re cap size


I would bet that your problems result from bad caps and bad gaps.
First, the caps.  Plate glass turns out to be lossy.  Also, the edges of 
the foil have fairly high corona losses.  Try running the thing in the 
dark.  You'll see the cap glowing purple.  (I used to use glass and foil 
too since I got it for free).  Probably your best bet is to make the 
Richard Hull style rolled caps.  They are very successful and very 
robust.  You should be able to down load the instructions at the ftp site 
ftp.funet.fi in /pub/sci/electrical/tesla.  The commercial caps are 
better I'm sure (still haven't verified that personally, but will 
eventually (still making my rotary gap)), but the RHRC (Richard Hull 
Rolled Cap) is fairly cost effective and you can probably have one sooner 
than a commercial.  Be sure to check the price total of all the materials 
first though, because some people have found that a commercial cap isn't 
much more expensive.  That's probably due to their materials suppliers.

Next are the gaps.  The best gap that I have used is an air blast gap.  
However, I already had the compressor.  You can get excellent results 
from a very simple static gap.
Here's what I did:  take a 1" rigid Cu tubing and cut it into 7 3" 
lengths.  Then take a piece of scrap wood that's about 6" wide and 8 to
10" long (2x6 works well).  Make some little "fingers" that hold the 
tubes down.  They will stick in the tubes.  These fingers need to be able 
to move a little so that you can adjust your gaps.  Next, set the spacing 
to about 0.030" between each piece of pipe using a feeler gauge.  
Finally, attach both your leads to the pipe using bulldog type clamps 
(Kind of like an overgrown alligator clip) or something reasonable and 
fire up the coil.  Most people will say "what about air cooling?".  My 
answer is "I didn't have a muffin fan when I made the gap, and it works 
for short runs anyway."  When the gap gets hot, the coil basically stops 
working and I need to wait a bit. C'est La Vie.