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They withstand heat because they are made of class and metal and not plastic so that nothing melts. Heat is dissipated through the air. Anode and cathode are both made of metal. Only the case (tube) is made of class to seal the vacum inside it.
Materials used in tubes are nickel for electrodes an wiring, thungsten for heating element, ceramics or mica for support and glass for enclosure. These materials are known as vacuum materials because they do not emit gasses or iones at high temperatures.