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PVC, Re. Bert's response on Pyrex




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From:  Edward J. Wingate [SMTP:ewing7-at-frontiernet-dot-net]
Sent:  Friday, March 06, 1998 5:16 PM
To:  Tesla List
Subject:  Re: PVC,  Re. Bert's response on Pyrex

Tesla List wrote:
> 
> ----------
> From:  Bert Hickman [SMTP:bert.hickman-at-aquila-dot-com]
> Sent:  Thursday, March 05, 1998 12:50 AM
> To:  Tesla List
> Subject:  Re: PVC,  Re. Bert's response on Pyrex
> 
> Tesla List wrote:

<large snip>

> Alfred,
> 
> My original reply was regarding an earlier post to answer a question
> about Pyrex from Jim Heagy. Although my response was requoted in the
> last post, it lost some of its context since the earlier posts were not
> also requoted. It was the earlier posting that directly compared Pyrex,
> PVC, and cardboard coilforms.
> 
> However, I still stand by that post. There's a BIG difference between
> using glass, Pyrex, or PVC as a dielectric in a tank capacitor and using
> it as a coilform. PVC and Pyrex make excellent coilforms based more on
> their dielectric strength and low leakage current than their dielectric
> loss characteristics. Dielectric losses in a coilform are simply more of
> a secondary (npi :^)) concern.
> 
> However, the simple fact is that Pyrex, with a loss tangent of 36-40 x
> 10^-4, has less than 1/20 the dielectric loss of PVC, at 800-1200 x
> 10^-4. Heck, Pyrex was even used as a coilform by Breit, Tuve, and Dahl
> for their oil-immersed 5 MV, 100 kHz coil ("High Voltage Laboratory
> Technique", page 109, Meek and Craggs). While they probably would have
> used PVC had it been available back then, I doubt that cardboard would
> have worked as well as Pyrex.
> 
> I also contend that, although you might be able to measure differences
> in instrumented Q between identically wound coils on these materials,
> there will be virtually NO performance difference when the sparks are a
> flyin', since other factors (streamer losses in particular) will easily
> drop the effective secondary Q by an order of magnitude.
> 
> However, if you've got a leaky coilform, that's a different story.  I'm
> not disputing that you can make wooden or cardboard coilforms that will
> work well after proper pretreatment. And for larger coils there may be
> no other readily available, affordable alternative. Heck, Ed Wingate
> gets great performance from large cardboard resonators with no
> pretreatment! However, I also suspect that cardboard's voltage-standoff
> and leakage current characteristics are not as good as some of the other

Bert,

I didn't snip the last paragraph. That's the way the post came off the
list.

I have to say that I definitely DO treat all of my sonotube coilforms
before use. I first set them near a heater until I'm certain they are
dry, and then I apply at least three coats of shellac both inside and
out. I also make sure that the edges are well covered. After the wire is
wound on the form I then apply another coat or two of shellac to hold it
in place. I prefer shellac to clear polyurethene because it doesn't
crack and peel with age as the polyurethane does. Shellac also has the
advantage of drying faster which makes the whole job go much quicker.

Ed Wingate