Acorn tube

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 effect by arranging cathode, grid(s) and anode as closely spaced together as possible.

The 954 and 956 types are sharp and remote cut-off pentodes, respectively, all with indirect 6.3 V, 150 mA heaters.

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.