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Acorn tube
An acorn tube, or acorn valve, refers to any member of a number of vacuum tubes produced in the USA starting just before World War II. They were named after their resemblance to the acorn, specifically due to the glass cap at one end of the tube that looked similar to the cap on an acorn. The acorn tubes were designed to work at high frequencies, and found widespread use in radios and radar systems. The introduction of the EF50 was the first serious competition for the acorn design, and replaced the acorns in many roles, especially post-war when millions of surplus EF50s were dumped on the market.
The range included about a dozen valves, designed to work in the VHF range. The 955 is a triode. The 954 and 956 types are sharp and remote cut-off pentodes, respectively, all with indirect 6.3 V, 150 mA heaters. Types 957, 958 and 959 are for portable equipment and have 1.25 V NiCd battery heaters. The 957 is a medium-mu signal triode, the 958 is a transmitting triode with dual, paralleled filaments for increased emission, and the 959 is a sharp cut-off pentode like the 954. The 957 and 959 draw 50 mA heater current, the 958 twice as much. In 1942, the 958A with tightened emission specs was introduced after it turned out that 958s with excessively high emission kept working after the filament power was turned off, the filament still sufficiently heating on the anode current alone.
==References==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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