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I'm not sure which W6SAI handbook you're
looking in but the twenty-third edition
gives a input impedance value of 55 ohms
for the 8874 with 26 watts of drive,
pp15-18.It also gives a value of 54 ohms
for the 3cx800a7 which I have confirmed
on the bench using similar operating
conditions as listed in the handbook.
If you have a set of the constant
current curves the formula for the
approximate driving impedance is also
given in the Orr handbook, pp15-14.
Paul
PAUL HEWITT
WD7S PRODUCTIONS
QRO HOMEBREW COMPONENTS
http://wd7s.home.att.net
Since this thread started, I tried to
locate the magic
drive Z value with mixed bag and varied
results. An
amps member reported ~95 ohms for a
single tube,
a W6SAI Radio Handbook says ~160 ohms
and now
your 2 tube value. Someone has to have
the real value,
it's not listed anywhere in plain sight
on the Eimac data
sheet I just looked up on the web.
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