AETHERPHON, global theremin family

Philip,

Is a schematic of the Keppinger theremin available?

Do all the oscillators have this problem or is it just your variable  
frequency oscillator?

If you disconnect the oscillator from the detector does it oscillate  
strongly?  One wonders if there is a leaky capacitor or something that  
is robbing energy from the tank.

Normally, one expects the oscillators to couple -- is it possible that  
instead of coupling they are phase-cancelling each other out?

I've not spent appreciable time with vacuum tube circuits however I'd  
check the individual oscillators (that is, I'd deactivate all but the  
oscillator I'm testing) and determine if the issue is with an  
individual circuit or is some kind of interaction.

All the best... keep us posted.

-- Kevin

Quoting Philip <kmg5443@...>:

> When I was building my Keppinger theremin, I recall that it sounded   
> wonderful. What was not wonderful was the issue that the oscillators  
>  would not keep oscillating. I could momentarily restore function by  
>  grasping the large coils. The suckers would work for a minute or so  
>  and then quit. I believe the X factor, be it the transconductance  
> of  the tubes or my own unique and extensive personal magnetic   
> bio-field, somehow prevented my theremin from working correctly   
> (perhaps an unrealistic expectation). Permanent dunking of the unit   
> in the nearby Atlantic Ocean was almost realized due to engineering   
> frustration on my part.
>
> I have a WW2 era tome, Radio Engineer's Handbook, by Frederick E.   
> Termen, Sc.D. I wonder if he was related to Lev? Anyway, one of the   
> items discussed under in oscillator chapter was that the time   
> constant formed by the grid shunt bias resistor Rg and the coupling   
> resistor Cg to the tank was important.
>
> The text stated that if intermittent operation of a Hartley   
> oscillator is experienced, then reducing the time constant Rg x Cg   
> may be helpful. In essence, the tank is not getting enough "kick" to  
>  keep it running. The text also stated that the preferable method of  
>  accomplishing the reduction was via decreasing Cg, and NOT Rg.    
> Reducing Rg would change the bias operating point of the tube.
>
> The initial values were Rg = 100 Kohms and Cg = .001 uF. I had   
> reduced Rg to around 80 Kohms and voila!, the sucker worked.
>
> Now memory being what is, I seem to recall that the theremin sounded  
>  better when it was intermittent. I posit that my changing of the   
> grid shunt resistance, not only shifting the bias point of the tube,  
>  but also changing the tone of the instrument. In retrospect, I   
> perhaps should have reduced the value of Cg. My napkin calculations   
> indicate values in the neighborhood of 750 pF with the original   
> 100Kohm shunt. Obtaining a variable cap in that value is not   
> practical. So, one needs to tack solder a range of values from 470   
> to 750 pF and see if the intermittent operation is cured and the   
> sound is better. Mica caps in that size are cheap.
>
> My primary complaint has been lack of character in the tone, i.e.   
> the waveforms closely resemble a sine wave at some points. The high   
> frequency harmonic issue has been largely cured by careful selection  
>  of the two oscillator frequencies so any mixing products are  
> outside  the human range of hearing (i.e greater than 20 KHz), and  
> careful  selection of the volume oscillator bias point. If the bias  
> of the  volume oscillator circuit is advanced too much, i.e. more  
> volume for  a given hand position, "fuzzing" of the tone at low  
> frequencies  (approximately 500 Hz and lower) is noticed and is  
> objectionable,  rather than what one would expect from pleasant  
> overtones.
>
> I am a quintessential Electrical Engineer. I read this text while on  
>  the throne at my day job.
>
> Opinions?
>
> Thanks
>
> Philip Neidlinger
>
>
>
>
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