Transmitting tubes

Large transmitting tubes have tungsten filaments containing a small trace of thorium. A thin layer of thorium atoms forms on the outside of the wire when heated, serving as an efficient source of electrons. The thorium slowly evaporates from the wire surface, while new thorium atoms diffuse to the surface to replace them. Such thoriated tungsten cathodes routinely deliver lifetimes in the tens of thousands of hours. The claimed record is held by an Eimac power tetrode used in a Los Angeles radio station's transmitter, which was removed from service after 80,000 hours (~9 years) of uneventful operation. Transmitting tubes are also claimed to survive lightning strikes more often than transistor transmitters do. For RF power levels above 20 kilowatts, vacuum tubes are commonly more efficient and reliable than similar solid-state circuits.

Information from Wikipedia.