Here's an idea to try.
Add an external tuner to the input of the amplifier (leaving
the input pi network in place). Run up the amplifier to the
required operating conditions and use the external tuner to
get good VSWR at the exciter output. Do this at the power
you plan to use because the impedance changes with drive
level.
Shut everything down and connect 50 ohms to the external
tuner input (instead of the exciter), remove the tubes and
use the MFJ259 to measure the impedance at the tube socket.
If you reverse the sign of the reactive part, you get the
tube impedance (for example, if the 259 reads 25 ohms and
16ohms inductive, the tube input impedance is 25 ohms and
16ohms capacitive).
Steve
-----Original Message-----
From: dj2hz <wsz@ginko.de>
To: amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
Date: 01 August 1999 23:31
Subject: [AMPS] Input pi-net
>
>Hi all,
>
>I designed a switchble input pi-net for my 'two-holer' (a
pair of 8877's). The
>calculated Z is aprox. 23 ohms. The circuit is located
close to both input
>blocking caps and tubes. I tested this circuit with both
tubes connected to
>the sockets and a 23 ohms resistor cross to both blocking
caps. I tuned this
>pi-net for all bands. As source generator I used the MFJ
Antenna Analyser 259.
>All settings was nearly SWR 1:1 and I was satified. Later,
under real power
>conditions all values are away up to 2:1 and higher. I know
my first test was
>made under static circumstance and in the real world it
works under dynamic
>conditions. But I do have no chance to tune the network
under operation
>conditions (no possibilities).
>Is there to go another way to fix it right under 'could
conditions'? Otherwise
>I have to make a new metal design to operate the input
pi-net parts under
>power conditions.
>Maybe it would be better to test this circuit with more
r.f. power than the
>MFJ 259 produces???
>73, Werner DJ2HZ
>
>
>
>--
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http://www.contesting.com/ampfaq.html
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>
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