RE: First light with 833 tube coil, then silence

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Cameron B. Prince" <cplists@xxxxxxxxxx>

Hi John,

I'm am aware of and agree with your statements. However, this is a G3 mica
transmitting cap rated 12A @ 300kHz. The diode David is using is only rated
at 750mA and would normally go long before the cap would. I guess stranger
things have happened, but I would certainly rule out every other possibility
before buying a new cap.

This is the cap David is using:

http://www.teslauniverse.com/images/vttc/DSC02483.JPG

Notice it's almost as big as the tube...

David is, if I'm not mistaken, running dual MOT's with the secondaries in
series. I think it would be prudent to verify that one of those secondaries
hasn't opened and would consider this much more likely than cap, diode or
tube failure.

David, let us know what you find.

Thanks,
Cameron


> -----Original Message-----
> From: Tesla list [mailto:tesla@xxxxxxxxxx]
> Sent: Saturday, September 23, 2006 7:49 PM
> To: tesla@xxxxxxxxxx
> Subject: Re: First light with 833 tube coil, then silence
>
> 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
> >
>
> Snip ---- Snip ---- Snip