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Ensuring the emissive reliability of microwave vacuum-electronic devices (MVED) is closely related to knowledge of the main operating characteristics of cathodes, primarily of cathode emissive ability and potential (resource) life. In research practice it is convenient to perform an estimate of cathode emissive ability via pre-heated curve (Tsarev and Nikonov, 1958) from which the knee temperature Tkn is found. The knee temperature Tkn, defining the minimum temperature at which the cathode provides operating current density jop required for rated operation of MVED, is directly proportional to the effective work function of cathode and, thus, one-to-one corresponds to cathode emissive ability at the given current density jop (Nabokov nd Dudkin,1968). Information ability of the pre-heated curves method for ensuring emissive reliability of MVED was noted by Vorozheikin and Kozlov (2002). Long term (more than 30 years) investigations of cathodes emissive ability both in experimental models and in real devices allowed us to determine its dependence on cathode operating mode (on cathode operating temperature Top and operating current density jop) and operating time (under conditions excluding an effect of environment factors on operation). In this paper some dependences are given for oxide cathodes (OC), metal-porous (dispenser) M-type cathodes coated with film on the basis of Os (MC), scandium metal-porous cathodes (ScC).