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RE: MMC Question.

Original poster: "Lau, Gary" <Gary.Lau@xxxxxx>

Hi Glen:

One thing that posts in this thread have failed to address is the
importance of setting the static gap width.  The gap width sets the
maximum voltage that the cap charges to before the gap fires.  If you
have a 15kV NST and a cap that's close to the mains resonant value, the
cap could charge up to 60kV (WAG) or more, if you open the gap too wide.
If you set the gap width so that it fires at a voltage that's safe for
the NST, say 21kV, then using a mains-resonant cap is no more hazardous
than an LTR cap.  As far as mains resonance goes, it's low Q and broad,
so a .0106uF cap is not significantly different than a .0094uF cap.

You may wonder - if it's as simple as just setting the gap width, why do
folks worry about LTR and safety gaps and filters?  The answer is
largely due to human nature.  Setting the gap width has the unfortunate
correlation of giving increasingly longer sparks with wider gap widths.
Who doesn't want that, right?  Also, setting the gap width properly
isn't quite as easy as one might hope.  There's no simple rule that
states for a 15kV NST, use a .xxx inch gap width.  It must be set
experimentally, with just the gap across the NST, so that it just starts
to fire at the maximum Variac setting.  But I doubt there's a one of us
that hasn't widened the gap beyond that point to some degree, to enjoy
the added performance.  Using an LTR cap, to a degree, may limit that
resonant rise.

The other benefit to using an LTR cap is that coil performance has been
shown to correlate to "bang size" - the amount of energy stored in the
cap when the gap fires.  The energy is determined by the gap firing
voltage and the cap uF size.

Regards, Gary Lau

> Original poster: "Glen McGowan" <glen.mcgowan@xxxxxxxxx>
> Will the filter Terry developed protect the NST in the event of
> resonance or would the filter resonate as well along with the NST?
> As I understand it I'm currently in the neighborhood of less
> desirable capacitance. As Scott,Barton and others have pointed out
> it's not a particularly good idea to let the NST resonate but with my
> current MMC (.0094uF) would I suffer any adverse effects?  I guess
> the question is more along the lines of being that .0094uF is pretty
> close to .0106uF would  it resonate or would my capacitance need to
> match .0106uF before NST resonance would kick in?  How exact do I
> need to be is what I'm asking.
> Looking back at the statement I made about the NST "preferring"
> .0106uF it seems I was quit mistaken and the .0106uF should be
> treated as a boundary rather than a goal. I'm learning.