In Europe, the principal method of numbering vacuum tubes ("thermionic valves") was the nomenclature used by the Philips company and its subsidiaries Mullard in the UK, Valvo(de, it) in Germany, Radiotechnique (Miniwatt-Dario brand) in France, and Amperex in the United States, from 1934 on.
The special quality could be anything, from rugged designs for military and industrial use, through devices with exceptionally low noise and microphony, to designs primarily optimised for long life without cathode poisoning when used for switching in a digital computer (but not necessarily with exceptional qualities as an amplifier).
It was the usual practice for power transformers to have a 5 volt insulated winding for rectifier filaments, and a 6.3 volt winding for all the other heaters; virtually all valves with 5V filament are rectifiers with cathode connected to heater, in practice full-wave (usable as half-wave by strapping both anodes together), e.g. GZ34.
For lower-voltage lower-power requirements, rectifiers with 6.3V heaters and insulated cathodes such as the EZ80 were used, connected to the common filament supply.
There is no special nomenclature for EHT rectifiers for cathode-ray tubes; the EY51 and EY86 were rated at 17kV with an average current of 350 microamps.
[5] The first types assigned using this sequence (in the mid to late 1930s) were less systematic and sometimes would append the US "G" and/or "GT" suffixes for octal base versions, although the base type was not always knowable from just the type number: Prior to 1934, Phillips numbers were based on a sequence of one letter to indicate filament current range, followed by one or two digits for the filament voltage, then two digits that gave either the amplification factor (for triodes) or a code beginning with 41 to indicate tetrodes, pentodes and so on.
The second letter broadly indicated the type of device, roughly following the tube designation, without indicating the semiconductor material: From 1966 the new Pro Electron standard codified type numbers for solid-state active devices using initial letters "A", "B" and "C" (rarely used heaters) for germanium, silicon and other semiconductors.
Most existing European valve type number allocations were compatible with the new system, but sometimes ambiguities could only be resolved by checking the digits in the name.