> I had just got a brand new torroid (20"). I happily placed it  
 > on the top of the secondary, turned on the power, and...       
 > and...lots of noise, but no spark! At all!

 > What happened? 

 > This coil is powered by 15KV -at- .06A (neon sign trans). The     
 > secondary is  8" diameter, 3 feet tall, (500KC resonant freq). 
 > The primary is 5 turns  1/4" copper tubing around a 16" diam   
 > form. The primary cap is .007uf. 

OK, you have a few things going on. First, the toriod is a bit
large for the low inductance of this particular secondary.
My guess would be that this coil is space wound with magnet wire,
or close wound with PVC insulated stranded wire (maybe even space
wound with PVC insulated wire?). The coil specs that I would
recommend to take best advantage of this new toriod is more like
a close wound magnet wire coil that resonates at 250KC or lower.
Look at a close wound 6" x 24" secondary with a resonate
frequency around 250KC to work best with this toroid. 

Next, the addition of a 20" toroid on a coil such as yours drops
the resonate frequency of the secondary coil below that which can
be reached with your present capacitor and 5 turn primary. You
need a primary that can be tapped out over 10 turns to regain
your system tune. You would be surprised how much the resonate
frequency drops in a low to medium inductance secondary coil,
such as yours, when you add a toroid.

Last is the peak power factor. Your coupling is, or is going to
be, too loose. The peak power produced and transferred by the
tank circuit is not exciting enough resonate rise in your
secondary w/new toroid to get break out. The toriod shape breaks
down at a much higher voltage than other dischargers (which means
much larger sparks), but the coiler must be prepared to change
other things around as well to produce the higher voltages.

What to do?

First, patch in an extension on your primary so your tank 
circuit can reach down to the lower frequency of the secondary 
with the new toroid. When you are patching in this extension keep
in mind you need to snug up your coupling to drive more energy
into the secondary.

Then retune the system. If the spark won't break out you can
judge system tune by placing a low pressure bulb near the
discharger and monitoring the glow (brighter = closer in tune),
or you can pull spark off the terminal with a grounded probe.

Last, you can wind a new secondary with higher inductance per
unit volume. Use the rule of thumb guidelines I have posted. You 
will need a larger primary too.

Remember! A toroid ALLOWS the coiler to produce a longer spark
from a smaller system. It does so by breaking down at a higher
voltage. But it is up the the coiler to produce these higher
voltages. The toroid by itself does not MAKE bigger sparks. 

Good Luck!

Richard Quick

... If all else fails... Throw another megavolt across it!