<P> To prevent gases from compromising the tube's vacuum, modern tubes are constructed with "getters", which are usually small, circular troughs filled with metals that oxidize quickly, barium being the most common . While the tube envelope is being evacuated, the internal parts except the getter are heated by RF induction heating to evolve any remaining gas from the metal parts . The tube is then sealed and the getter is heated to a high temperature, again by radio frequency induction heating, which causes the getter material to vaporize and react with any residual gas . The vapor is deposited on the inside of the glass envelope, leaving a silver - colored metallic patch which continues to absorb small amounts of gas that may leak into the tube during its working life . Great care is taken with the valve design to ensure this material is not deposited on any of the working electrodes . If a tube develops a serious leak in the envelope, this deposit turns a white color as it reacts with atmospheric oxygen . Large transmitting and specialized tubes often use more exotic getter materials, such as zirconium . Early gettered tubes used phosphorus - based getters, and these tubes are easily identifiable, as the phosphorus leaves a characteristic orange or rainbow deposit on the glass . The use of phosphorus was short - lived and was quickly replaced by the superior barium getters . Unlike the barium getters, the phosphorus did not absorb any further gases once it had fired . </P> <P> Getters act by chemically combining with residual or infiltrating gases, but are unable to counteract (non-reactive) inert gases . A known problem, mostly affecting valves with large envelopes such as cathode ray tubes and camera tubes such as iconoscopes, orthicons, and image orthicons, comes from helium infiltration . The effect appears as impaired or absent functioning, and as a diffuse glow along the electron stream inside the tube . This effect cannot be rectified (short of re-evacuation and resealing), and is responsible for working examples of such tubes becoming rarer and rarer . Unused ("New Old Stock") tubes can also exhibit inert gas infiltration, so there is no long - term guarantee of these tube types surviving into the future . </P> <P> Large transmitting tubes have carbonized tungsten filaments containing a small trace (1% to 2%) of thorium . An extremely thin (molecular) layer of thorium atoms forms on the outside of the wire's carbonized layer and, when heated, serve 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 usually deliver lifetimes in the tens of thousands of hours . The end - of - life scenario for a thoriated - tungsten filament is when the carbonized layer has mostly been converted back into another form of tungsten carbide and emission begins to drop off rapidly; a complete loss of thorium has never been found to be a factor in the end - of - life in a tube with this type of emitter . WAAY - TV in Huntsville, Alabama achieved 163,000 hours of service from an Eimac external cavity klystron in the visual circuit of its transmitter; this is the highest documented service life for this type of tube . It has been said that transmitters with vacuum tubes are better able to survive lightning strikes than transistor transmitters do . While it was commonly believed that at RF power levels above approx. 20 kilowatts, vacuum tubes were more efficient than solid state circuits, this is no longer the case, especially in medium wave (AM broadcast) service where solid state transmitters at nearly all power levels have measurably higher efficiency . FM broadcast transmitters with solid state power amplifiers up to approx. 15 kW also show better overall mains - power efficiency than tube - based power amplifiers . </P> <P> Cathodes in small "receiving" tubes are coated with a mixture of barium oxide and strontium oxide, sometimes with addition of calcium oxide or aluminium oxide . An electric heater is inserted into the cathode sleeve, and insulated from it electrically by a coating of aluminium oxide . This complex construction causes barium and strontium atoms to diffuse to the surface of the cathode and emit electrons when heated to about 780 degrees Celsius . </P>

Why did solid state devices replaced thermionic devices
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