[Amps] Discrepencies in Pi-net calculators

[Jim Thomson]

Date: Thu, 21 Nov 2013 17:02:10 -0700
From: "Steve Flood" <kk7uv at bresnan.net>
To: <amps at 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