Tube VCO uses Theremin Techniques

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I wanted to do something with vacuum tubes again. This example works like a Theremin but is voltage controllable, so any regular synth controller can be used (as opposed to waiving your hands at it). The part that makes it voltage controllable is a frequency modulation circuit. This works by putting a "reactance tube" in parallel with the tank network of an oscillator. The reactance tube is essentially a voltage controlled source of reactance, so the frequency of the oscillator changes linearly to the control voltage. The oscillator and reactance tube in the schematic are made from U1A and U1B, respectively. U2A is the reference oscillator.

The two oscillators run in the hundreds of kilohertz. To get an audible tone from them they need to be mixed. Mixing the two signals is achieved by U3A, which is a Pentagrid converter. It effectively multiplies the two signals, giving the sum and difference frequencies. The sum frequency is filtered out by C13, leaving only the difference. Because the two oscillators are nearly the same frequency, the difference is low and is therefore in the audio range.

I mentioned before the input to the reactance tube is linear. There's a problem with that; musical scales aren't linear. For every octave, the frequency doubles. While it would be possible to tune a keyboard to a non-linear scale, there's a lot more flexibility in using a 1V per octave linear input. How do you turn a linear scale (1V per octave) into an antilog scale? The V/I curve of a diode happens to be exactly the sort of curve we need. This means, if we apply a linear voltage scale to the diode, the current flowing through it will be exponential. Converting back from current to voltage to control the reactance tube is achieved by U2B, which is configured as a high gain amplifier with negative feedback. This operates much like an op-amp with a diode.

First attempt at getting a sound

More stable but no tuning

A little better, but using silicon to convert the scales