[Elecraft] Transmitting low power through the PA

[Dave G4AON]

Julian the problem with running low power through a high power PA is 
that you get poor efficiency. A typical HF transceiver running 5 Watts 
output from a 100W PA may be drawing 8 Amps to do so. The Elecraft 
solution nicely improves efficiency and works well for most situations.

73 Dave, G4AON
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I believe that the K3 and K2 are unique in that they switch the PA out of
circuit when low power (below 12W in the case of the K3) is used. Why do
they do this? Would it not generate a more linear signal and result in less
stress on the low power PA components if the high power PA was used, where
fitted, even for power levels of 10W or less?

I have recently become interested in the WSPR beacon mode
(http://physics.princeton.edu/pulsar/K1JT), which involves transmitting a
continuous signal for periods of 2 minutes. At a level of 5W the underside
of the K3 gets uncomfortably hot. Since this is the sort of thing you want
to leave running all day, I'd prefer the 100W PA with its big heatsink and
pair of fans to take most of the strain. Is this possible?

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Julian, G4ILO. K2 #392 K3 #222.