Studies of a cyclotron autoresonance maser (CARM) are presented. The measurements are carried out at a frequency of 35 GHz using a mildly relativistic electron beam (1. 5 MeV 300 A 40 ns) generated by a field emission electron gun. Perpendicular energy is imparted to the electrons by means of a bifilar helical wiggler. Amplification measurements give a small signal growth rate of approximately 65 dB/m. The saturated power output is ''-''-33 MW and the corresponding electronic efficiency is ''-7.