I got a variety of suggestions, and decided to go the easiest route
(since it worked) of removing the current limiting resistor and
switching the diode to a schottky diode. The extra diode is required in
my case because the PTT also keys an internal TX preamp whose keying
circuit is run on 13.8V. Without the diode this circuit (unless I chose
another suggestion of creating two separate PTT drivers on the
transceiver) would see the floating 5V in the KXPA100 as a "relative"
ground, so just connecting the KXPA100 key line would have caused the
internal TX preamp to be keyed.
It was clear that a current limiting resistor was not required for
either the internal TX keying circuit or the KXPA100 (they key current
is about 1 mA for each of them). I decided if a current limiting
resistor is required for keying another amp I can build an external
adapter cable that includes the resistor for that use case.
With the schottky diode in place the Hermes transceiver pulls the
KXPA100 key line down to about 0.39 volts, which should be enough to
trigger it with hopefully a comfortable margin. Everything appears to be
working properly now, so now I get to play!
Anyway, thanks to all for the helpful suggestions.
73,
John
AC0ZG
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