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Re: [Amps] Grid Dipping the Pi network in a new amp? --- ( It nowFunctio

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Subject: Re: [Amps] Grid Dipping the Pi network in a new amp? --- ( It nowFunctions)
From: "Chuck Curran" <>
Date: Sat, 21 Jul 2007 14:57:08 -0500
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To All:

I am now getting good results, that at least respond in a logical manner to
all adjustments made.  Story is below:

This morning I inspected and cleaned all connections.  I then made up a
plate load resistor with short leads and clipped it across the input cap.  I
used a measured 2080 ohm resistor, to simulate the anticipated plate load.

I followed the suggestion made to employ my MFJ-259B antenna analyzer,
connected directly to the output of the PI-NET circuit.  With the amp set up
for 80 meter operation, I quickly found a minimum SWR around 4.8 MHz.  I
then started cranking in more tune and load capacitance, but both were
already near maximum.  The minimum SWR, which was 1.3, occurred at 4.6MHz.

I added a 75 pf doorknob padding cap to the tune cap, so along with it's 284
pf I now had 359 pf available on the tune cap.  I added a measured 420 pf to
the load cap, which gave me a total of 1640 pf on the load side.  The 80
meter inductance checked at 7.0 uH  

Now I got my minimum SWR at about 3.740 MHz, much higher than expected.
While the use of the MFJ-259B worked very well, the results I got were
somewhat disappointing.  I expected to see a low SWR down around 3.6 or so,
without the padding caps.  I did expect that I might have to add padding, so
this was not a total surprise.  

I went back and re-checked my calculations, as a result of some of the
points made on the Amps list server this last week.  Actually, I decided to
purchase a new ARRL Handbook and simply load up the PI-NET calculators
included with the Handbook.  Ya, I am lazy, should of just made up my own
spreadsheet.  When I adjust Q (to 16.4) in the Handbook PI_NET calculator
until it gives 7.0 uH as the required inductance, it is suggesting 294 pf on
tune and 1699 on load.  Since I only had 1640 pf on load, possibly that is
why I had to jam in 359 pf on tune and still just got to 3.740 MHz

At this time I do feel I may realize an advantage if I increase the
inductance, since the impact will be a significant reduction in the required
tune and load cap values.  I could finish the amp without any padding caps
and the associated switch, if I simply go from 7.0 uH to about 9.2 uH.  I
will ponder that option tonight.

The grid dip meters never worked.  I did fail to mention I was placing the
grid dip pick-up coil just above the tank coil -- I was not coaxial with it
and the pick-up was not inside the coil at all.  I possibly had poor
coupling with this attempt.

Thanks for all your comments,

Chuck WA9POU

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