[Amps] Cold testing cathode driven GG input circuit

[Jim Thomson]

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