> 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.
73, Tom W8JI
W8JI@contesting.com
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