THE GREAT MULLARD MAGIC BLOG — overheated valve

This is an interesting one as a number of insulation failure modes exist for ageing valves.   Often poor insulation may be caused due to cathode emissive material or metallic deposits which have evaporated from the  electrode cage being deposited on various parts of the valves interior.  Deposits can build up on mica separators or the glass pinch where the connection pins/leads enter the envelope and even sometimes the inner envelope glass - remember those manky EL84 which had been run hard and hot and built up those yukky black deposits on the glass.........? In manufacture, the mica separators are...