Can minor increases in input circuit lead length or stray capacitance make
a major difference in how the amplifier behaves on one low HF band but not
on others?
My specific puzzle -- last year I installed a vacuum relay T/R circuit in
my SB-220. The input side relay is a miniature Hi-G vacuum unit. The
system has performed flawlessly, except that I noticed that when I used the
amplifier in 40 meters, both plate current and output power were unusually
high, even though the driving power had not changed and tuning behavior was
about normal. Since, as usual, I had not followed the scientific method
and only changed one thing at a time, I had also significantly shortened
the coax from the transceiver (a TS-930), so the first thing I did was to
lengthen the input coax by 6 feet or so. The amplifier's behavior
immediately returned to normal.
I had never noticed this phenomenon before I modified the T/R circuit,
which makes me wonder if it is to blame. Mounting the Hi-G relay involved
extending the input coaxial lead by approximately one inch, which I did
with miniature teflon coax that came with the relay. I was unable to
ground the shield of this length of coax on the end where it joined up with
the original input coax. Could this slightly kludged arrangement be
responsible for the odd behavior on 40?
The relay circuit and the amplifier work just fine otherwise, and even this
isn't a problem as long as I keep the long input cable, but not
understanding it bugs me. Anyone have ideas?
73, Pete N4ZR
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