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The design requirements affecting power tubes at high frequencies are discussed and their application in the development of three new water-and forced-air-cooled triodes are outlined. For water-cooled operation these three types have the following ratings: Characteristics and constructional details of the new tubes are described. The use of a re-entrant anode in the largest type is of particular interest. By this design, in which the anode is folded back on itself twice, the length of internal leads to the grid and filament structures is reduced to approximately one quarter the length required in conventional designs. Fin-type coolers for forced-air cooling of the anode are described.