From 2015:

On Apr 15, 2015, at 7:48 AM, Edward R Cole <kl7uw@...> wrote:

One question is how low voltage can be adjusted.  Amps are designed for 12.0 volts.

Ed,

All Elecraft 100-W amplifiers are designed for optimal performance at about 14 V (as measured at the DC jack on key-down). This includes the K3's and K2's internal 100-W amps as well as the KXPA100. They will certainly run from lower voltages -- down to about 10.5 to 11 V -- but for home use, we recommend setting the DC supply to 14.0-14.2 V. They'll run from as high as 15 V, though again, around 14 V optimizes power output, IMD levels, and efficiency.

73,
Wayne
N6KR

de Rick NK7I

I remember a comment from Elecraft saying that the K3 will transmit a cleaner signal with voltage around 13.8 to 14 volts.   I don't remember the exact comment.