Isn't there also the issue of the current rating of the blocking cap?
At higher frequencies, the tubes stray C can carry quite a bit RF
circulating current, thus the tank will draw an equivalent amount
of circulating current thru the blocking capacitor (in opposite
phase to the circulating current in the tube C of course). Thus
I would expect the blocking cap may need to carry significantly
more RF current than would be implied by the plate impedance
of the tube if the stray C makes up a significant portion of the
tune C in the PI network (as in a 6 meter amp for instance).
73 de Mike, W4EF...........
----- Original Message -----
From: "2" <2@vc.net>
To: "Tom Rauch" <W8JI@contesting.com>; <amps@contesting.com>;
<jstrohm@texas.net>
Sent: Wednesday, November 28, 2001 5:44 PM
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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