Re: VTTC capacitor

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Area31 Research Facility by way of Terry Fritz <twftesla-at-qwest-dot-net>" <rwstephens-at-hurontario-dot-net>

Steve,
 
You simply described a type of capacitor as a 'pulse cap'.  There are pulse
caps out there that cannot tolerate sustained reversal at high rep rates. 
There are admittedly others that can.... for a while.  As a generalization,
with 'pulse caps' rep rates of 1 KHz are considered high.  You are talking up
to 1000 times this rep rate at 100% reversal.  I was leaning on the side of
caution.  I was merely suggesting that such capacitors may not be the best for
this application because of the reversal concerns which are covered nicely in
the old Maxwell Corp. brochures.  I believe this info may now be found from the
company that took them over called General Atomics.
 
I have never seen a 'pulse cap' (i.e. like a Maxwell) in the tank circuit of a
commercial RF power application employing transmitter tubes.  Mica, vacuum and
air insulated plate types are most common.  It was these that I recommended,
although I forgot to mention the vacuum cap, but you really don't need to go to
that expense.
 
Having said all that one must  remember that in the hobby of Tesla coiling it
is commonplace to ignore product specifications and pour the coals to whatever
components and materials one has at their disposal until the smoke comes out. 
If you already have a capacitor which you think will work then you should use
it.  
 
r stephens
>
> ----- Original Message ----- 
> From: <mailto:tesla-at-pupman-dot-com>Tesla list 
> To: <mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com 
> Sent: Tuesday, February 12, 2002 23:52 
> Subject: Re: VTTC capacitor
>
> Original poster: "Steve White by way of Terry Fritz
> <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> <<mailto:slwhite-at-zeus.ia-dot-net>slwhite-at-zeus.ia-dot-net>
>
> I do not understand your comment about pulse caps handling only DC. Even 
> when used on a disruptive coil, every time the spark gap fires this closes 
> the circuit on the primary tank circuit. The tank rings with a damped 
> sinusoid (AC) each pulse. Thus, the pulse cap is subjected to AC. I happen 
> to have a 0.0015 uf pulse cap rated at 15 KV. This should be a very good 
> size for a tube coil. I am still seeking an answer to my question about 
> frequency applicability of a pulse cap. I suspect that it it should not be a 
> problem because most tube coils seem to operate at under 1 MHZ frequency.
>
> Steve: Coiling in Iowa
>
> ----- Original Message ----- 
> From: "Tesla list" <<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com> 
> To: <<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com> 
> Sent: Monday, February 11, 2002 4:46 PM 
> Subject: Re: VTTC capacitor
>
>
>
> > Original poster: "Area31 Research Facility by way of Terry Fritz 
> <<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>"
> <<mailto:rwstephens-at-hurontario-dot-net>rwstephens-at-hurontario-dot-net> 
> > 
> > Steve, 
> > 
> > Tetrodes are poor choices for self excited RF power oscillators.  This 
> tubes 
> > triode counterpart, a 3-1000 would be an excellent tube.  All commercial 
> > industrial RF induction heating systems that use tubes employ triodes for 
> a 
> > good reason. 
> > 
> > Pulse capacitors are usually designed for so many 'shots' or discharge 
> cycles 
> > with minimum percentage voltage reversal to none at all.  For this reason 
> I 
> > would have to recommend not using them in a CW RF tank circuit where 100% 
> > reversal at the RF frequency is the norm. 
> > 
> > Relatively large values of tank cap are not normally used in VTTC circuits 
> > because the oscillator tube plate represents a much higher source Z than a 
> > spark gap switch.  The tank circuit must have an impedance at the Fo that 
> > matches the plate impedance of the selected tube(s).  Tube circuits, even 
> in 
> > high powered coils in the tens of kilowatts often use cap values in the 
> merely 
> > 0.002-0.005 uF range.  More L is needed in the tank coil to compensate 
> compared 
> > to a disruptive TC with usually larger capacitance.  Mica transmitter 
> > capacitors are ideal in this tube oscillator application.  'Cheese slicer' 
> > mechanical variable capacitors with large plate spacings from commercial 
> > transmitters and antenna tuners are also suitable as a tank cap.  They can 
> even 
> > be submerged in oil to increase their operating voltage and capacitance, 
> or to 
> > allow a lower voltage cap otherwise unsuitable, to handle the voltage.  I 
> > suspect that the latest rage, the MMC using polypropylene/foil or 
> deposited 
> > foil would be fine as long as the relatively high tank circuit circulating 
> > current ratings were respected in its design. 
> > 
> > The 4-1000 makes a very pretty display tube.  I have one in my display 
> case. 
> > 
> > r stephens 
> > > 
> > > ----- Original Message ----- 
> > > From: <<mailto:tesla-at-pupman-dot-com>Tesla>mailto:tesla-at-pupman-dot-com>Tesla list 
> > > To:
> <<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com>mailto:tesla-at-pupman-dot-com>tesla-at-p
> upman-dot-com 
> > > Sent: Monday, February 11, 2002 08:59 
> > > Subject: VTTC capacitor 
> > > 
> > > Original poster: "Steve White by way of Terry Fritz 
> > >
> <<<mailto:twftesla-at-qwest-dot-net>twftesla-at-qwest-dot-net>mailto:twftesla-at-qwest-dot-net>
> twftesla-at-qwest-dot-net>" 
> > > <<mailto:slwhite-at-zeus.ia-dot-net>slwhite-at-zeus.ia-dot-net> 
> > > 
> > > Are pulse caps suitable for use with a VTTC? I am particularly 
> interested in 
> > > their high frequency suitability. Also, has anyone ever built a VTTC 
> with a 
> > > 4-1000A tube? Since this tube will handle over 1000 watts, it seems like 
> a 
> > > good 
> > > candidate. 
> > > 
> > > Steve: Coiling in Iowa 
> > > 
> > > 
> > > 
> > > 
> > > 
> > 
> > 
> >
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