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Originally Posted by astral highway
Nice demo, Russell!
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Thanks, Al.
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I like the breakout pattern of mechanically-excited coils, it is distinctively more random than with solid-state modulated or valve-powered circuits, breaking out above, below and to the side of the top-load (toroid, typically). What did you use a a spark-gap?
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The spark gap is three spherical copper RF knife-switch contacts on the end of the 50Hz supply (photo) so two gaps in series, and I put a vacuum cleaner nozzle adjacent to suck out the arc and leave the spark. It's very noisy! I have an old tumble-drier motor and had planned to make a rotary spark-gap, but life (and work) gets in the way sometimes. Manana...
I wound the primary spiral to resonate at fundamental frequency, which is about half-way around, and which allows adjustment for the corona loading the secondary and pulling the resonant point when it goes over. I get corona with a tighter tap on too, which, from bridging, I presume is from the second resonance point.
It was only afterwards I decided to stick the thing on a VNA to see what it looked like.