While the site quoted has the info in there, it is kind of hard to
follow/read.
Here is an example of how to size a plate padding capacitor.
Let's assume we have an amp with a plate load impedance of 2000 ohms that
puts out 1500 watts. To find the voltage across the plate tune or padder
capacitor;
V=square root of power times resistance. So RMS RF voltage would be square
root of (2000 x 1500) = 1732 volts across the capacitor.
If you use a 100 pf capacitor at 1.8 MHz it will have a reactance of 884
ohms.
1732 volts divided by 884 = 1.96 amps of current thru the capacitor.
If you look at the Hi Energy web site at 7.5 kv capacitors, a 100 pf
capacitor has a current ratting of 3.6 amps at 1.8 MHz. that should work
fine.
http://www.highenergycorp.com/ceramic/ht50.html
However if you were to use a 400 pf capacitor in its place, the reactance of
the 400 pf cap would be 221 ohms at 1.8 MHz.
with the same 1732 volts of RF energy there would be 7.8 amps of current
thru the capacitor.
Looking again at the current rating for that capacitor you find that it is
only rated at 3 amps! Yes it will get hot.
So using smaller capacitors in parallel is the better choice.
73
Gary K4FMX
> -----Original Message-----
> From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
> Behalf Of Mike McCarthy, W1NR
> Sent: Sunday, February 22, 2009 8:20 AM
> To: evert@pa2kw.com
> Cc: amps@contesting.com
> Subject: Re: [Amps] Doorknob quality
>
> Take a look at http://www.qsl.net/i0jx/capacit.html for some advice.
> Too bad I just sold a 500pf type 859 at a flea market recently...
> I would try and find a surplus vacuum capacitor and a vacuum relay to
> switch it in as I did for 160M in my 4-1000 amp.
>
> Mike, W1NR
>
> Evert Bakker (PA2KW) wrote:
> > Hello Jim en Colin,
> >
> > Thanks for info.
> >
> > It will be an extra capacitor switched in parallel with the variable
> plate
> > capacitor (so within the tuning circuit)
> > I "cooked" already two of these (different) doorknobs in just a few
> minutes.
> >
> >
> > I guess Colin is right, it must be the high currents.
> >
> > I have no idea what the currents are, and often they are not stated on
> the
> > websites which makes selecting the proper device difficult.
> >
> > I looked at
> > http://www.surplussales.com/capacitors/Trans_Coup_Caps/cap_trans.html
> > And saw the TDK and Murata capacitors are labeled as high current.
> >
> > Any suggestion before making an "investment"?
> >
> > 73's, Evert PA2KW
> >
> >
>
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