>
>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?
>
// The 220's tuned inputs need to have a Q of 2 -3 instead of the orig.
Q of c. 1. Also, the inputs need to be optimized for a particular
length of coax with a specific solid-state output radio.
This subject is covered in "Circuit Improvements for the Heath SB-220
Amplifier", November-December 1990 *QST*. The orig. manuscript for this
article is available on my Web site.
cheers, Pete
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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