Re: First light with 833 tube coil, then silence

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Dr. John W. Gudenas" <comsciprof@xxxxxxxxxxxxx>

Cameron
Be cautious with just voltage ratings on mica caps (or any caps for
that matter).
They need to handle high current too If the duty cycle is high, such
as operating in self rectification without a staccato controller.
The current in the tank circuit can get get up there.  With my dual
833A VTTC which produces 24" corona without a staccato controller
I can run for 10 to 15 minutes before things get hot. This coil
destroyed at least three new 12,000 volt mica transmitting caps
(typical ones about 3" x 2" x 4" with the screw terminals on top).
One blew out the bottom and left a very unpleasant mess. I switched
to very high current composite micas in an MMC arrangement.
These are .0024 mfd. They are black cylinders about 3" in diameter
and 5" long with large brass connectors on each end with 1/4 -20 bolts.
The rating was 20kv too. I tried one and had very poor spark output,
I saw this before cap failure. I put two in series then and
paralleled this with another string of two in series.
The coil worked great with no tank cap heating. The coil operates
around 250 kHz.  Cap problems ended completely. Good RF ceramics do a
decent job too.

Cap dielectrics tend to get quite lossy at high currents and voltages
operating in the RF ranges.  If they can't handle the current they
die a rapid death.
I have also destroyed MMC's with my DRSSTC that I didn't
sufficiently  design for long duty cycles. Not from over volting, but
from too much current.

Professors tend to get on soap boxes and preach, so today I am just
saying consider voltage and current in capacitor choice (obviously
dielectric too).
David will solve his problem as intersecting all our responses, we
suggested every component could be bad as well as wiring errors. Good
luck David.
Cheers!
John


On Sep 23, 2006, at 3:08 PM, Tesla list wrote:

> Original poster: "Cameron B. Prince" <cplists@xxxxxxxxxx>
>
> Hi David,
>
> I seriously doubt that mica cap is bad... They are built to handle
> a lot
> more than you are throwing at them with your VTTC. When I ran it on
> mine it
> stayed ambient temperature.
>
> Notice that 12kV rating is not peak or maximum, it's nominal and
> you are
> only at 50% of that with the VTTC.
>
> Cameron

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