An investigation of the failure of a modified 1B24 TR (transmit-receive) tube revealed the cause to be the short circuiting of the pulsed keep-alive cathode to its anode, due to the formation of an unusual deposit on the cathode insulation and on the nearby anode surfaces. Analysis of the deposit showed that it originated at the cathode. To facilitate the investigation, diodes were constructed in which such parameters as distributed capacity, cathode material, current density and wave-form, and water vapor content of the gas fill could be readily varied while maintaining constructional features of the modified 1B24 relevant to the problem. The study disclosed that the glow discharge normally present breaks into a momentary arc, which by local heating of the cathode frees the metal that forms the deposit. Critical factors in the processes were determined, and by their proper control the deposit could be reduced or under some circumstances eliminated.