Looking at a tube theramin design

Posted: 5/28/2019 5:04:55 PM
Wrnchbndr

Joined: 5/28/2019

Is the 1961 Electronics Illustrated theramin a worthwhile design? It took me about an hour to find sources for all of the parts. I've built about seven different guitar tube amplifiers using little more than the available on-line schematics so I feel perky enough to follow these 5 pages but I faced with making an investment is a lot of parts that otherwise I'll have little use for if the project goes south. Old designs can have limitations but half of my incentive is to be able to say its an old "vintage" design for what ever that is worth. Does this design work well? I like the idea of needing to do this from scratch.

Posted: 5/28/2019 5:24:55 PM
Wrnchbndr

Joined: 5/28/2019

Doing a search for this subject in this forum, I found a few references to the 1961 design that mention crystal oscillators. I don't see a crystal oscillator in the parts list or the schematic. The only crystal mentioned is a crystal diode IN34A which I assume is just a plain diode in function. Is an IN34A equivalent appropriate? and... This is what I'm looking at for the oscillator coils:
https://www.tubesandmore.com/products/coil-oscillator

Posted: 6/8/2019 10:39:03 PM
jgdpeng

From: Canada

Joined: 3/7/2019

My quick review of the 1961 design uncovers the following:
·[font=Times New Roman]        [/font]L1 and L2 are standard oscillator coils, 455kHz centre of range.  A vintage RCA Theremin operates around the 170kHz range.  I have recently completed a theremin based on these 70-OSC coils but would build the next one resonating around 170kHz as I think this frequency would give better control range similar to the RCA Theremin design.
·[font=Times New Roman]        [/font]C1 is not tunable as a front control therefore, you will find it hard to adjust the tuning on the fly as the need to adjust will change with component drift and the proximity of the musicians body to the instrument.
·[font=Times New Roman]        [/font]The power supply voltage will be approximately 300VDC.  DANGEROUS considering the antennas are exposed electrodes.  In addition, the chassis is used as the circuit neutral.  There has been a major revolution in grounding and neutral practices since 1961 and a good resource to study this is in “Building Valve Amplifiers’ by Morgan Jones.  The topic is called ‘Earthing’.
·[font=Times New Roman]        [/font]And the antenna geometry can be whatever you want to do within reason, although I do find the design shown in the 1961 article rather bizarre.  Be aware the 1961 design does not include antenna coils!!!  This is setting you up for failure.  One needs to ‘tune’ the antenna to the frequency of the oscillators to maximize the sensitivity you will crave.  (Vintage Theremins all have antenna coils).

Study the schematics of the RCA Theremin and see if you can model a design based on it.  Or consider the Keppinger Theremin design offered on Theremin World.  I do not have experience with either of these designs but I would consider their attributes.

Finally, I do offer the design I completed and you will find it at www.waytoomuchtime.ca however, please consider the improvements and design changes I offer.

I don’t mind offering my experiences to you.  Let me know if you wish to discuss further. 


Posted: 6/20/2019 6:06:39 PM
rupertchappelle

From: earth

Joined: 5/8/2017

Arthur Harrison Tube Theremin:

https://www.theremin.us/126/126revised.html

You cannot go wrong with Harrison.

Be sure to use plate antennas if you wish to play music. Otherwise - vibrato, vibrato, vibrato.

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