The metals and alloys suitable for anode material are, in general, also useful for grid structures. Like anode materials, a good grid material should have reasonably low gas content, should be easy to degas, and should have sufficient mechanical strength to hold its shape while operating at very high temperatures. A very «mall change in the shape of a control'grid structure results in a relatively large change in tube characteristics. Grid material should be suited for drawing into wire, because grids are often formed of spirally wound wire supported by metal side rods.
An important consideration in the choice of grid material is the electron' emitting characteristic of the material, especially in the presence of other elements which may be used in tube manufacture. In most types of r-f service the grid is driven positive part of the time, so that the grid is bombarded by electrons and must dissipate some power. If the grid material is active enough, or the grid temperature gets high enough, primary grid emission may take place. This effect should be minimised in tube operation because it may result in loss of grid bias if a grid leak is employed. Grid structures are sometimes pretreated in various manners to reduce primary grid emission.
When the control grid is driven positive, the primary electrons which bombard it may dislodge secondary electrons. This effect, called secondary emission, may also cause a loss of grid bias, and must be minimized by proper choice of grid materials and by suitable processing methods.
Some of the metals used for grids are tungsten, molybdenum, tantalum, and also nickel alloys, such as magno-nickel. The latter is an alloy of nickel and manganese. Alloys of molybdenum and tungsten are also employed. Grid materials in some cases are coated with carbon to reduce secondary-emission effects and to increase thermal emissivity.
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