Mullard Code (Used for UK and European valves):

This consists of a string of letters followed by a string of digits. E.g. ECC83, EL34 First letter gives heater rating

A - 4V
C - 200mA series connection
D - 1.4V (Normally directly heated)
E - 6.3V (By far the most common)
G - 5V
H - 450mA series connection
K - 2V
O - cold cathode/semiconductor device
P - 300mA series connection
U - 100mA series connection

Rest of the letters give the types of device in the valve. They are normally listed in alphabetical order.

A - signal diode
AA - 2 diodes with separate cathode
B - double diode with common cathode
C - signal triode
D - power triode (e.g. TV shunt stabiliser)
E - signal tetrode
F - signal pentode
H - hexode/heptode (Hexode structure)
K - heptode or octode (octode structure)
L - output tetrode, beam tetrode, or pentode
M - magic eye (seeing eye ? ) tuning indicator
N - gas filled triode / thyratron
Q - Nonode
X - gas-filled full-wave rectifier/double diode
Y - half wave rectifier/single diode
Z - vacuum full-wave rectifier/double diode

Digits indicate the base (first digit) and a code to distiguish valves that would otherwise have identical numbers (e.g. EL84 and EL85 are both output pentodes with a 6.3V heater on a B9A base. They are otherwise different).

First digit
0,1 - misc base - P base, side contact, etc
2 - B8B Loctal
3 - International Octal
4 - B8A
5 - B9G, B9D, misc
6,7 - Subminiatures
8 - B9A
9 - B7G