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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