Keppinger theremin tuning

Posted: 4/9/2021 1:04:52 PM
Ancient

Joined: 3/29/2021

Good afternoon, dear colleagues!

I had some free time, and I built (as a first approximation) a cabinet for my musical instrument:

Keppinger theremin cabinet

Thick copper tubes were purchased in the store, in the near future I will decide on the method of mounting the antenna in the cabinet!

with best regards,
Oleg

Posted: 4/30/2021 3:13:10 PM
hypergolic

From: Richmond Hill, Georgia

Joined: 9/18/2005

You must select the volume and tone resting frequencies so that harmonics of the tone oscillators (multiples) will not interact. Example: A multiple of the tone oscillator is not close to the volume oscillator frequency. If you don't do this, you will get all kinds of garbage in your sound. The basic design works well BUT it must be tuned correctly. All the tubes on the sound chassis must be shielded types. Use octal tubes! Substituting different style/number tubes will alter the design in terms of transconductance, impedances, MU, etc. Stick as close to the original design as possible or you will run into issues. Mark Keppinger is still around on Facebook so you may be able to interact with the Master.

Did you tightly twist your filament windings? You may also wish to investigate a 'Hum bias" circuit if you have AC hum riding on the output waveform. Remember, these tubes are AC heated filaments.

Orient the chassis mounted coils 90 degrees to each other to prevent inductive coupling or they will interact. An oscilloscope combined with a freek counter are essentials. IIRC my tone freek is around 196 KHz and the volume freek is 443 KHz.

Also, look up "concert tuning" and "bass tuning". Some of caps in the resonant circuits are changed out. I find that concert tuning gives me the best octave range. 

I use full size RCA reproduction aerials so the hand response is excellent. If you use small whip aerials, there is less surface area for interaction. Capacitive coupling is all about surface area and distance and of course the dielectric constant the insulating medium (air in this case).

Fleep

Posted: 4/13/2022 7:58:54 PM
mraroid

From: Salem Oregon, USA

Joined: 2/15/2011

Hello...

I am so pleased to read this thread.  I hope to learn how to tune mine once finished.
Great stuff here!  Thanks all.

mraroid

Posted: 6/5/2022 2:55:59 PM
bcourson

Joined: 4/25/2010

how do I measure the resting vol and tone oscillator frequencies?

Posted: 8/12/2022 2:20:35 PM
Ancient

Joined: 3/29/2021

how do I measure the resting vol and tone oscillator frequencies?

Good afternoon, colleague!

In order to measure the frequency of the oscillator, you can use a frequency meter, or use an oscilloscope.

Even if your oscilloscope does not have a built-in function for displaying the signal frequency, the frequency can be more or less accurately calculated.

Usually any, even the simplest analog oscilloscope has a horizontal frequency limit switch (the "time / division" parameter). If we know in what position this switch is set, we can always estimate how many whole "divisions" fit in one period of our signal under study.

During the measurement, it is best to remove the lamp of the second generator from the panel, and connect the measuring device after the capacitor C15 (pin 5 of the radio tube V2)

with best regards,
Oleg

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