Editing Triode (section)

=== High-power triodes ===
High-power triodes generally use a [[electrical filament|filament]] which serves as the cathode (a directly heated cathode) because the emission coating on [[Hot cathode|indirectly heated cathodes]] is destroyed by the higher ion bombardment in power tubes. A [[thoriated tungsten]] filament is most often used, in which [[thorium]] added to the tungsten diffuses to the surface and forms a monolayer which increases electron emission.  As the monolayer is removed by ion bombardment it is continually renewed by more thorium diffusing to the surface.  These generally run at higher temperatures than indirectly heated cathodes. The envelope of the tube is often made of more durable ceramic rather than glass, and all the materials have higher melting points to withstand higher heat levels produced. Tubes with anode power dissipation over several hundred watts are usually actively cooled; the anode, made of heavy copper, projects through the wall of the tube and is attached to a large external finned metal [[heat sink]] which is cooled by forced air or water.