Recently, I became concerned that the hommemade plate choke in my SB-1000 had
series
resonances near some of the bands. I thought that was responsible for the fact
that
I was unable to get full output on the higher bands -- for example, while I
easily
got 800+ watts out on 40 and 750 on 20, I could only get about 600w on 15
meters, and
less on 10. The homemade choke had a primary resonance at about 17.2 mHz, and
lots
of little ones scattered about above that.
The way I measure this is to short the choke (removed from the amplifier), and
then
use a GDO to look for resonances. I'm coupling a counter to the GDO to be sure
of
the frequency.
Anyway, I ordered a replacement AL-80B choke from Ameritron. Before installing
it, I
decided to see where it was resonant -- imagine my surprise to find a big
primary
series resonance at about 21.2 mHz!
Now, before I do anything, I want to get some advice.
1) Am I measuring the resonance point properly? Should the choke be installed
in the
amp? If so, should it be connected to the plate and other circuitry or just
sitting
there? (obviously the power will be OFF!)
2) If I have measured it correctly, then probably I should take a few turns
off of
the new choke to move the resonance point away from the band. What frequency
should
I aim for? I don't want to reduce the inductance too much, or it will mess up
160-meter performance.
This choke seems superior to my homemade one because it only has one big
resonance.
The old one has a number of them. However, I really want to do it right this
time!
Vic K2VCO
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