The first operation of a plasma maser /1/, utilizing High-Current REB showed the possibility of Cherenkov radiation of intense microwaves from plasma waveguides and therefore the possibility of constructing single-mode High - Power microwave generators using REB. The parameters of the Plasma Cherenkov Maser such as threshold plasma density, startup current of the utilized relativistic electron beam (REB), efficiency and gain were calculated in the high magnetic field limit in previous works /2,3,4/. The estimate conditions of beam kinetic energy transform into a plasma coaxial waveguide TEM mode were analyzed in /2/. By now we have developed this project and improved the quality and mode composition of the output microwave power. We'll discuss two groups of new experimental results corresponding to various geometries of the PCM.