The heater inside the cathode

An oxide-coated cathode can't heat itself, and it has to be hot to emit electrons. So, a wire filament heater is inserted within the cathode. This heater has to be coated with an electrical insulation that won't burn up at the high temperatures, so it is coated with powdered aluminum oxide. This is an occasional cause of failure in such tubes; the coating rubs off or cracks, so the heater can touch the cathode. This can prevent normal operation of the tube. And if the heater is running from AC power, it can put some of the AC signal into the amplifier's output, making it unusable in some applications. Good-quality tubes have very rugged and reliable heater coatings.

By Eric Barbour
Information from www.vacuumtubes.net