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Re: [TCML] Chokes and Terry filters

I also never run my total main gap at more than 200 mils (0.200") when
running with nst power.  I usually set each safety gap off each nst bushing
at 180 mils max using a calipers.  I use the McMaster-Carr 5/8" dia brass
balls for each hot and also for the central ground electrode.  If the tuning
isn't sharp some flashovers can occur on the safety gap.  I usually tune
with an oscope & sig generator, and then use a variac for 35% power tuning
before opening up to full power.

I also get rather cautious with the coeff. of coupling.  I've noted that in
many cases JAVATCs suggested value (rather tight) always seems to produce
shorter spark outputs than a slightly looser coupling value.  Personally, I
like 0.10 to 0.13 range, and usually never more than 0.14 with a classic
type spark gap.  This range usually gives me the longest sparks with normal
coil design in the 4 to 10 inch sec diameter range.

The DRSS type coils seem to tolerate a value of 0.14 to 0.18.  Anything over
0.18 is usually asking for problems especially with coils that will be
operated each and every day.

After reading Ritchie's excellent report, I also bumped all my cap values up
to 1.4 x Cres.

I like to unpot my nsts and get them in oil even if only operating at normal
values.  The xmfr oil seems to be a good secret to long life with your nst.
Most experimenters also obtain used nsts (good price is why) and one can
never tell how long they have been stressed in previous service.  Sometimes
only for 6-12 months and sometimes for an eternity.  This makes qualitative
comparisons rather difficult.

Seems to perform well.


D.C. Cox

On Tue, Jun 17, 2008 at 6:52 PM, bartb <bartb@xxxxxxxxxxxxxxxx> wrote:

> I think all of us are preaching proper gap spacing via NST and gap setting
> (workbench approach) which eliminates the various electrodes chosen. Another
> aspect to consider is that DC typically runs at about 1.3 x Cres for his cap
> size. So he is taking advantage of some resonant charging as far as bps is
> concerned. The static gap still clamps the voltage regardless and I agree is
> the main problem with NST failures.
> If I were to weigh in on the filter discussion, I would say the chokes are
> unnecessary with a filter. However, I have even pushed my gap setting wide
> enough to kill an NST even with a Terry filter installed. This is when I
> realized the safety gap was useless due to my gap setting and sort of what
> drives me to stress the margin between safety and main gap needs to be large
> enough to allow the safety gap to work, otherwise, it's just a waste of
> labor and electrodes.
> I do push my transformers with what I can get away with. If it fails, I'll
> just fix it or replace it. There are probably a few like me out there on
> that subject. With that in mind, I've even been thinking about doing away
> with the safety gap (my static gap coils only). The reason is I have little
> margin and it's an annoyance when set correctly and useless when it's not an
> annoyance.
> The fact is, the probability of a decently built static gap failing is slim
> to none. So, what's the point of the safety gap in that type of system?
> Take care,
> Bart
> Lau, Gary wrote:
>> Indeed.  I think the reason that DC has been successful in not killing
>> NST's is his conservative approach to keeping static gap width small.  That,
>> more than using any kind of filter, is the key to long NST life, and that's
>> what we should be preaching.  But human nature being what it is, the desire
>> to increase performance has us opening the gaps to the max, and that's where
>> Terry filters become useful.
>> Regards, Gary Lau
>> MA, USA
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