiplication factor..

If we add the cost of one large pitch coil (and to truly do the job the old way one needs large volume coil), this would probably clock in at about the cost of the whole circuit board.. Add to this the extra cost of the larger cabinet, extra costs for storage and postage of the larger theremins, and the EW would probably be selling at double its retail price - and I do not believe there would be any noticable improvement for this added expense.

To me (as a designer) the EQ coils are a pain in the backside - whatever form they take! Fortunately, I now think that eliminating them entirely is possible without reducing performance - a few little kinks to bend out, but with a few modifications to Lev's oscillator I believe this can be turned into a self-linearizing oscillator..

But in the meantime, I think the Hammond and Miller (which I have expierience with) coils do the job adequately - I have found no advantage from big air coils.

Fred.

(ps - I dont see any reason why a tube theremin would not work with the smaller Hammond or Miller coils if the components are carefully chosen - But lets face it - Anyone going to the trouble of building a tube theremin isnt going to want small coils! - You cant build a tube theremin into an enclosure the size of an EW - So you may as well put big coils in - One is talking about a whole different philosophy and greater parts cost when one does anything with tubes)

"To me (as a designer) the EQ coils are a pain in the backside..."  - FredM

If you pick mode 0 (series tank, LPF) they aren't nearly the problem they are in mode 1 (parallel tank, BPF) because there is only one resonance controlling things (EQ coil & antenna + hand capacitance).  I believe most Theremins benefit from an EQ coil in the pitch side because the human ear is more sensitive to frequency and can therefore use as much SNR as you can throw at it.  The volume side can probably get away without one (in mode 0 anyway).  (I really wonder about designs that magnetically couple the tank and EQ inductors together.)

I have a kind of love/hate thing going with coils at the moment, I need one in my AFE playing nice (mouser order comes Monday).

" I believe most Theremins benefit from an EQ coil in the pitch side because the human ear is more sensitive to frequency and can therefore use as much SNR as you can throw at it." - Dewster

Oh I agree wholeheartedly - The EQ coil confers great improvement in performance, not just linearity... One of the "kinks" in my ideas / experiments with a self-linearizing Lev oscillator is the loss of the extra antenna EMF which the EQ coil gives.

However, I am not going to indulge and discuss this here - its not related to the topic or Roys question really.. (EDIT -> I am taking discussion about Mode 0 to your Digital Theremin thread..) Apart from which I am not yet certain that I really know what I am talking about! ;-) LOL.. Once I have got further and have a self-linearizing Lev oscillator FULLY running, I will post it on the scratchpad thread.... At this time I really shouldnt have mentioned it, or the fact that I find (and have always found) the EQ inductors a pain..

And yes, I had eliminated them in my early digital designs - but returned back to this linearization method (and analogue implementation) with my tail between my legs.

Fred.

Fred, thanks for the explanation regarding the EQ coils.
To be honest, I hadn't taken the design of the whole unit as a consideration but do see the philosophy that says that if one is building a valve instrument then aesthetically the bigger ail coils provide that much more visual 'va-va-voom' than the smaller ones would.

dewster-modes 0 and 1...I'm not going there but thanks for the offer...I remember a wee while ago asking why the non-linearity in the approach to the antenna and almost regret it...I'm a chemist and make fiddles in my spare time damn it...:-)

Roy