We present data related to the repetitive pulse performance of Reltron HPM tubes. While the tubes using thermionic cathodes are usually limited only by the power supply and/or cooling system, the repetition rate for the high-peak-power tubes (which use explosive emission cathodes) is limited by the rise in the vacuum pressure. In particular, if the tube pressure significantly exceeds 10-4 torr, the tube fails to operate properly, and the voltage pulses are significantly degraded. We have developed a simple model to describe the phenomenology. We have also begun to explore alternate cathode materials which evolve less gas than velvet. A CsI-coated graphite-fiber cathode appears most promising at present.