ANODES

Tungsten

Tungsten was one of the first materials employed for anodes in air-cooled tubes. From the standpoint of gas content, ease of degassing, vapor pressure, and maintenance of mechanical shape at high temperatures, tungsten is a satisfactory material for anodes. However. from the standpoint of workability, tungsten has a serious disadvantage. That is difficult to fabricate into the desired shapes and is, therefore, little used at present as an anode material.

Molybdenum

The characteristics of the metal molybdenum make it suitable for use as an anode material. Although its thermal emissivity is rather low, molybdenum degases readily and is much more workable than tungsten. The heat-dissipating ability of molybdenum anodes is improved by the addition of fins (e.g., such as in type 852). which increase the radiating area of the anode. Further improvement is obtained when the anode surface is roughened by means of carborundum blasting.

Graphite

Graphite is used as an anode material in many radiation-cooled cube types. Although graphite contains considerably more adsorbed gases than either tungsten or molybdenum, these gases can be largely removed by suitable manufacturing technique. This includes pretreatment of graphite anodes before the tubes are assembled.

The thermal emissivity of graphite depends on the treatment the surface has received. Compared with molybdenum anodes, graphite anodes operate at a visibly lower temperature for the same power dissipation. Some users of transmitting tubes find it convenient to judge the operating efficiency of a tube by observing the color temperature of the anode. With tungsten, molybdenum, and tantalum anodes this is easily possible, because at the normal operating temperature the anodes are distinctly cherry or orange-red in color. With graphite, however. practically no color can be seen in normal operation so that it is very difficult to judge visually how much energy is being dissipated by the anode.

Graphite anodes are made with relatively thick walls for mechanical strength. They are not subject to warping and have the further advantage that their good heat conductivity, due to the thick walls, prevents "hot spots." The absence of hot spots means that the graphite anode radiates heat almost uniformly over its entire surface. Because graphite, as ordinarily termed, is a complex mixture of a variety of carbon forms, some of which produce undesirable effects in anodes, careful selection and processing of graphite anode material is essential. Mechanically, graphite presents no serious problems. It is a soft material and, therefore, can readily be formed into the desired shapes. The vapor pressure of graphite is low enough so that bulb blackening can be avoided during the exhausting of a tube.