[en] The characteristics of new prototype high gain photomultipliers having two microchannel plates in cascade for electron multiplication have been investigated. Measurements are given of the gain, dark current, quantum efficiency, anode pulse linearity, electron transit time, single and multiphotoelectron time spreads, and pulse height resolution of LEP PM137 8/II and PM137 7/I photomultipliers. The gain as a function of transverse magnetic field has been measured and is discussed. Emphasis is put on the determination of optimum photomultiplier operating conditions, particularly with respect to their pulse height resolution capability. Photomultiplier characteristics as a function of input pulse repetition frequency have also been investigated and discussed