Acorn tube

Common card · No. 493759

An acorn tube, or acorn valve, is any member of a family of VHF/UHF vacuum tubes 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 found widespread use in radios and radar systems. High-frequency performance is limited by (1) parasitic lead inductance and capacitance and skin 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 the phase 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 as Lighthouse tubes and Nuvistors attempt to minimize this...

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