Re: Van der Pol

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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