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Decoding Valve, Transistor and CRT Numbers
Commercial Valves

European Pro-Electron valve system, also known in Britain as the Mullard Code

Two codes are used, one for transmitting and industrial valves, the other for receiving valves. Both consist of two or more letters followed by a serial number, for example ECC83, EL34. The code for receiving valves was introduced around 1934.


First letter (general class of valve)

D -Rectifying tube (including grid-controlled tubes)
M - LF power amplifier or modulator triode
P - RF power pentode
Q - RF power tetrode
R - Rectifier
T - RF power triode

For tubes having dual systems, two of these letters are used, as in QQC04/15.

Second (or third letter for dual system tubes) (type of cathode)

A - Directly-heated tungsten filament
B - Directly-heated thoriated tungsten filament
C - Directly-heated oxide-coated filament
E - Indirectly heated oxide-coated cathode
G - Oxide-coated filament in mercury-vapour rectifier
V - Indirectly heated oxide-coated cathode
X - Directly-heated pure tungsten filament
Y - Directly-heated thoriated tungsten filament
Z - Directly-heated oxide-coated filament (except in mercury-vapour rectifiers)

Third (or fourth) letter

G - Mercury vapour filling
H - Helix or other integral cooler
L - Forced water cooling
S - Silica envelope
W - Water cooling
X - Xenon filling

The absence of a letter here indicates radiation cooling.

First number (anode voltage in kV ) followed by hyphen

05 - 0.5kV
5 - 5kV
12 - 12kV and so on

This is for transmitting tubes. For rectifying tubes the first group of figures is the approximate DC output voltage in kV in a three-phase half-wave rectifying circuit.

Second number (output)

For valves up to 5kW anode dissipation, the figures indicate maximum anode dissipation in watts.

For water-cooled valves above 5kW dissipation, the figures indicate the maximum output in kW.

For rectifiers, the figures indicate the maximum permissible rectified current per valve in milliamps or the approximate DC output power in W or kW per tube in a three-phase half-wave rectifying circuit.

Suffixed letters

B - Cables
E - Medium 7-pin base
ED - Edison base
EG - Goliath base
G - Medium 4-pin base
GB - Jumbo 4-pin base
GS - Super Jumbo 4-pin base
N - Medium 5-pin base
P - P-base


First letter (heater rating)

A - 4V AC
B - 180mA DC series connection
C - 200mA AC/DC series connection
D - 0.5-1.4V DC (normally directly heated, can be 0.625, 1.2 or 1.25V)
E - 6.3V series or parallel supply (by far the most common prefix)
F - 12.6 or 13V
G - 5V, later defined as ‘miscellaneous voltage, parallel supply’
H - 150mA series connection
K - 2V DC
L - 450mA series connection
O - cold device, not heated, e.g. cold cathode tube, voltage stabiliser or semiconductor device
P - 300mA AC/DC series connection
U - 100mA AC/DC series connection
V - 50mA AC/DC series connection
X - 600mA series connection
Y - 450mA series connection

The second and where applicable, third and fourth letters in multi-electrode valves indicate the types of device in the valve. They are normally listed in alphabetical order.

A - Signal diode (excluding rectifiers)
B - (=AA) double diode with common cathode (excluding rectifiers)
C - Signal triode (excluding power output triodes)
D - Output or power triode (e.g. TV shunt stabiliser)
E - Signal tetrode (excluding power output triodes)
F - Signal or RF pentode (excluding power output pentodes)
H - Hexode or heptode (hexode structure)
K - Heptode or octode (octode structure)
L - Output tetrode, beam tetrode, or pentode (including TV line output valves)
M - Magic eye tuning indicator
N - Gas-filled triode or thyratron
P - Secondary emission tube
Q - Nonode or enneode
S - Scanning valve for television
T - Miscellaneous (including thyratrons)
X - Full-wave rectifier or double diode (gas-filled)
Y - Half wave rectifier or single diode (vacuum)
Z - Full-wave rectifier or double diode (vacuum)

Digits indicate the base (first digit) and a code to distinguish 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 but are otherwise different). The last figure of tetrodes and pentodes (excluding power output tubes) indicates the type of characteristic; even figure denotes sharp cut-off characteristic, odd for variable-mu characteristic. With professional and industrial valves, serial numbers for prototypes always end in zero, those for variants in one of the figures 1 to 9.

