[Amps] TUNER

[Jim Thomson]

Date: Sun, 10 Oct 2010 07:03:52 -0700 (PDT)
From: Bob Gibson <w5rg at 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