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[AMPS] Another Stupid Question

To: <amps@contesting.com>
Subject: [AMPS] Another Stupid Question
From: jstrohm@texas.net (Jim Strohm)
Date: Wed, 28 Nov 2001 21:00:00 -0600
So in theory, if I had a capacitor that could pass the current and
withstand the DC, almost any value low enough not to "tweak" the palte
impedance would work.

This might be a way to use up all those ARC-5 50 pF vacuum caps that roll
around under the swap meet tables for 25 cents each.

Jim N6OTQ



>> 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.




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