. The thermionic vacuum tube and its applications . Fig. 131. Fig. 132. account of their low vapor pressure, the refractory metals such astungsten and molybdenum are very suitable materials for use inhigh-power tubes. Tungsten can, for example, be heated duringevacuation to about 2500° K. to drive out occluded gases, whileduring operation the tungsten anodes can safely rise to a tempera-ture of about 1500° K., thus making it possible to use a smalleranode area than is necessary when less refractory metals are used asanodes. On the other hand, high-power tubes that are to operate THE THKRMIOMC
. The thermionic vacuum tube and its applications . Fig. 131. Fig. 132. account of their low vapor pressure, the refractory metals such astungsten and molybdenum are very suitable materials for use inhigh-power tubes. Tungsten can, for example, be heated duringevacuation to about 2500° K. to drive out occluded gases, whileduring operation the tungsten anodes can safely rise to a tempera-ture of about 1500° K., thus making it possible to use a smalleranode area than is necessary when less refractory metals are used asanodes. On the other hand, high-power tubes that are to operate THE THKRMIOMC AMPLIFIER 245 on comparative!} low jilate voltages often require so much fila-ment that when the filament strands are properly spaced andsupported in a mechanically convenientmanner, they occupy a surface whichis so large as to necessitate a largeplate area. It is also necessaiy, in de-signing power tubes, to consider thesize of the bulb and make it large enoughto prevent overheating of the radiating area of the glass bulb,expressed in
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