Keppinger theremin tuning

Good afternoon, dear colleagus!

I've finished mechanical montage and soldering of my Keppinger theremin project. No cabinet at this time - cabinet in progress.

I have some question about tuning of theremin chassis:

1) At this moment instrument is worked, but it has very short scale - arround 100-150mm maximum:

First test

Q: How to increase instrument scale (distance between hand and antenna)

2) An attempts to increase the antenna length result in strong modulation of the 50 hertz tone generator.

This leads to incorrect work of the generator.

How can you solve the problem of high sensitivity of the antenna to electrical noise?

All the best!

Hi Ancient,
I do not know the Keppinger in detail nor have experience with it. Analyzing the YT video 1 by FFT there is a splitting of the overtones by additional +/- 100 Hz and about -20 dB amplitude. That means for me, that there is hum from your home installations or from voltage supply that modulates the mixing signal. 

What happens if you cut the big antenna coil and tune both oscillators within the audible difference range? Will the distortions then disappear?

I am afraid, aiding a project what is not located in the own laboratory framework is not easy. 

JP 

Good afternoon, JPascal!

Thank You with Your answer! Yes. The original schematic contains not very big capacity of electrolytic capacitors (47mkF) after kenotron.

Keppinger schematics

 I will to increase it. I think I will to have some free time for experiments after 1-2st apr.

with best regards,

Ancient

Good day!

Yesterday it was not possible to answer in detail - editing on the stage of our new musical does not leave too much free time yet.

Yes, shutting down the large inductor does the trick. Moreover, the result remains acceptable even if a large inductor is connected but no antenna is present.

I will try to conduct an experiment in the near future and replace the kenotron rectifier with a standard bridge rectifier with high values of the capacitance of the smoothing capacitors in the filter.

with best regards,
Ancient

Good day!

Yesterday it was not possible to answer in detail - editing on the stage of our new musical does not leave too much free time yet.

Yes, shutting down the large inductor does the trick. Moreover, the result remains acceptable even if a large inductor is connected but no antenna is present.

I will try to conduct an experiment in the near future and replace the kenotron rectifier with a standard bridge rectifier with high values of the capacitance of the smoothing capacitors in the filter.

with best regards,
Ancient

Wish you have enough time for the musical and look forward to your next tests concerning the Keppinger.

Good afternoon, dear colleagues!

So! After increase the capacity of electrolytic capacitor (power supply) nothing changed!

1) Picture with connected antenna:

test1

2) Picture with unconnected antenna (big coil is connected):

test2

P.S. In fact, I suppose that I need to finish the cabinet, install real antennas, and do all the final tuning on real antennas.

with best regards,
Oleg

"The devil is in the little things" - this is how folk wisdom says.

I suspect little things can play an important role in getting things done:

1) Soviet radio tubes instead of those indicated on the diagram (analogs)
2) Small difference in the diameters of the oscillating circuit coils
3) Small difference in supply voltage (within acceptable limits)

All these "small" little things together give a slightly higher (something about 210 kilohertz)
Perhaps I now have that generator frequency for which the sensitivity to harmonics of smallest amplitudes plays a significant role.

P.S. In fact, I would be interested to hear from a respected @hypergolic - how he solved this problem

with best regards,
Oleg

P.P.S. Adding capacitance from the hot end of the coil to ground (10pF) will stabilize the oscillator amplitude (temporary additional test)

An experiment with a connected coil from a loudness generator suggests that, indeed, the most correct solution is to split the large coil into two and place them in the instrument body perpendicular to each other.

test3

with best regards,
Ancient

Putting a proper small ceramic or air plate capacitor from the hot end to ground could / should be a permanent solution. Also the pitch behavior will be streched. The splitting of the big coil in two not magnetic coupled parts reduces the inductance. I suppose both things, additional capacitance or this splitting, lead to lower total Q. That might be good here, when too much hum is cached in.