>INPUT TUNING: What I found to be the hot ticket has been the MFJ 259B
>antenna analyzer. Remove drive cable from your amp and connect it to the
>259, key the amp and adjust the slugs. Just watch the meters as you crank
>the slug into resonance.
>
>Others, more astute than myself, will provide the answer to the 10 meter
>problem...
There is a problem using this analyzer to do input tuning:
The input impedance of a tube varies wildly over the drive cycle and
drive level. So in other words to really match it properly, you need to
apply full drive to the amp and then tune it. The MFJ analyzer really
puts out micro watts or so and that is not enough to drive the tube from
its cutoff region to active region. Yes, it may get you "close" but
"close" is not necessarily a good match at high power. Large signal
impedances are different from small signal impedances.
I didn't believe this at first when people told me, but I read about it
and then I watched it in real life. I watched as my SWR literally varied
significantly as I increased or decreased drive to the PA.
In the end, I was glad I didn't buy one of those analyzers.
Unfortunately, I did spend $90 on a noise bridge which really has the
same limitations.
73,
Jon
KE9NA
--------------------------------------------------------------------------
Jon Ogden
jono@enteract.com
www.qsl.net/ke9na
"A life lived in fear is a life half lived."
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