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Re: Van der Pol



Original poster: Ed Phillips <evp@xxxxxxxxxxx>

Hi Ed,

I don't have a schematic at hand right now, but as I remember it was a coil-less oscillator that operated because at a certain ratio of voltages between the grids the tube goes into a "negative resistance" region where increasing current causes decreasing voltage, then at a certain point switches back to positive slope causing oscillation. I think there was also a circuit in my dad's "Fundamentals of Radio", (Jordan et al c.1942) and there was a Federal Telephone & Radio reference book from the same era that had a schematic but no explaining text, IIRC.

Matt D."

I now know what you're talking about; it's a secondary emission phenomenon and, in answer to your original question, this would not be a suitable circuit for driving a TC; very inefficient and low power output.. The effect you're talking about is well understood and most tube makers went out of their way to eliminate it. It was a way to make a fairly stable and simple low power oscillator for use in a wavemeter or signal generator and had the convenient property that only two connections were needed to the LC circuit and you'll find circuits in old handbooks and text books. Often called a "dynatron" oscillator. In the US a typical tube for this application was the UX-222, the first commercial tetrode or "screen grid" tube. Used to play with them long, long ago.

Ed