Hello Gary,
Thank you for your comprehensive reply. Also greetings from your southern
neighbor; I am in CT.
> Primary tap selection is rarely super critical, you should get some output
even several taps away from the optimum point. Is the arc in the gap loud,
bright, and steady, or just buzzing and blue?
It's a high frequency bright loud banging, not the steady electrical arc
and hum associated with just NST. This occurs when I use the NST and the
MMC together, and I assume it from the MMC discharge. This is how I have
assumed the problem is not in the MMC or spark gap.
> For setting the spark gap width (both main and safety), this must be done
experimentally, with the primary tap disconnected. You want the spark gap
to *just* fire when the Variac is at maximum. Don't do it with the cap and
primary connected because mains resonant rise is at play, and voltage can
easily ring up to many tens of KV if the gap allows it. You want the gap
to fire at 12KV*1.414 V. There is no rule of thumb for total gap distance.
This is in fact how I have set my spark gap and my safety gap. They were
set without the MMC or primary coil connected, and adjusted to just fire
when full voltage was reached using the variac. In the case of the safety
gap I instead adjusted it from larger to smaller until it just fired at
maximum voltage, and then adjusted it a step backwards so that it was just
slightly too large for normal full voltage.
Joshua Thomas
On Tue, Nov 2, 2021 at 8:40 PM Gary Lau <glau1024@xxxxxxxxx> wrote:
> Hi Joshua,
>
> I'd like to chime in on a couple of points raised in this thread.
>
> If you wanted your MMC cap to run 24/7, year after year, you would want to
> use the AC voltage rating of the cap to exceed the peak voltage of your
> NST. But none of us on this List expect to use our coils anywhere near so
> long. Among hobbyists, it is common practice to set the MMC DC voltage
> rating to the peak voltage of the NST. CD caps are extremely durable and
> their failure mode is such that the capacitance just slowly diminishes.
> Catastrophic failures are uncommon unless the array is grossly under-rated
> or are poorly constructed. DC-rated MMC's are FAR more economical.
>
> I've never heard that bleeder resistors degrade the Q of anything. The
> resistance of the bleeders are many, many orders of magnitude higher than
> the reactance of the caps at Fo.
>
> I once tried to experimentally select the best primary tap by dragging the
> primary tap cable across the primary turns. The arc from the cable to the
> primary was loud and fearsome and I quickly powered down and abandoned that
> plan. I picked up the tap cable to attach it to the primary and
> ZAPPPPP!!!! I received a significant shock. Normally the primary coil
> provides a DC path to discharge the cap through the NST secondary, but with
> the primary disconnected, there is no discharge path. I'm a BIG proponent
> of bleeder resistors!
>
> Primary tap selection is rarely super critical, you should get some output
> even several taps away from the optimum point. Is the arc in the gap loud,
> bright, and steady, or just buzzing and blue?
>
> For setting the spark gap width (both main and safety), this must be done
> experimentally, with the primary tap disconnected. You want the spark gap
> to *just* fire when the Variac is at maximum. Don't do it with the cap and
> primary connected because mains resonant rise is at play, and voltage can
> easily ring up to many tens of KV if the gap allows it. You want the gap
> to fire at 12KV*1.414 V. There is no rule of thumb for total gap distance.
>
> Regards, Gary Lau
> MA, USA
>
>
>
>
> On Mon, Nov 1, 2021 at 11:20 PM Joshua Thomas <joshuafthomas@xxxxxxxxx>
> wrote:
>
> > Hi Alex,
> >
> > That's a very interesting technique, and definitely one I can attempt.
> > Thanks!
> >
> > Joshua Thomas
> >
> >
> >
> > On Mon, Nov 1, 2021 at 8:42 PM b alex pettit jr <a_pettit_jr@xxxxxxxxx>
> > wrote:
> >
> > > *WHAT YOU CAN TRY IS:*
> > >
> > > using a very low setting on the Variac and very small gap settings,
> > > Connect the Coil Tap wire to an insulated rod a foot or so long,
> > > energize the AC and touch the coil tap wire to various locations on the
> > > primary coil.
> > >
> > > *SOMEWHERE,* you should see some *Secondary Output *.. refining and
> > > stepping up the V is tedious this way,
> > > but Can Be Done.
> > >
> > > Alex
> > >
> > > On Monday, November 1, 2021, 07:07:01 PM EDT, Joshua Thomas <
> > > joshuafthomas@xxxxxxxxx> wrote:
> > >
> > >
> > > Hello Alex,
> > >
> > > Thank you for your reply.
> > >
> > > I have an oscilloscope but not a signal generator (the pitfalls of
> being
> > > on a tight budget!). If I can source a signal generator I will try your
> > > technique to find resonant frequency; thank you for that.
> > >
> > > I am somewhat surprised to read that the primary coil needs to be tuned
> > > with with-in 1/4". The other sources I have been reading who have had
> > > successful coils started out with a "guessed" turn to tap the primary
> > coil
> > > and then advanced or retarded the tap point by an entire turn of the
> > coil,
> > > which is far more than a 1/4". Others seemed to have generated sparks
> > even
> > > with a sub-optimal tap point, using the tuning to get optimal length.
> > > Perhaps you could elaborate more on this?
> > >
> > > Thank you very much,
> > > Joshua Thomas
> > >
> > >
> > >
> >
> > --
> > Joshua Thomas
> >
> > My new email address is: joshuafthomas@xxxxxxxxx
> > Please update your information if you have not already done so.
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Joshua Thomas
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Please update your information if you have not already done so.