Valve Numbering Systems

The following information is derived from various sources, including Radio Bygones magazine, Volume 9 (February/March 1991).

Pro-Electron/Mullard Code

This are probably the most commonly encountered numbering system in the UK - and also the most informative. It consists of two or more letters followed by a number (normally two digits).
Examples - UL41, ECC85, UABC80.

The first letter gives heater rating.

CharacterHeater Rating
D0 - 1.5V (previously 1.4V)
GMisc. (previously 5V)

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

CharacterDevice Type
ASignal diode
BDouble diode
CSignal triode
DPower triode
ESignal tetrode
FSignal pentode
HHexode or heptode (hexode type)
KOctode or heptode (octode type)
LOutput tetrode or pentode
MMagic eye (tuning indicator)
NGas-filled triode (thyrathon)
XGas-filled full-wave rectifier
YHalf-wave rectifier
ZFull-wave rectifier

The first digit indicates the base type. Where there is only one digit this is assumed to be the second digit, and be preceded by a zero. For example, EM4 should be interpreted as EM04.

DigitBase Type
0 and 1Miscellaneous bases (P-base, side contact etc)
2B10B (previously B8B/B8G (Loctal))
3International Octal (8-pin with centre locating spigot)
4B8A (8 pin with locating pip on side)
5B9G and B9D (wire ended)
6 and 7Subminatures
8B9A (9-pin glass)
9B7G (7-pin glass)

The remaining digit(s) are used to differentiate between valves that would otherwise have identical numbers:-
One digit for early valves
Two figures for later entertainment valves
Three or Four figures for later professional types

GEC Code (also used on Marconi and Osram valves)

This consists of one or two letters followed by a number (normally two digits). Examples L63, KT88.

The letters have the following meaning.

CharactersDevice Type
AIndustrial valve (could be anything!)
BDouble triode
GUGas-filled rectifier
GTGas-filled triode (thyrathon)
HSignal triode (high impedance)
HSignal triode (medium impedance)
KTKinkless tetrode (beam tetrode)
KTWVari-mu RF kinkless tetrode
KTZSharp cut-off RF kinkless tetrode
LSignal triode (low impedance)
MUIndirectly heated rectifier
NOutput pentode
POutput triode
PXOutput triode
QPQuiescent push-pull double pentode
VSVari-mu tetrode
WVari-mu pentode
XTriode-hexode, heptode, octode (frequency changer)
Y'Magic eye' tuning indicator
ZSharp cut-off HF pentode

The digits are simply to distinguish similar valves and cannot be decoded:-
One digit for early valves
Two figures for later valves

Note: Suffix 'M' indicates external metallising

Mazda Code

These can be confused with the US code (below). The codes consist of digits, then letters, then digits. Examples 10D2, 6F18.

The first digits give the heater rating.

DigitsHeater Rating

The letters indicate the type of valve. Mazda codes do not normally double-up the letters - so for example 'D' would be used for single and multiple diodes.

CharactersDevice Type
CFrequency changer
DSignal diode
FSignal tetrode or pentode
KGas-filled triode (thyrathon)
LSignal triode
M'Magic eye' tuning indicator
POutput tetrode or pentode
UHalf-wave rectifier
UUFull-wave rectifier

The final digits distinguish between valves which would otherwise have identical codes.

Miscellaneous Early British Codes

A selection of some of those codes from the era of British 4, 5 and 7-pin based valves, and also Mazda Octals. Some codes were used by one manufacturer only, some by more than one.

We have tried to limit this table to those codes that, in general, always had the same meaning, though some had different shades of meaning under different brand names. Sometimes code letters were combined to identify a multiple valve, for example the AC/2PenDD, a double-diode output pentode with 4V heater from Mazda.

CharactersDevice Type
AC4-volt heater
DSingle or double diode
DDDouble diode
DDTDouble-diode triode
FCFrequency changer
HHigh-impedance triode
M4-volt heater
ME'Magic eye' tuning indicator
PenOutput pentode
PPPower (output) pentode
PTOutput pentode
RFull-wave rectifier
SPStraight RF pentode
THTriode-heptode or triode-hexode
URectifier (usually half-wave)
UUFull-wave rectifier
VPVari-mu RF pentode


The US codes consist of digits, then letters, the digits, then possibly further letters. Examples 6V6GT, 5Z4G, 5R4GY

The first digits give heater rating.

DigitsHeater Rating
0Cold Cathode
1Up to 1.6V
54.5 - 5.6V
65.6 - 6.6V
75.6 - 6.6V with Loctal base
1411.2 - 13.2V with Loctal base

Figures other than those in this table represent the nominal heater working voltage. For tapped heaters the figure indicates the rating with the sections in series.

The next letters do not give much helpful information about the device function.

'S' often indicates a single-ended (no top cap) version of an earlier valve with such a cap. 'L' often indicates a lock-in type in the battery range.

The letters were allocated in sequence starting with 'A' and omitting 'I' and 'O'. Rectifiers follow the sequence backwards starting at 'Z'. When all single letters are used, the combinations continue with double letters starting at 'AB' (pairs of identical letters are seldom used).

The second digits give either the number of active electrodes, or the number of external connections.

The final suffix letters often specify the type of envelope. Devices with no final letters have metal envelopes.

'A', 'B', 'C' etc indicate a later/modified version which can be substituted for a previous one but not vice-versa. Other letters have these meanings.

GLarge glass envelope
GCGlass compact
GTGlass tubular
MMetal-coated class envelope
WAHigh quality version
WMilitary version
XLow-loss base
YMedium-loss base

Special Quality Valves

These are electronically similar to a number of standard types, but have improved mcehanical construction to reduce microphony and vibration failures. They are also more closely controlled in manufacture and testing.

Numbering Systems


System 1: A 4 figure reference number. For example, a 6060 is a special quality 12AT7

System 2: The RMA system followed by the suffix 'W' indicating a military type. For example, a 12AT7WA is a special quality 12AT7.


The prefix 'Q' is added to the standard type number. For example, the QZ77 is a special-quality Z77.


System 1: Uses the Pro-Electron code, but with the figures for base type and serial number placed after the letter for filament or heater rating. For example, the E88CC is a special quality ECC88.

System 2: An initial letter 'M' followed by a 4-figure serial numler. For example an M8088 is a high-quality EF91.

UK Military

Most special quality valves used in military equipment have 'Common Valve' numbers in the CV4000 group. See the 'RSGB Equivalents' section of this CD-Rom for commercial equivalants.

Military Codes

Please refer to the 'RSGB Equivalents' section on this CD-Rom for an explanation of the various codes used and lists of commercial equivalents.

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