1 to 10 - side contact
11 to 20 - footless 8-pin Telefunken
21 to 30 - B8G (Loctal)
31 to 39 and 300 series - International Octal
40 to 49, 140 to 149 - B8A (Rimlock)
50 to 59 - B9G (Magnoval)
60 - Subminiatures
70 - Miscellaneous
80 to 89, 180 and 800 series - B9A (Noval)
90 to 99 - B7G (Miniature 7-pin)
200 series - B10B
500 series - B9D

Britain: GEC Code (Marconi/Osram and EMI brands also)

Consist of Letter (or Letters) followed by digits (e.g. L63, KT88). The digits are simply to distinguish similar valves (like all triodes), and cannot be decoded. The letters have the following meaning.

A - Industrial valve (May be almost anything – power triode, etc.)
B - Double triode
D - Diode or double diode
GU - Gas-filled rectifier
GT - Gas Triode (Thyratron)
H - Signal triode (high impedance)
KT - Kinkless tetrode (beam tetrode)
L - Signal triode (low impedance)
M - Metallising (when used later in code)
MU - Indirectly heated rectifier
N - Output pentode

P - Output triode
QP - Quiescent push-pull double pentode
S - Tetrode (Screen Grid Valve)
U - Rectifier
VS - Variable mu tetrode
W - Variable mu pentode
X - Triode hexode, heptode, octode, frequency changer
Y - Tuning indicator
Z - Sharp cut-off screened HF pentode

Britain: Ediswan and Mazda Code

Consists of digits, letters, digits. Do not confuse with a US code.

First digits give heater voltage: 1 = 1.4V, 6 = 6.3V whilst 10, 20, 30 indicate 100mA, 200mA, 300mA for series connection.

Letters give type of valve

C - Frequency changer
D - Signal diode or double diode
F - Signal amplifier, tetrode/pentode
K - Thyratron (small gas triode or tetrode, according to another source)
L - Signal triode or double triode
M - Tuning indicator
P - Output (Power) tetrode/pentode
U - Half wave rectifier, without first figure
UU - Full wave rectifier, without first figure

Mazda codes tend not to double up letters – L is used for multiple triodes also.

Final digits distinguish between otherwise identical codes.

Britain: STC Brimar (British-Made North American Range) codes

Three codes are used; if the valve is for export, the American RTMA code is adopted, sometimes with a prefix SV. Valves for the home market employ a code consisting of a figure or figures such as:

1 - Half-wave rectifier
8 - RF pentode
9 - Variable µ pentode
20 - Triode hexode

followed by a letter and a serial number

A - 4-volt heater
B - 2-volt heater
D - Indirectly heated, other than 2V or 4V cathode

If special valves, the code consists of a number, such as:

2 - Diode
3 - Triode
4 - Tetrode
5 - Pentode

22, etc. - Double diode, etc.

followed by a letter giving maximum anode dissipation and type, followed by a serial number, followed by one or two letters indicating base type and special features.

Britain: BVA numbers

BVA (British Valve Association) numbers were assigned to valves used for civilian replacements in WW2.

Britain: Ericsson Valve Code

The code consists of a group of letters followed by 1 or 2 numbers (separated by a '/'), and then another letter. Some devices have a suffix consisting of another letter, preceded by a '/'.

Examples : GC10B, GC12/4B, GTR150M/S

First letter gives the construction of the device:

G - Gas filled
V - Hard vacuum

Remaining letters in the first group give the type of device:

C - Counter (Stepping tube where not all electrodes are available on separate pins)
D - Diode (stabiliser, etc.)
TE - Tetrode (trigger tubes, etc.)
T - Triode (trigger tubes, etc.)
TR - Triode (trigger tubes, etc.)
S - Selector (Stepping tube with all outputs available on separate pins)
R - Register display tube (e.g. Digitrons, Nixie tube-like devices)

First or only number gives the most important characteristic of the device:

Counters, Selectors - Number of states
Register displays - Number of display electrodes
Diodes/Stabilisers - Operating voltage
Trigger tubes - Striking voltage on trigger electrode.

Second number is used for counters only, and specifies the number of available output pins (if more than 1).

