Just come back from C5&6W where I used a KX3. Unfortunately there were several power cuts whilst there and I used both a main fed psu and a motor cycle battery (floated this off the power supply). The mains when present varied from 180V to 260V and the output varied a little too.
Having seen a catastrophic failure of another QRP rig when an over voltage occurred in the past I protected my KX3 when running off a non-battery external supply. The circuit is very similar to the FT817 voltage conditioner from AD5X http://www.ad5x.com/images/Articles/Condx817RevA.pdf from where I obtained inspiration but this time I used a fixed voltage LD1085V120 low voltage drop regulator which is a 12V T0220 version and with a heatsink will easily sink 3A, a KX3 will draw about 2A at 10W. I used a 600W 15V zener across the input, I added a 3 amp fuse on both the gnd and +ve side. 16V 10uF capacitor between in and ground and another between out and ground. No switch required, no resistor network.
Note I tested a couple of devices to make sure I really did get 12V, some were a little low at 11.6-11.9V when 13.8V in.
I also added a little push to make switch and LED to indicate when power was available.
This meant that the supply into the KX3 is both reverse and over voltage protected and I use a set of power poles on the Elecraft supplied leads into my little gizmo.
One less thing to worry about when I was the DX station.