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Anacorn tube, oracorn valve, refers to any member of a family ofVHF/UHFvacuum tubes starting just beforeWorld War II. They were named after their resemblance to theacorn, specifically due to the glass cap at one end of the tube that looked similar to the cap on an acorn. The acorn tubes found widespread use inradios andradar systems.
High-frequency performance is limited by (1)parasitic lead inductance and capacitance andskin effect, and (2) electron transit time (the time required to travel from cathode to anode). Transit time effects are complicated, but one simple effect is thephase margin; another one is input conductance, also known as grid loading. At extremely high frequencies, electrons arriving at the grid may become out of phase with those departing towards the anode. This imbalance of charge causes the grid to exhibit a reactance that is much less than its low-frequency "open circuit" characteristic. Acorn- as well asLighthouse tubes andNuvistors attempt to minimize this effect by arranging cathode, grid(s) and anode as closely spaced together as possible.[1]
The original range included about half a dozen tubes, designed to work in the VHF range. The 955 is atriode. The 954 and 956 types are sharp and remote cut-offpentodes, respectively, all with indirect 6.3 V, 150 mA heaters. Types 957, 958 and 959 are for portable equipment and have 1.25 VNiCd battery heaters. The 957 is a medium-μ 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 specifications 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.[2][3] After the introduction of the miniature 7-pin base, the 954, 955 and 956 were made available with this base as 9001, 9002 and 9003. Other acorn tubes include:
Larger, higher-power types such as the 316A, 368A, 388A, and 703A triodes[6] and the 713A and 717A pentodes were referred to asDoorknob tubes.
The introduction of theEF50 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.
