mica cap death


Well, after my dissappointing show at the UK Teslathon where my coil
refused to work, I have concluded that the 0.004 uF mica cap that I was
using has blown. This was a Sangamo 20KV cap in a ceramic case with
massive end plates. Yesterday, Mike Tucknott, Steve Rodway and I got
together in Swindon and I took the cap apart. Unfortunately its potted
in bitumen and rubber so I wasn't able to detect any burning etc. It
appears to have failed open circuit rather than short. I'd had maybe a
total of 5 minutes run-time on this cap, so I think its fair to say that
on *my* set-up, mica caps are a pretty poor choice.

This was confirmed yesterday when we ran my primary, secondary and
toroid with Mikes RQ gap and OBIT using two other mica caps that I had.
We paralled two 0.0015 12KV Dubilier caps (from 1933!) and proceeded to
tune. All was well for a couple of short runs and we got 15" discharge.
Next time we fired we had to bring the breakout point to 4" from the
toroid. Oh dear, another one bites the dust.  :-(   It may be of
interest to point out that I am using a flat strap primary with about 27
turns, I believe we were tapped at about 22.

We then dragged out Mike's beer bottle caps which proved to be capable
of taking much more abuse. Played around with different toroids and
generally made ozone ;-) I managed to get some cine (Super 8) film of
the day which I'll convert to video and hopefully get onto a website at
some point.

Anyway.........  The upshot is that I'll be in the market for some of
these wonderful MMC beasties, and as I've got a 25KV Transformer, I
think I'll be needing rather a lot.....


Steve Crawshaw

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