I figured it out!
After looking at the problem and using admittances it became obvious why it
works. The rf choke and bypass cap are part of the plate susceptance and the pi
network with the higher L, C values, help matching down to 50 ohms (at 160m).
This is a good example where admittances help you see the problem better than
using impedances. I should have thought of it sooner.
chris kb3bf
-------------- Forwarded Message: --------------
From: rfdude@comcast.net
To: amps@contesting.com
Subject: [Amps] RF choke for 160m amp mod
Date: Mon, 30 Jan 2006 15:40:44 +0000
Hi,
I added another 90microH mounted orthogonally to the existing (also 90microH)
when modifying my 30S-1 for 160m.
The unmofidied amp calls for 1.5nF 6KV bypass after the (original) RF choke.
I mounted 5nF 10KV after the second choke and I was going to switch the 1.5nF
(now between the chokes) in and out of the circuit when going from the regular
bands to 160m.
Well it turns out I don't need to, and with it in place I still get about 10%
extra output on 160m, so that's one less thing I need to worry about. I have
the
amp on the air for many hours with no signs of heating or fireworks.
Why am I even bothering with the second choke? However the numbers say I show
use it.
If the PA output has a (magnitude) Z of about 5-6K, and the choke and bypass
reactances are about j1000 and -j100 at 2MHz respectively, why shouldn't I
observe a lower efficiency output on 160m, instead I get more! What gives?
thanks for any inputs on this surprising output (pun intended).....
73s
kb3bf chris
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