The type number consists of two figures followed by two letters (e.g. 90AV).

First figure (tube base)

2 - Loctal 8-pin base
3 - Octal 8-pin base
5 - Special base
8 - Noval 9-pin base
9 - Miniature 7-pin base

Second figure (serial number)

First letter (type of cathode)

A - Caesium-antimony cathode (blue-sensitive)
C - Caesium on oxidised silver cathode (red-sensitive)
T - Tialkali cathode
U - Caesium-antimony cathode with quartz window

Second letter (class of phototube)

G - Gas-filled
V - High vacuum

Third letter (always P for photomultiplier).

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