Electron multipliers And Vacuum tubes in the 21st century

The highest possible vacuum is desired in a tube. Remaining gas atoms will ionize and conduct electricity between the elements in an undesired manner. In a defective tube residual air pressure will lead to ionization, becoming visible as a pink-purple glow discharge between the tube elements.
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

Getter in opened tube; silvery deposit from getter

help free any remaining gases from the metal parts. The tube is then sealed and the getter is heated to a high temperature, again by radio frequency induction heating. This causes some material from the getter to

Tube tester manufactured in 193evaporate, reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside

Voltage regulator tube in operation. Low pressure gas within tube glows due to current flow.

of the envelope of the tube. The getter continues to absorb small amounts of gas that may leak into the tube

70 watt tube audio amplifier selling for $2,680^[23] in 2011, about 10 times the price of a comparable model using transistors.[24during its working life. 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.