Re: My VTTC: The Quest For Longer Sparks

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Shad <shenderson@xxxxxxxxxxxxxxxx>

Hi Chris,

It's good hear your coil is running!

Without going to a staccato controller, it can be difficult to get long
sparks from a VTTC.  You're on the right track with the voltage doubled
MOT though.

You state that your tank cap is 5.6nF x2.  Are those caps in series or
parallel?  Generally, I've found that the less plate current a tube can
supply, the better it likes a small tank cap.  I'm usually in the 1-3nF
range on the tank cap with a few more primary turns (around 30 or so)
for most of my coils.  Very small tubes (usually sweep tubes wired in
triode mode) can get by with .5nF or less.

How many turns are on the feedback coil, and where is it physically
located?  What's the values for the grid leak C and R?  The lower the
grid R, the hotter and thicker the sparks are, and the hotter the tube
runs.  I was taught to wind the grid coil physically below the primary
on the form to expose it to less voltage stress and lessen the chance of
a secondary/primary arc finding it's way to the grid.

The MOT itself may be a limiting factor in what you're seeing, also.
MOTs saturate easily (made to the cheapest limits possible), and a
voltage multiplier's output voltage sags greatly under a heavy load.
You may try two MOTs in series without the doubler and just feed the
plate raw AC.  When hooking 2 MOTs in series, be sure to mount the
second mot on standoffs because it's core will be floating at the output
voltage of the first MOT.  It's also a good idea to use an isolation
xformer between the mains and the second MOT, in case of insulation
breakdown on it's secondary that shorts to the core/primary.  Of course
fuse everything, make sure the first MOT core is solidly grounded, and
don't touch 'em in use. :)

Do you have RF bypass caps on the filament leads to ground?  I've lost a
filament xformer recently to stray RF coupled back into it from the
filament.  A few nF from each leg to ground does the trick.

Also, I recommend a choke on both the plate and grid leads at the tube.
I generally use an 8 ohm non-inductive resistor wound with 12-15 turns
of 20ga wire right at the connection to the tube.  Those smush out
parasitic oscillations that suck up power that helps heat the plate.

VTTC's can be quite picky and a PITA to get running well... But I prefer
'em to the disruptive coils.  More hum and hiss, less crash and bang. :)

Shad H.

On Sat, 2005-10-29 at 17:22 -0600, Tesla list wrote:
> 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.....
>
>