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TC construction hints for beginners (long post)

Hi Guys,
We seem to have lots of new people getting on the list so I decided to write
up some of the tips that I have discovered in the last 2 years of fooling
with coils.  I have only built and refined 1.5 coils, but I have been
fortunate enough to hang out with lots of really gifted local coilers,
including some professionals.  The following is written with the _average
hobbyist_ in mind (not quite sufficient for museum coils, or professional
coils that go on the road or have long duty cycles and run times)

Capacitors:
Build a MMC.  You can't make a less expensive, better performing capacitor
than a MMC.  Look for Metalized Polypropylene caps in the .05uF, 1000V
range.  Size it so that the max AC voltage is the same as the max DC rating.
This has been shown to work well for the average hobbyist coil that has
short run time.  The MMC is better than rolled LDPE or plate caps because
1.  It's only a fraction of the weight and volume of oil-filled caps
2.  No messy oil to leak out, catch fire, or sealed containers to explode
3.  The Ctotal is easily adjustable and it's easy to check and repair
http://users.better-dot-org/roverstreet/components/cap.html

Stand:
A great way to make a cheap and easy stand is to go to Home Depot or Eagle
Hardware and purchase pre-cut wooden circles.  You can get 3/4 inch thick
Plywood or Birch 22 inch diameter circles for about $7-15 dollars each.
Mount the NSTs, MMC, and gap on the bottom circle.  Mount the primary and
secondary on the top circle.  Separate the circles with (3) 1.5-2" PVC legs.
Mount PVC end caps on the inside of the top and bottom circles and then all
you have to do is insert legs of whatever length you want.  I sanded the
bottoms of the
end caps flat and used #8 bolts to secure them to the circles.  In order to
make it really pretty, it's best to mount everything, drill all holes, then
disassemble, sand, prime, paint, and reassemble.  You can then use black
felt
to make a curtain to fit all the way around the bottom of the coil.  This
makes it look really sharp and shield the gap so it doesn't overexpose
photos when you start taking time exposures of the arcs.
(I don't have pics of my new stand on my page yet)

My original coil required a huge stand to support the (4) NSTs and (3)
rolled LDPE caps.  The new stand is much more manageable and should put the
toroid close enough to the ground that I can get direct ground strikes.


Power Supply:
Neon sign transformers are the standard for the typical hobbyist coil.  I
recommend depotting the NST and removing some shunts.  I'm currently running
a 12KV unit that has been reworked to produce 200ma at 120V input.  It's
running dry, although that is a bad idea.  I recently got it so hot by
running it for about 2 minutes that it was making a frying sound and molten
tar and some sort of liquid (gasoline?) was oozing out of the secondaries.
It still works great.  It performs better than the 130lb stack of 4 NSTs
that I used to have banked together.
http://users.better-dot-org/roverstreet/components/psu.html

Gap:
The traditional design where you place sections of copper pipe along the ID
of some 6" PVC.  I like to mill slots between the electrodes and use a
muffin fan to force air through the gap.  I'm currently using 7 pipes spaced
at .030" each for a total gap of .180".
http://users.better-dot-org/roverstreet/components/gap.html

Secondary:
Stick with aspect ratios of around 4 or 5 (one expert on the list recommends
4.5, ie, make the windings 4.5 times as long as they are in diameter).  Use
a wire that will give you somewhere in the neighborhood of 1000 turns.  John
Freau has found that he can get slightly longer arcs with more turns -
somewhere in the neighborhood of 1500 turns, I think...

I'm not sold on the idea of rigorously preparing PVC coil forms before
winding secondaries.  I just make sure my forms are clean, ends are squarely
cut, and they are really dry.  People who live in humid areas may have to
spend more time getting the moisture out of the PVC.  I really like using
some sort of 2 part clear epoxy to coat the secondary after winding.  It
provides superior abrasion resistance and provides resistance to flash-over.
People around here use "Pour-On" by Environmental Technologies, "Behr
Super-Build 50", or "Glaze Coat".  You just mix it up and apply it with a
foam brush while the coil is being rotated on the winder.  Let it rotate
while it dries.  It makes a super hard, super clear, 1/8" thick coat over
the windings.  It may reduce Q a trivial amount, but it will really save you
the 1st time someone lets your secondary fall out of the truck door.

Also I've started routing the top of the wire into the secondary form.  All
you have to do is install 2-3 baffles along the interior of the coil form
and it works out great.  Make the baffles out of plexi or something and use
epoxy and silicone adhesive to hold them in place.  I put one below where
the "top turn" wire enters the secondary, one in the middle, and one above
the grounded end of the secondary.

I like to attach the secondary to the wooden circle that the primary is
mounted on with a friction-fit wooden plug.  I cut 3 or 4 circles out of
3/4"
plywood that just barely fit inside the ground end of my secondary.  Then I
glued and screwed the circles together to make a 3" high plug.  Mount the
plug in the center of the primary.  Secondaries larger than 8 or 10 inches
in diameter may require screws to penetrate the secondary side-walls and
sink into the wooden plug to ensure they don't fall over.
(No pics of this on the page yet - coming soon)

Primary:
I recently decided to quit using strike rails and oversized primaries.  It
has been shown that extra turns on the primary rob power and decrease arc
output.  Also, current theory seems to suggest that primary strikes are not
all that fatal to MMCs.  Figure out the smallest primary capacitance and
largest top load that you ever intend to use and calculate how many turns
you need on your primary to be resonant.  Build the primary with about 1.5
turns more than this number.

Don't make conical primaries - Build all primaries as flat spirals.  Conical
primaries will be overcoupled and you will wind up raising the secondary
ridiculously high.  I like to use 6 primary supports.  Build the supports
out of LDPE (old cutting board) and use long bolts to secure them to the
plywood circle.  I will elaborate more on this on when I get pics on my
page.

Hope this is of some help.  Keep an eye on my page, a major revision of my
"components" and "model 1B" section will happen in a few weeks.

Cheers,
Ross Overstreet