In order that vacuum tubes or thermionic valve numbering had some reason to it, numbering schemes were developed.
Using these it was possible to gain an approximate idea of the tube and its functions.
There are two schemes that were adopted, one used in Europe for valves primarily manufactured and used there, and the other for vacuum tubes from the USA. Although the numbering schemes were different, most valves or tubes were available with numbers in both systems.
European System
FIRST LETTER INDICATES THE HEATER SUPPLY
A 4V AC
C - 200mA AC and DC
D - Battery supply 1.2 to 1.4V
E - 6.3V AC and DC
G - 5V AC
K - 2V battery supply
P - 300 mA
U - 100 mA AC and DC
SUBSEQUENT LETTERS INDICATE THE TYPE OF VALVE
A - Single diode
B - Double diode
C - Triode
D - Triode output valve
E - Tetrode
F - Pentode
H - Hexode or heptode
K - Octode
L - Pentode output valve
M - Tuning indicator
N - Thyratron
Q - Nonode
T - Miscellaneous
X - Gas filled full wave rectifier
Y - Half wave rectifier
Z - Full wave rectifier
FOLLOWING FIGURES INDICATE THE BASE
20 - 29 B8G (Loctal)
30 - 39 Octal
40 - 49 B8A
50 - 59 Miscellaneous constructions
60 - 79 Subminiature constructions
80 - 89 B9A
90 - 99 B7GIf a number is greater than a hundred then the first figure should be disregarded to determine the base.
American system
FIRST FIGURE INDICATES THE HEATER VOLTAGE
0 - Cold cathode
1 - 0 - 1.6V
5 - 4.6 - 5.6V
6 - 5.6 - 6.6V
7 - 6.3V Loctal
12 - 12.6V
35 - Around 35V
SECOND AND OTHER CHARACTERS ARE SERIAL NUMBERS
SUFFIX LETTERS
G - Large glass envelope
GT - Small glass envelope
M - Metallised
X - Low loss base
W - Military type base