I always used what was most economically available, based on dielectric
performance and not capacitance value...if there was a great sale on
.001/10kV doorknobs, that was the obvious choice! Ditto for 500pF, or
3000pF, or whatever. Evidently, transient energy can take out any kind of
blocking cap if the surge is big enough...my Henry 3K Premier used four
large doorknobs in parallel and two exploded one day...
WB2WIK/6
"Each success only buys an admission ticket to a more difficult problem." --
Henry Kissinger
> -----Original Message-----
> From: 2 [SMTP:2@vc.net]
> Sent: Wednesday, November 28, 2001 5:44 PM
> To: Tom Rauch; amps@contesting.com; jstrohm@texas.net
> Subject: Re: [AMPS] Another Stupid Question
>
>
> >
> >> What determines the capacitance value for a plate-blocking capacitor?
> >> By this I mean the capacitor (usually a doorknob cap) between the
> >> plate choke and anode connection, and the start of the tank circuit /
> >> pi network.
> >
> >Mostly it seems people just guess at what to use, and never think
> >about why a certain value is required. Most of the sizes used are
> >actually just plain silly..being far too big in capacitance..and too
> >low in current rating.
> >
> >Technically, all the capacitor has to do is effect the impedance of
> >the tube anode a minimal amount.
> >
> >If the anode load impedance is 3000 ohms (you have to include the
> >choke, and do this on the lowest frequency), a 300 ohm reactance
> >blocking capacitor would have minimal effect on the system.
> >
> >That means you only need about 300pF blocking C for 160
> >meters, if the anode system's operating impedance is 3000 ohms.
> >
> >I remember laughing at a friend of mine who stuck a 100pF
> >blocking capacitor (by mistake) in his 3-500Z amplifier for 160.
> >When he changed the part to 1000pF, nothing happened except
> >the tuning capacitor slightly changed position. Efficiency and power
> >out remained the same.
> >
> // Amen, Mr. Rauch, Another Ham radio old wives' tale bites the dust.
>
> - R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
> end
>
>
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