[Amps] Cold testing cathode driven GG input circuit
Jim Thomson
jim.thom at telus.net
Sat Dec 2 12:41:07 EST 2017
Date: Sat, 2 Dec 2017 15:15:54 +0100
From: Conrad PA5Y <g0ruz at g0ruz.com>
To: amps at contesting.com
Subject: [Amps] Cold testing cathode driven GG input circuit
<I know how to cold test an anode circuit by connecting a resistor
<between anode and grid using a resistor with the value of the design
<plate RL. I presume that I can do the same cold test for the input by
<using a resistor of the value of cathode Z in. Its a 3CX800A7 is
<supposedly 54 ohms for my design parameters. I am assuming that if I
<connect a resistor of this value between the cathode and grid I can do
<exactly the same thing as for the anode?
<Is this correct or did I miss something?
<Regards
<Conrad PA5Y
## Thats exactly how I do it.... except you must also factor in the stray C
from the tube or tubes themselves. IE grounded grid to cathode C is directly
in parallel with the C2 cap of any PI tuned input. It just means the actual value
of C2 is reduced by the value of the stray C.
## What I have found is... even when the digital lcr meter is used for the exact
PI tuned input values, that when the amp is placed in service, the actual C1 + C2
values may well be off slightly, keeping the coil the same.
## Typ the harder the tube is driven in GG, the input Z will start to drop, and in some
cases, quite a bit. So allow for some wiggle room. But the basis test, using a load resistor
and a MFJ-259 works good. Just tweak the C1 and c2 for flat swr on each band of interest.
Jim VE7RF
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