Date: Thu, 21 Nov 2013 17:02:10 -0700
From: "Steve Flood" <kk7uv@bresnan.net>
To: <amps@contesting.com>
Subject: Re: [Amps] Discrepencies in Pi-net calculators
Carl WS7L wrote:
>>Elmer Wingfield, W5FD, wrote it up nicely in the Aug 1983 QST.
Thanks Carl! The Wingfield article is terrific and satisfies my curiosity.
73, Steve KK7VU
## watch out. Although the Wingfield calcs are correct, they don’t factor in
stuff like
stray C between and grid on any GG amp. They also dont factor in stray uh
between anode and
the C1 tune cap. By inserting just a tiny bit of uh, like .4uh to .7uh,
between plate block caps and
the c1 tune cap, the plate load Z will drop like a rock on the high bands.
That effect can be used to great
advantage. IE: a plate load Z of 1900 ohms can easily be reduced to 1000
ohms.... on 15 and or 10m.
Then the KW PI or PI-L net can then be designed around a 1000 ohm plate load Z.
That then allows one
to use a much lower loaded Q on the higher bands. With a low min C tune
cap, like a 2 section type, with one section
being really low, like 5-25 pf etc, will allow for an even lower loaded Q.
Ditto with a vac variable. Lower loaded
Q means broader tuning, less having to tweak the tune + load when QSYing.
Lower loaded Q also means way less
circulating current in the band switch and also the tank coil... = way less
heat generated.
## good news is the GM3SEK PI and PI-L free software will factor all this
stray C and stray L into the equation.
You also get to juggle what value you want to use for the loaded Q. IMO, a
loaded Q of 8-10 is more than plenty
for a PI net. On my 3CX-3000A7 amp, it’s a 160-15m affair. Q on 15M is
8.......which rises to 12 when the 15M
coil is re-used on 17M. To pull that off, I used a .66 uh coil, made from
.375 inch OD cu tubing, 7 turns on a 1.5 inch ID,
and inserted it between the plate block caps and the C1 vac tune cap. The .66
uh coil is also at right angles to the 20-17/15m
coil. I have also implemented the same scheme by tapping in 1 turn into the
main coil with the C1 vac tune cap. Both methods
work superb. Bottom line is..there is no reason to run a sky high loaded Q on
the upper bands..... including 6M.
Jim VE7RF
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