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[AMPS] Tesla coil effects in open turns of coils

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Subject: [AMPS] Tesla coil effects in open turns of coils
From: (John Lyles)
Date: Sun, 10 Jan 1999 15:37:33 -0700
Was reading the comments about shorting the unused turns on an RF power
inductor. Comments that leaving unterminated open turns (coupled) leads to
Tesla Coil effects. I would have to guess that it would need a parasitic to
fire off an extreme high voltage at the end of a couple of turns, in an
otherwise large inductor at HF. If you calculated the first open-ended
resonance of a coil, say the 1/4 wave mode with high field at one end, like
an aircore transformer arranged as a Tesla Coil, it would be fairly high
freq wouldn't it? I haven't measured, but would suspect 30 MHz or so. I
suppose that could get excited with harmonic voltage alone. There is a
higher likelyhood in things like RF chokes, feeding DC to the tubes.
Here's a scenario where such ideas really could explain the mitigating
cause of failure:

A few years ago, at Sri Lanka, the Voice of America lost a 500 KW Marconi
B6128 transmitter, as the modulator burned up. Nothing conclusively was
ever explained (to me) as to the exact cause, but there was some thought at
the site that somehow the modulator became unloaded (loose connection) and
it was a pulse width class D modulator. The switching frequency itself was
60 KHz. In the output of the modulator TH537 tetrode makes PWM voltge which
feeds through a high power PDM filter before going to the RF final PA
plate. This filter has some very long inductors in it, several feet long.
There were also oil-filled HV caps nearby in the cubicle. Whatever
happened, the fire destroyed the transmitter and part of the building.

It appears that the tube modulators could get dumped in favor of a solid
state "digital" modulator, so there may be a glut of TH537s on the surplus
market someday! Surplus of Nebraska needs to make some room.
These pups will produce an easy 300 KW or so at HF, with 15 to 20 kV of
plate voltage. Stand back guys, you'll need a lot of water capacity for
your shack.

Our recent amplifier for 2.8 MHz runs with a 15 uH Multronics inductor in
the pi-network output. I've got a little over a turn unused, and did not
bother to short it. It hasn't caused any problems.


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