Re: Redesigning/tuning a coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "John Richardson" <jprich-at-up-dot-net> 

Hi Mike,

The trans sounds good.

Try the caps, and see how things go. You may find that after some short runs
that a bottle capacitor might work a little better.  Your narrow time frame
doesn't allow ordering anything else.

You may want to go for another turn or so on your proposed primary to bring
the total number up to 16 or so, if you are planning to build the primary
without tuning with a scrap wire setup first.  I would keep it flat, ie not
conical.

Your secondary sounds fine.  If there are no internal baffles in it, you may
want to cut some out of an old poly cutting board or similar plastic and
cement them a few inches in from each end.

I live in Upper Michigan, and the big chain stores don't carry the Al flex
duct outside of 4".  If that's also the case with you, go for it, with
something like an OD of 30 to 36" or so.  Not too critical.  Beg an old
piece of paneling from the lumber yard, cut a 24-26" circle, smear on some
wood glue and lay strips of Al foil on the glue.  When it's dry, follow the
OD with a razor to trim the foil, and do the same to the other side.  I've
found that even ugly toroids can work well.

Your proposed gap sounds good.  Be sure to put a safety gap across the NST
outputs.  Stiff scrap wire will do.

I don't feel that grounding to the steel will present any problems at all.

Good luck, and try to borrow a variac to power up slowly while tuning.

John Richardson

 > The current setup (all inch measurements are approximate):
 >
 > 15/60 NST
 >
 > Primary: about 19 turns of insulated wire, I'd guess around AWG 12
 > or 14, 18" diameter bundle
 >
 > Secondary: 8.75" OD, 36.5" wound length, 1500-1550 turns of
 > AWG 22 varnished wire
 >
 > Topload: currently either a metal bowl upside-down over the top or
 > a 8" x 23" toroid of aluminum flex ducting
 >
 > main cap: 16.2 nF (6 caps in parallel, each 2.7 nF -at- 40 kV--the
 > doorknob type, probably ceramic)
 >
 > Spark gap: a single gap using several nails
 >
 > Ground: I'm not sure if he has a good ground--I don't think so
 >
 > My main ideas for improvement based on what I've read here:
 > -switch the primary to 1/4" bare copper tubing, around 12-14 turns
 > to allow for plenty of flexibility when tuning, in a cone angled at 30
 > degrees for better coupling
 > -build a new toroid, 6" x 26" (aluminum flex ducting)
 > -main cap: see if I can persuade him to try something else, since I
 > know the doorknob caps are really lossy and their C changes with
 > temperature
 > -spark gap: an RQ/TCBOR style multiple spark gap
 > -connect a good ground
 >
 > Main questions:
 > -Will the 6" x 26" ducted toroid be too big for this coil?  I was also
 > considering 5" x 20", but someone my contact talked to said it only
 > comes in the even sizes, e.g. 4" and 6" but no 5", so I figured the 6"
 > was better than trying 4".  I thought this was strange, since many
 > people have mentioned 5", but perhaps they weren't using ducting.
 > Any thoughts on how appropriate this size is?
 >
 > -How acceptable are the ceramic doorknob caps?  I know they're
 > not the greatest, but I'm thinking we may just stick with them, since
 > we already have plenty, and replacing them would cost a fair
 > amount.  Also, the current C seems pretty reasonable; any
 > recommendations on either front?
 >
 > -The coil is currently located underground in a large warehouse-
 > style area (though I don't know if it'll stay there), so driving a
 > grounding rod isn't feasible--would connecting the ground to
 > structural steel members be acceptable?  (I don't think the water
 > pipes would be a good idea, as they're connected to the normal
 > electrical ground--is the structural steel also connected to the
 > electrical ground?)
 >
 > Any other thoughts/comments?
 >
 > Thanks!
 > Michael Johnson
 >
 >