The TL922 I have would not load properly at 1810 kHz without adding
capacity. Although now it works, it is less efficient than on other
bands and it probably should have more inductance. I couldn't find an
easy way to do this neatly, so I left it alone. I suspect it was
designed for 1.9 mHz.
It is a Japanese model -- you can set it up for 200-240 V but not 120,
and it came with 10 meters installed. I wonder if they are different in
any other way?
On 3/11/2014 9:33 AM, Carl wrote:
Add a carbon pot at the anode end and feed an antenna analyzer into the
output connector. Find out what the difference is between calculated and
tested and do the math to find the K factor used.
Ive often found even commercial amps to be shy on the 160 and 80M load C
especially when a lot of fixed C is used and the measured C is well on
the low side of what is printed.
One amp that is shy on 160M C and L is the Clipperton L and its variants.
The SB-200 is also low on C at the low end of 80 with a typical antenna.
For my own amps I modify and make the pi network load the various160 and
80M antennas since some have a high VSWR at places and I refuse to waste
money on an external tuner. Ive been doing that since the 60's.
Carl
KM1H
----- Original Message ----- From: "Jim Garland" <4cx250b@miamioh.edu>
To: <amps@contesting.com>
Sent: Tuesday, March 11, 2014 11:44 AM
Subject: [Amps] Pi-L Network Question
Hi all, I've built a tank circuit using the on-line Pi-L network
calculators by VE3OZZ and also G3SEK. These are both based on the
equations
published in an article in August 1983 QST (by W5FD). Althought the G3SEK
calculator is somewhat more sophisticated (it corrects for tube and stray
inductances and capacitances), both calculators give about the same
answers
for 80m and 160m. I'm finding that the predicted values for C1 and L1 are
very close to what I need to tune the amplifier, but the predicted
values of
C2 are far lower than what is required to load the amplifier properly.
I'm
wondering if there could be an error in the W5FD formulas, and if anyone
else has experienced the same problem? (If there is an error, it probably
wouldn't be noticed on the higher frequency bands, because the load cap
would most likely have enough tuning range to compensate for the error.)
I've computed the Pi-L network values over a range of plate
impedances. (My
amp uses bandswitched L1 and L2, so those values don't change.) What I
find
is that as the plate impedance increases (e.g., tuning the amp at a lower
power level), the equations predict that Q goes from 10 to 18, C1 doesn't
change, C2 increases only about 5 percent. In other words, according
to the
on-line calculators, tuning to a lower power requires a minor tweaking of
the load capacitance, but that's all
At 3.5 MHz, for my amp, the equations predict a load capacitance of
1057 pF
and on 1.8 MHz, a load capacitance of 2057 pF. I'm finding that, in
practice, those predicted values are more than 1000pF too low.
Here are some details of the actual tank circuit:
The design plate impedance is nominally 720 ohms (2500V@2.0A, with k=1.7)
<mailto:2500V@2.0A,%20with%20k=1.7)> , Q=10, and I've computed network
values for a range of plate impedances from 720 to 1440 homs. The actual
tank circuit is:
80m: L1=8.4 uH, L2=3.8 uH
160m: L1=16.3uH, L2=7.4uH
I'm using two paralleled 1000pF doorknobs for a plate blocking
capacitance.
The plate choke is 225 uH, bypassed at the base by 7700 pF. The safety RF
choke is 470uH, with an 18 ohm DC resistance. The tune and load caps are
30-240pF and 33-1000pF air variables padded with doorknobs, as required.
The tank seems to tune smoothly, with no heating or quirkiness. THe only
problem is that I need much more C2 capacitance than the formulas
predict.
At this point, I'm at a loss to explain the discrepancy, other than
wondering if there's an error in the formulas somewhere.
73,
Jim W8ZR
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