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Re. Help...I'm having a lot of problems here...

>Original Poster: "Kent & Kim Schaffer" <santoken-at-bright-dot-net> >
>Hello all...
>About six months ago my step son and I decided to build a TC for his science
>fair project.  Boy, was that a mistake...
>First I'll tell you that this is my first coil project, so I'm sure we've
>made a lot of mistakes in our design.  I think that one of my biggest
>mistakes is that cap we are using (I thought I would try to save some time
>and money).
>
>Power source: Furnace transformer or NST (I'm not sure): 10KVA -25 milliamp
>
>Cap (set back for a good laugh): 12x 2KVDC .01 uf ceramic caps soldered
>in parallel for a total of 24KVDC -submerged in mineral oil in plexi box
>
>Spark gap (another joke):  -plexi box with three adjustable gaps in series
>
>Primary: -3/8" copper tubing -ID 7.7" -Wire spacing ..875" -5.5 total
>turns -wound flat (0 degrees)
>
>Secondary: -4.2" diameter (thin wall PVC drain pipe) -21" long winding
>-22 ga wire - enamel coated -shellaced before and after winding
>
>Toroid:  -4" dryer duct -12" diameter center of tube to center of tube
>
>Now here is the problems we're having:
>After hooking up the cap the way all the schematics show I get no discharge
>at the spark gap at all.  BTW, the way I read the schematics, one terminal
>of the cap attaches to the output of the transformer and the other attaches
>to the ground.  Now, if I attach the cap in a series (one terminal input,
>the other output) I get spark at the gap.
>
>I have gotten absolutely no discharge at the torid whatsoever...sure does
>light a flourescent tube nicely though :-)
>
>Does anyone have a suggestions...I'm sure some of you must!  Your response
>is greatly appreciated, since the science fair judging in Thursday...Yes,
>this Thursday.
>
>Kent

While the components in general could be made to work, they don't work
together, i.e., the primary circuit is not in tune with the secondary.

For a coil powered by your small 10KV/25mA transformer, 3/8" tubing is
overkill for the primary, and the .875" spacing is too wide to give you
enough inductance.  You also want more than 5.5 turns.

Your ceramic cap is not unreasonable for a small coil.  Assuming you
meant that they were wired in series (not parallel), they would give you
833 pF.  Ideally you'd want .006 uF, or about 7 times as much, but still
probably workable.  Once you get this cap working, you'll probably have
to keep run times short as self-heating will cause ceramic caps to change
capacitance and shift the primary tank out of tune.  The oil immersion
will help this though.

Your secondary and toroid are fine by most standards, but they're too big
for your current coil.  

The secondary you describe wants to resonate at about 514 KHz.  When you
add your toroid, that figure goes down to about 293 KHz.  However, your
primary (11.7 uH) and cap (833 pF) want to resonate at 1.6MHz.

Since I assume you don't have time to make a larger cap, the most
expediant thing would be to scrap your (no doubt) handsome primary and
replace it with something quick and dirty with a lot more inductance.
I'd recommmend getting some insulated 12 AWG solid copper wire and
wind a new flat primary with about 20 turns.  Stagger the taps about 1"
from each other so they don't arc.

You'll also probably have to replace your (no doubt) handsome toroid with
something smaller, to raise the secondary's resonant frequency.  A foil
covered baseball may work.

As far as circuit hookup, the spark gap should be in parallel with the
transformer.  The cap and primary should be in series with each other,
and free end of cap to one side of gap, and free end of primary to other
side of gap.  IMPORTANT:  Neither end of the gap, cap, or primary should
go to ground.  Only the transformer case (connected internally to its
center tap) and secondary base should go to "RF ground", and this is NOT
your 3rd prong on your outlet.  Nothing should go to your 3rd prong
ground.  Violating this is hazardous to your building's wiring and any
other electronic appliances plugged in nearby.

Without any test equipment, tuning is going to be interesting.  Ceramic
caps often have wide tolerances, so unless you are able to measure what
you have, be willing to try a wide variety of taps and top loads to
achieve tuning.  Good luck!

Regards, Gary Lau
Waltham, MA USA