[Elecraft] queer behaviour !

[Paul Christensen]

The K3's KEY OUT jack is rated at +200V @ 5A.  I owned an SB-230 back in the 
mid '70s and I believe it uses a relatively HV negative key line.  So, while 
the K3's keying circuit is very robust for positive key lines, it acts as a 
short on negative key lines.  I doubt any damage has been done to the K3, 
despite the high negative key line of the SB-230 amp.

The Jackson Harbor Press "Keyall" uses a pair of HV MOSFETs, floating above 
circuit ground.

http://wb9kzy.com/keyall.htm

This allows for positive, negative, and even AC amplifier keying, all from 
one circuit.  A commercial version, already wired/assembled is shown on a 
web link and is manufactured by Gary, KD9SV.  There are other commercial amp 
keying interfaces available, but not all of them will accommodate 
high-voltage, negative key lines.

If you operate much CW, I would go one step further and find an Ameritron 
QSK-5, designed by W8JI.  It's really an excellent design.  I use one 
(highly modified) between my K3 and SPE 1K-FA amp.  The mods take care of 
some additional design deficiencies within the SPE amp.

Paul, W9AC