>
> You didn't mention if the problem is with the fixed or the variable
> oscillator...
>
> Interesting that it oscillates when you touch the coils -- to touch
> the coils would reduce the frequency of the oscillator, correct?
>
> Could you use a 'scope to check the grid bias? Would be interesting
> to see what happens when you touch thc coils and, in particular, to
> see what happens as the oscillation stops. Is the bias hitting "the
> rails"? -- might suggest that the bias resistor value needs a tweek.
>
> It seems, though, that to tweek the capacitor value (as you wrote)
> would be a little more direct than to mess with the grid bias
> resistor. Would be interesting to put a variable resistor there and
> to observe the effect on a 'scope.
>
> It is really amazing that the early electronics pioneers figured all
> this out without 'scopes.
>
> (As you can see, my approach is that of a tinkerer -- I just hook up a
> scope, tweek parts, and see what happens.)
>
> -- Kevin
>
>
>
> Quoting Philip <kmg5443@...>:
>
> > Just my particular setup. Probably something somewhere didn't gel
> > quite right, wiring layout, who knows.
> >
> > However, excessive shunt resistance is a well documented phenomena
> > for oscillator irregular operation.
> >
> > The schematic is on the kepptheremins yahoo group, and Marks
> > original is on my site at
> >
> > http://wwww.neidlinger.us/keppinger.htm
> >
> >
> >
> > --- In aetherphon@yahoogroups.com, kkissinger@ wrote:
> >>
> >> 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
> >> >
> >> >
> >> >
> >> >
> >> > ------------------------------------
> >> >
> >> > AETHERPHON, the global thereminist community
> >> >
> >> > To contact the moderator, e-mail porphyrous@
> >> >
> >> >
> >> > Yahoo! Groups Links
> >> >
> >> >
> >> >
> >> >
> >>
> >
> >
> >
> >
> > ------------------------------------
> >
> > AETHERPHON, the global thereminist community
> >
> > To contact the moderator, e-mail porphyrous@...
> >
> >
> > Yahoo! Groups Links
> >
> >
> >
> >
>
