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Size of secondary, freq, and pri. surge impedance





---------- Forwarded message ----------
Date: Sat, 11 Oct 1997 20:06:36 -0400 (EDT)
From: FutureT-at-aol-dot-com
To: tesla-at-pupman-dot-com
Subject: Size of secondary, freq, and pri. surge impedance

All,

I converted my 42" spark TC back into a classic coil, but installed
a narrow 4 1/4" by 23" secondary in place of the original 6 1/2" by
23" secondary.  This has approximately doubled the frequency, and
required a shift in the pri. tap point from 33 turns to 20 turns.  I didn't
measure the coupling yet, but I placed the sec 1 1/4" below the 
center of the primary compared to 1 3/4" above the pri. when using
the wide secondary (now I'm using the narrow one).

Bottom line is; I lost about 1 or 2" of spark length with the narrow
secondary, or about 2.5 to 5%.  By using the narrow secondary,
I'm degrading the TC in three ways; higher frequency results in 
greater radiation losses and corona production, lower surge
impedance increases gap and tank losses, and the smaller 
secondary has a smaller isotropic capacity.

Although I didn't measure the coupling yet, I suspect that it is in
the ballpark, both from experience, and from the behaviour of the
spark.  Interestingly, when I first installed the narrow secondary,
the spark tended to occasionally surge to longer lengths at times.
It was possible to hear the sound of the gap change as the spark
grew at a slow steady pace.  But these long sparks were still
shorter than 40" since at this time the coil was not yet optimized.

"another day, another experiment"

John Freau

P.S.   I bought a 4-400A vacuum tube at a hamfest, and it gave a
proper 13" spark when installed in one of my tube coils.  But the
next time I turned on the coil, it gave only 9" of spark, then the
next time, only 7", now  it's down to 6".  When I install my original
good tube, the spark goes back to 13", so it must be the tube 
that is changing, not the coil.  I've never seen this mode of tube
degeneration, any ideas, anyone?