Re: Capacitor and coil form questions (fwd)

---------- Forwarded message ----------
Date: Thu, 15 Jan 1998 12:16:15 -0700
From: Bill Lemieux <gomez-at-netherworld-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Capacitor and coil form questions (fwd)

Tesla List wrote:

> Has anyone actually seen evidence that coil form losses are significant?

Quite the contrary- everyone talks about it, but I've never seen it in
experimentation.  Despite someone who was supposedly quite knowledgeable
(someone on this list in fact- I'll not name names) once telling me that
both PVC and dried-&-sealed cardboard were "just no good" for coil
forms, I've used both and obtained excellent performance.  I've since
learned that this same person uses PVC himself!

Although I built my second Tesla coil ever (the rather poorly designed
BTC-5 plans from Information Unlimited) using Sonotube, I would never
use wax-impregnated cardboard again if I had the option- it resembles a
candle too much for my peace of mind!  Pluse, I don't believe the wax
they use in concrete form tubing is as good as urethane at impregnating
the cardboard and preventing future ingress of moisture.

Also, urethane-impregnated "cardboard" of the carpet-tubing type makes a
surprisingly light and strong form, and is eminently affordable to most

More recently, I built a prototype coil using a 6"x24" secondary form. 
The first one was made from carefully dried & sealed cardboard to keep
our costs down.  I heated it with a 100W light bulb inside the form over
several very dry days during the winter, then sealed it immediately with
four coats of Varathane inside, outside, and on the ends.  (it was
carpet tube, not concrete form, so it had no wax inside)

I built another coil secondary in all respects except that the form
material was
thin-wall fiberglass "pipe", which according to my research ought to be
close to polyethylene in loss characteristics. (after all, epoxy/glass
is good enough for high-frequency RF PC boards) I saw no improvement in
performance.  This machine is a ~0.9KVA unit that makes 3-5' streamers,
depending on the customer site's altitude.

We're going to try both foam-core (DWV service) PVC pipe and ABS pipe
next.  The ABS is nice just for cosmetic reasons, since it's black.

> I can understand not wanting RF-lossy materials for your capacitor
> dielectric, but it seems a bit anal to exclude them from being coil
> forms.  I have used PVC and cardboard, both urethane sealed, and I see
> absolutely no performance differences.  I am interested in hearing from
> those who have compared PVC forms to say, LDPE or Acrylic forms.

When I was in high school, I built a vacuum tube coil using acrylic for
the form.
Later, I rebuilt the thing into a spark-gap machine, and I built
identical secondaries out of sealed paper/glue ("cardboard") tubes and
one out of polyethylene- I saw no difference in performance.

This is yet another area where the standard RF theory doesn't hold up
well in the face of real life Tesla coil work.

> >For the secondary tube, go with the PVC if possible.  I have used
> >cardboard and it punctured rather easily and rendered my very first coil
> >useless.

I'd use foam-core (DWV / non-pressure service) PVC on general
principles, since it should be less lossy, and besides, it's lighter
weight and costs less.

> Punctured by an arc?  Something else was wrong if you had arcs anywhere
> near the secondary itself.  I am having great success with my latest coil
> wound on 8" sonotube (heavily urethaned).

I agree- there should never be arcs going toward the _inside_ of the
secondary form. 

My guess is that the material was not carefully dried and/or sealed well
enough and that it carbon-pathed because of bad tuning or during the
tuning process, when there tend to be bad inter-turn voltage transients
and "nodes" along the secondary winding.


.                personal email: gomez-at-netherworld-dot-com                .
.          personal web page: http://www-dot-netherworld-dot-com/~gomez       .