I have seen some valves, all of Chinese manufacture, go into a state of thermal runaway, in which the anodes go from just visible red, through orange to a damn great hole in about 2 seconds. The first was a 6146 in an hf transmitter, and subsequently several el34 and 6l6 in guitar amplifiers. The best theory I have heard to date is that the Chinese are happy if they get most of the oxide coating on the cathode, and are in too much of a hurry to care where the rest goes. That which ends up on the control grid will begin to emit electrons if the valve is run anywhere near its maxmum dissipation, turning the valve further on and getting it hotter.
Has anyone any better ideas? I bet Nigel has something to say on this one
Has anyone any better ideas? I bet Nigel has something to say on this one