Gunn oscillators on diamond heat sinks have produced up to 780 mW c.w. with 5.1% efficiency at 9.9 GHz. The devices were operated in full-height waveguide cavities. Similar devices on copper have produced up to 700 mW with 4.1% efficiency and typical powers of 600 mW with efficiencies of 3.9%. The effect of varying the 2nd-harmonic loading on devices mounted in fully-reduced-height waveguide has also been investigated, but no enhancement in output power at the fundamental frequency was observed.