Final letter gives the base:

A,B,C,D,E - Plastic base shell (Octal, B5, duodecal, solder tags, etc.)
G,H - 26-pin B26A, B27A
M,P,Q - B7G
T - B9A
W,X,Y - Wire-ended


/S - tested to military (Services) specs
/R - tested for resistance to vibration

France: Mazda

The code comprises two letters followed by three or four figures.

First letter (heater voltage)

A - 1V
B - 2V
D - 4V
E - 5V
F - 6V
G - 7V

Second letter (heater current)

Z - 0.05A
Y - 0.1 to 0.14A
X - 0.15A
W - 0.2A and above

First two figures

Amplification factor (one digit only if less than 10)

Last two figures

Internal resistance (in kilohms)

France: Miniwatt

This is a subset of the European system. Either two or three letters are used, followed by a sequential number.

First letter (heater)

A - 4V AC
B - 180mA DC
C - 200mA AC/DC
D - 1.4V battery
E - AC and 6.3V car battery
F - 13V car battery
H - 4V battery
K - 2V battery
U - 100mA AC/DC

Second and optional third letter (type of valve)

A - Diode
B - Double diode
C - Triode (oscillator, detector, amplifier)
D - Audio power triode
E - Pentode
F - RF pentode
H - Hexode or heptode
K - Octode
L - Audio power pentode
M - Visual tuning indicator
X - Dual-anode rectifier (gas-filled)
Y - Single anode rectifier (vacuum)
Z - Dual-anode rectifier (vacuum)

France: Visseaux

The code comprises two letters followed by four figures.


RO - directly heated
RS - indirectly heated

First figure (heater voltage)

4 - 4V

Second figure

No particular significance

Last two figures

Amplification factor

Germany: Telefunken

This original code had just two series, RE and RS, standing for Röhren-Empfangen (receiving valves) and Röhren-Senden (transmitting valves). The three-digit number following indicates first the heater current (first two digits, in hundredths of an amp) then the heater voltage (4, 3, 2 or 1 for 4V, 3V, 2V or 1.5V). The final letter indicates the valve base pattern, t for Telefunken, d for Europa-base with side contact for space grid, and no letter at all for Europa-base.

The later V series valves used the first letter V to signify 0.05A heater current, followed by one or two letters denoting the function of the valve (following the standard European Pro-Electron/Mullard system). The numeric suffix indicates base type; 11 is an 8-pin Telefunken-pattern. Thus VCL11 indicates a combined triode and output pentode with a Telefunken-style base.

Netherlands: Philips

The early Philips sets used tubes (not the rectifiers) coded by a letter plus three or four digits, like A428, E434, etc. The coding was used before the introduction of the European (Mullard) codes.

The letter gives current as follows.

A - 0.06 to 0.10 amp
B - 0.10 to 0.20 amp
C - 0.20 to 0.40 amp
D - 0.40 to 0.70 amp
E - 0.70 to 1.25 amp
F - 1.25 amp and more.

First digit(s): heater voltage, from 1 to 20 volts.

Last two digits:

...give the gain if the tube is a triode.

41 is a tetrode
42, 52, and 62 are HF tetrodes
43, 53, 63 are output pentodes
46 is an RF pentode.

Examples: E443 is an output pentode, A415 is a triode with gain 15.

USA: (pre-RMA code)

Until about 1932, tube designations included two letters and three numbers, as in ‘UV199’.

First letter: brand, e.g.

C - Cunningham
D - Duovac
N - Philmore

There were others.

Second letter: base type, e.g.

X - 4 pins
Y - 5 pins

First digit: licensee

Last two figures: type number.

Some designations appear not to follow this code. Another authority asserts the two letters indicate base configuration, and the first digit denotes the designer or manufacturer. ["The ‘UV’ denotes 4-pin stub base and the 1xx is Western Electric. The ‘PA 250’ does not follow this completely – it denotes the RCA design, but has PA instead of UX for the base designator."]

USA: (Radio Manufacturers Association, Radio Television Manufacturers Association)

There are two codes, which is confusing to some,

The more commonly encountered code [Standard ET-110] consists of digits, a group of letters, digits plus an optional suffix. A variant is used by the Russians.

