A "getter" removes or absorbs any stray air in the vacuum tube. Vacuum tubes only operate with a near complete absence of air inside the tube. As the tube ages small leaks may develop, the getter handles this situation if the leak remains small. The getter has no effect on the output of the tube but does maintain the integrity of the vacuum tube for long operation.
The getter of most glass vacuum tubes is flashed at the factory. After the tube is assembled and as much air as possible is pumped from, the tube an electrical current is introduced to the getter. The getter "flashes" absorbing any oxygen remaining in the tube. The getter usually turns a silver color during this process. If the vacuum of the tube is broken and additional oxygen enters the tube the getter turns white indicating a seal has broken.
Glass Power Tubes
Power tubes, tubes that operate at high temperatures, do not use the flashed getter but rather have a small component of materials that absorb oxygen at high temperatures. These tubes often glow a red color indicating extreme heat. These types of tubes are designed to operate at these temperatures.
Flashed getters are commonly made from pure barium. This material is known for its ability to absorb oxygen in its pure state. Zirconium, titanium and graphite are used in tubes that do not use a flashed getter. Graphite is most heat resistant and most expensive making it the ideal material for the highest powered vacuum tubes.
Ceramic tubes, vacuum tubes with an exterior housing of ceramic materials rather than glass, commonly use zirconium getters. Because it is impossible to see into the tubes it is impossible to observe the getter and determine if any oxidization has occurred. These tubes must be tested by checking the grid current to determine the operational status of the tube.
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