Date: Sun, 10 Oct 2010 07:03:52 -0700 (PDT)
From: Bob Gibson <w5rg@yahoo.com>
Subject: [Amps] TUNER
If you put the tuner at the back of the Amp..Why not just use the tuner in the
Radio? Bob
## You folks keep forgetting the cathode RF return current pulses are via the
C2
cap of any PI tuned input. Those pulses are typ 3 times the plate current
for
AB-2... and 4 times the plate current for class C. I already know one guy
here
in town who has fried the auto tuner in his FT-990..from using it as a tuned
input.
IE: the auto tuner has to hanle the xcvr's normal RF.... PLUS the 3 x times
plate current pulses.
IF your built in PI net tuned circuits in your amp have a bit of swr on band
edges
then it's ok to use the auto tuner in the xcvr to take the swr down to 1:1,
otherwise,
don't mess with the auto tuner. Besides, when you switch to barefoot, the auto
tuner
now see's the ant swr, and has to re-tweak itself.
## If you mount the tuned input any distance from the cathode, eff will go
to hell
really fast. You want the C2 cap as close to the cathode as practical.
Putting
a small fixed cap right at the cathode will shift some of the C2 directly to
the
cathode, increasing eff on the higher bands... but it can't be too high,
otherwise
you won't be able to adjust 10m. 50-150 pf NPO is usually ample.
## another trick is to use < 50 ohm coax between C2 and the cathode, esp if
the coax length is > 12-14". 2 x RG-59's in parallel...or 2 x RG-58's will
do the
job. That's in effect, like moving C2 closer to the cathode. That works
good on the
upper bands.
Jim VE7RF
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