955 Acorn Triode

955 Acorn Triode

The type 955 thermionic valve/electron tube is a small triode designed primarily to operate at high frequency; although data books specify an upper limit of 400–600 MHz, some circuits may obtain gain up to about 900 MHz. Interelectrode capacitances and Miller capacitances are minimized by the small dimensions of the device and the widely separated pins. The connecting pins are placed around the periphery of the bulb and project radially outward: this maintains short internal leads with low inductance, an important property allowing operation at high frequency. The pins fit a special socket fabricated as a ceramic ring in which the valve itself occupies the central space.

The 955 is one of about a dozen types of "acorn valve", so called because their size and shape is similar to the acorn (nut of the oak tree), designed to work in the VHF range. The 954 and 956 types are sharp and remote cut-off pentodes, respectively. Types 957 and 958 are triodes with 1.4 volt heaters, and the 959 is a sharp cut-off pentode like the 954.