[TCML] VTTC Coupling

["Dan Elyea" <fsiyfr@xxxxxxxxxxxxxx>]

There is a really fun thing that can be done with an overcoupled VTTC.  You
can't take this to really high powers or you'll chance breakout from the
secondary below the top of the winding, but it's fascinating for the
experimentally inclined.  Also, the necessary close physical spacing for the
tight coupling raises voltage breakdown issues between windings, and that
limits how far you can push the system.

 

You have to deliberately overcouple the primary and secondary, and achieve
the classic double-humped response.   Get the coupling strongly into this
mode with two very prominent humps.  Then, with careful adjustment of the
primary tuning, you can get the circuit oscillating on two frequencies
simultaneously.  A scope is very helpful for this.  However, the appearance
(and sound) of the output discharge changes dramatically when you get the
two frequencies going at once, so a scope isn't absolutely necessity. The
discharge looks a lot more like multi-forked lightning rather than the
typical VTTC brush.  With a scope, you can see the two frequencies in the
waveform.  As you bring up the HV, one frequency kicks in first, and you
have the typical "regular" VTTC discharge appearance and sound.  Going up
higher in voltage, the second frequency kicks in (and the first one keeps on
running, too).  You can see it happen on the scope, and simultaneously see
and hear it in the discharge.

 

Try it-you'll like it.  It may not be as long of a discharge, but it is much
more interesting, in my opinion.  Running paralleled 811As and paralleled
MOTs, I got a loud, dramatic discharge.   Very snappy sound in the
discharge.  The main problem I ran into with this was breakdown between the
primary and secondary.  Eventually, I resorted to filling the space in
between them with RTV silicone sealant so I could run full power.  (The
physical spacing between windings was a little less than an inch.)
Secondary was wound on a four-inch polystyrene form with #26 enameled wire.
About 24 inches or a little more of winding, as I recall.

 

Dan

 

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