>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.
Efficiency is probably not a hv-rfc issue, except maybe on 160m. .
>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!
>
The only resonances that seem to count are the ones that exist when the
choke is mounted and wired in the amplifier. A resonance within 5% of an
operating frequency may be problematic. If there is a HF resonance
problem, one will probably have a choke fire. However, if there is a vhf
resonance near the anode's vhf self-resonance, if there is a vhf
parasite, a few turns on the choke may smoke, but there will be no fire.
>Now, before I do anything, I want to get some advice.
>
>1) Am I measuring the resonance point properly?
the dipmeter coil winding should be semi-parallel to the choke winding.
......
>
>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.
You wud have to take off plenty of turns to mess it up for 160m.
>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
>
When measuring power out, it's important to measure anode current. If
the anode current is low, input swr may be causing the problem. If the
load is other than 50 plus or minus j zero ohms, all power measurements
are suspect.
cheers
Rich...
R. L. Measures, 805-386-3734, AG6K
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