The first digits give the approximate filament or heater voltage rating in bands, a point misunderstood by many. Digit 0 denotes cold cathode, 1 between 0.1 and 2.1, 2 between 2.1 and 2.9, 3 between 3.0 and 3.9, 5 between 5.0 and 5.9, 6 between 6.0 and 6.9, and so on. Another interpretation states that 6 indicates a voltage between 5.6 and 6.6 volts, while 5 indicates a voltage between 4.6 and 5.6 volts; 1 indicates a voltage in excess of 0 and including 1.6 volts, whilst 0 indicates a cold cathode.

The exception is that codes 7 and 14 are ‘nominal’ voltages and are used to indicate 6.3V and 12.6V valves with a loctal base. In the case of tapped filaments or heaters the first figure indicates the total voltage with both sections in series.

The second symbol is a letter which is allotted in sequence commencing with A, except that I and O are not used; rectifiers follow the sequence backward commencing with Z. When all the single letters of a group are exhausted, the system then proceeds with two letters commencing with AB; combinations of identical letters are not normally used. The single-ended (no top cap) alternating current has a first letter S while the second letter may be that of the nearest equivalent in the double-ended range – e.g. type 6SK7 is the nearest single-ended equivalent to type 6K7. Another special case is the first letter L which is used for lock-in (loctal) types in the battery range. P as a second letter indicates a cathode ray tube. The rectifiers all start at the end of the alphabet like 5Z3, 35Z5, 5X4, 5U4 etc.

Second digits give either the number of active electrodes or elements, or else the number of external connections. In metal tubes the shell counts as one element. Shielding by or in the base does not count as an element.

Whenever possible the 12V equivalent of a 6V tube retains the same letters, thus 6SK7GT and 12SK7GT are similar except for the heater rating.

The suffix letters specify the type of envelope or base. The code is:

G - large Glass envelope on octal base
GC - Glass Compact, a tubular glass envelope
GT - Glass Tubular, the smallest glass envelope
GT/G - Glass Tubular, interchangeable with G and GT types
LM - Loktal-metal
LT - Locking base
M - Metal-coated glass envelope on octal base
MG - Metal-glass
ML - Metal-loktal
S - Spray-shielded (zinc-clad)
W - Military type assigned on behalf of armed forces, ruggedised
X - Low-loss base for HF use (ceramic)
Y - Intermediate-loss base (mica-filled phenolic)

A, B, C, D, E, F modified version of a type without other suffix (note 2)

Note 1: Valves used specifically for industrial purposes have a sequential all-numeric designation. RCA used 1600 numbers for special purpose/industrial tubes, and Sylvania used 1200 numbers.

Note 2: Devices calling for the A versions of the tube may really need them. The canonical rule is that higher suffixes replace lower suffixes (or no suffix at all), but not vice-versa. Then again, that might just of been the current version at the time of design. In some A tubes, the A indicates things like higher anode dissipation, particularly in the case of power tubes. In other cases, the A means controlled heater warm-up time for series-string filaments.

USA: RMA code for special purpose tubes (devised about 1942)

This is the code which covers the tubes which appear not to follow the rules; for instance 2C22 which has 6.3V, not 2-volt, heaters.

First Number: Heater Power (Watts)

1 - none (no filament)
2 - up to 10
3 - 10-20
4 - >20-50
5 - >50-100
6 - >100-200
7 - >200-500
8 - >500-1000
9 - >1000

Letter: Type of device

A - single element (ballast, etc.)
B - diode (e.g. 1B24, 2B21)
C - triode (e.g. 2C39, 2C51)
D - tetrode (e.g. 4D21)
E - pentode (e.g. 2E22, 2E26)
F - hexode (e.g. 2F21)
G - heptode (e.g. 2G21)
H - octode
J - magnetically controlled (e.g. 4J30)
K - electrostatically controlled (e.g. 2K26)
L - vacuum capacitor (e.g. 1L21)
N - crystal rectifier (e.g. 1N21, 2N706)
P - photoemissive (e.g. 1P25)
Q - cavities (e.g. 1Q21)
R - ignitrons (e.g. 1R21)
S - switches (e.g. 1S22)
T - storage
V - photoflash
W - travelling-wave
X - X-ray
Y - thermionic converters

It may be that letters T to Y were never used. Some others were only used once.

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