>For example, I can use a MFJ-259 Antenna analyzer (or the new
>MFJ Antenna Analyzer that measures R and X, return loss, reflection
>angle, and other parameters) or any other signal source to excite the
>cathode (or anode)
Tom,
The small signal level present on one of these instruments will do
nothing to tell you the impedance of the cathode of a tube. I took the
suggestion from a ham article to use an RX noise bridge to do it. I
spent 80 bucks and got nothing. When a tube is driven by 100 Watts or
even 10 Watts it is in large signal mode. All that the normal test
equipment and even the expensive network analyzers excite things with
small signal modes (In my commercial design days we did make up high
power NA setups but it is complicated). You and I both now that small
signal parameters of a devictally different from the large signal params.
In class A amps, you can *maybe* get away with measuring input impedance
or such with a small signal but certainly not in AB or B where the
driving signal effectively biases the tube.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@webspun.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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