My VTTC: The Quest For Longer Sparks

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Original poster: "Chris Fanjoy" <zappyman@xxxxxxxxxxx>

  After much tweaking and other work, I have my dual 813 VTTC 
producing some nice thick sparks of about 9" or so in length. But 
from what I've read, I should be able to get considerably more from 
this coil. Here are the specs of what I'm using now:
  -Power supply is a large MOT with voltage doubler
  -Secondary: 4" x 23", 27AWG, fr=157KHz
  -Primary: about 23 turns #12AWG, on 8" form
  -Tank Cap: .0056uF /40KV (TDK UHV-6A x 2)
  One theory I have is that the primary coil coupling isn't high 
enough - a narrower primary may be needed. Or perhaps it should be 
redesigned to use a larger tank cap.
  I'm powering the coil with a Variac, and it's worth noting that 
the sparklength peaks when the Variac reaches about 70 volts, and 
cranking to 120 doesn't seem to make any difference.
  Any ideas? I'd be happy with even some 12" sparks.....