Re: My VTTC: The Quest For Longer Sparks

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: FutureT@xxxxxxx 

In a message dated 10/29/05 6:28:42 PM Eastern Standard Time, 
tesla@xxxxxxxxxx writes:

Chris,

I agree with John G. that your coil is most likely not tuned
just right.  When a VTTC is not tuned correctly it behaves
exactly as yours is.  Yours is tuned for 9" sparks.  You need
to tap the primary so more turns are used, or increase your
tank capacitance.  You may need to add primary turns if you're
already at the last turn.  Or you may need to add more capacitance
in parallel with your tank capacitor.  I prefer to add capacitance
rather than adding primary turns.  This is because adding primary
turns will raise the tank impedance and will limit the power input
into the coil.  Adding primary turns will also increase the chances
of voltage flashover inside the vacuum tubes which is a bad thing.

A dual 813 coil should be able to produce at least 24" sparks.
You may also need to
fiddle with the grid feedback and grid leak resistance, but
this sounds exactly like a mistuning problem.  If you retune
by adding capacitance, you probably need to increase the
capacitance by about 50%.  As a test you can add or remove
a toroid.  If you are using a toroid, removing it may increase
your spark length if you're mistuned now.  If you're not using
a toroid, then adding one may reduce the spark length even
more.  The toroid tricks are to verify what I'm saying about the
mistuning.  If you add a toroid, be sure you have a tall (4")
breakout point above the toroid, or the breakout point can be placed
on the edge of the toroid pointing upwards.  Running with a
toroid but no breakout point can stress the components too much
on a VTTC.

Adding capacitance is an easy thing to do usually and should
be tried first in this case.  Use the toroid tricks to gain a clue
about what the situation is regarding the tuning.

John



>    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.....