[Elecraft] Requirements for keying a KXPA100

[John Marvin]

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