As part of the Electron Cyclotron Heating (ECH) Facility upgrade at the DIII–D National Fusion Facility, a regulated AC power supply for the filaments of a 110 GHz Gyrotron was designed, tested and integrated into the new ECH power system.1 A precision regulated filament power supply is essential for maintaining the stability of the cathode temperature and consequentially the gyrotron beam current and rf production.2 This paper provides a description of the design of the filament power supply, presentation of test results, and an assessment of its performance.