Hi Jan, Elecraft designed the KX twins to run with maximum efficiency, so up to 10 watts, it is the more efficient PA in the KX3 which supplies the output power with the KXPA100 in bypass, and when more than 10 watts is dialed in the KXPA100 is enabled, and the KX3 throttles back to make the KXPA100 output whatever is desired. Cheers & 73, Howie - WA4PSC