The type 955 thermionic valve/electron tube is a small triode

Triode

A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...

 designed primarily to operate at high frequency, although data books specify an upper limit of 4-600 MHz some circuits may obtain gain up to about 900 MHz. Interelectrode capacitance

Capacitance

In electromagnetism and electronics, capacitance is the ability of a capacitor to store energy in an electric field. Capacitance is also a measure of the amount of electric potential energy stored for a given electric potential. A common form of energy storage device is a parallel-plate capacitor...

s, and Miller capacitances are minimized by the small dimensions of the device. The connecting pins are placed around the periphery of the bulb and project radially outward, this maintains short internal leads with low inductance

Inductance

In electromagnetism and electronics, inductance is the ability of an inductor to store energy in a magnetic field. Inductors generate an opposing voltage proportional to the rate of change in current in a circuit...

, an important parameter 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 valves", so called because their size and shape is similar to the nut of the oak tree

Acorn

The acorn, or oak nut, is the nut of the oaks and their close relatives . It usually contains a single seed , enclosed in a tough, leathery shell, and borne in a cup-shaped cupule. Acorns vary from 1–6 cm long and 0.8–4 cm broad...

, designed to work in the VHF range. The 954 and 956 types are sharp and remote cut-off pentodes respectively. Types 957 and 958 were triodes with 1.4 volt heaters and the 959 is a sharp cut-off pentode like 954.

Pin Connections

When viewing the device from above, (the end without the exhaust tip), the pins are arranged in a group of three and a group of two, starting with the centre pin in the group of three and going in a clockwise direction, the pins are, cathode, heater, grid, anode, heater.

Ratings

The 955 is an indirectly heated triode which means that the heater is electrically isolated from the cathode. The heater has a 6.3 volt rating, which it shares with many other common types of thermionic valves/electron tubes, and it draws about 150 mA.

The maximum anode voltage is 250 V, with an anode current of 420 microamperes and anode load 250 kiloohm, and the maximum anode current is 4.5 mA at a voltage of 180 V with an anode load of 20 kilohm. The 955 is designed to be used in the frequency range of 60-600 MHz, (5-0.5 metres wavelength).
The amplification factor obtained is between 20 and 25 depending on details of the specific stage design and operating voltage.