Pioneered by RCA in the 1930's, the 'acorn' valves were an designed for VHF service. In order to operate at very high frequencies, valves must be as small as possible. The design must reduce inter electrode capacitances; reduce transit time and reduce the inductance of the connections to the electrodes. At VHF and UHF even a few centimetres of wire have a significant impedance. It is also necessary to provide shielding between input and output circuits to avoid self oscillation. RCA miniaturised the valves and arranged the pins radially in a ring seal (visible on the example above). This design was not improved on until the advent of the tiny Nuvistors in the late 1950's. RCA recommended maximum frequency was 250 Mc/s, but they can be used up to 400 MHz with care.
This 955 triode has a 6.3 volt indirectly heated cathode and operates at a maximum anode ratings of 250 volts, 8 mA and 1.6 watts. Amplification factor is 25.