  State of the art high-current superconducting accelerators require efficient
RF sources with a fast dynamic phase and power control. This allows for
compensation of the phase and amplitude deviations of the accelerating voltage
in the Superconducting RF (SRF) cavities caused by microphonics, etc. Efficient
magnetron transmitters with fast phase and power control are attractive RF
sources for this application. They are more cost effective than traditional RF
sources such as klystrons, IOTs and solid-state amplifiers used with large
scale accelerator projects. However, unlike traditional RF sources, controlled
magnetrons operate as forced oscillators. Study of the impact of the
controlling signal on magnetron stability, noise and efficiency is therefore
important. This paper discusses experiments with 2.45 GHz, 1 kW tubes and
verifies our analytical model which is based on the charge drift approximation.
