Pro-Electron/Mullard Code

This are probably the most commonly encountered numbering system in the UK - and 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.

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

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.

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.

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, and then digits. Examples 10D2, 6F18.

The first digits give the heater 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.

The final digits distinguish between valves that 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.

I 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.

US Code

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

The first digits give heater voltage rating, with the exception that '7' and '14' are used to indicate 6.3V and 12.6V valves (respectively) with Loctal bases.

The next letters indicate the type of valve, but there is no real consistency of coding.  'S' often indicates a single-ended (no top cap) version of an earlier valve with such a cap.

The second digits give either the number of active electrodes, or the number of external connections. Again though this does not always work out!

The final letters often specify the type of envelope.

Chas Miller commented:

There never has been a suffix "M" for a metal tube. All the original octal tubes were metal cased and their nomenclature, e.g., 6K8, 6K7, 6Q7, 6V6, 5Z4, etc., indicates this. It was only when glass versions were produced that the suffixes "G" (glass) and "GT" (glass, tubular) appeared. I have a painful recollection of writing an article for Practical Wireless some years ago concerning a military receiver that used metal valves. I referred correctly to these throughout as 6K8, 6K7, etc., but the then editor in his wisdom decided to add the erroneous "M" suffix, resulting in my receiving scathing letters from readers.

Loctal valves. Those with the first digit "7" originally were rated at 7V on the heaters, and those with the first digit "14" at 14V. These voltages were chosen to correspond with those of nominally 6V and 12V automobile storage batteries whilst on charge from a dynamo. This remained for only a few years before the ratings were changed to 6.3V and 12.6V respectively.

Norman Leal disagrees:

Here in the US there were metal tubes with an "M" after the number, 6A8M, 6L7M, even "MG", 5Z4MG. These where larger versions of the small metal tubes. In some cases a metal shield was actually built right over a "G" type tube. These tubes were also built in other countries. I've attached a picture of a 6A8M tube made in France.

So does Rob Jones:

Regarding Chas Miller's comments at the end of this listing, his paragraph 2 is misleading. The second figure group of a valve type No in the American RETMA designation system refers to the number of ACTIVE ELEMENTS in that valve. Note that in the case of a metal valve, the outer shell is counted as an active element.

Taking his own examples:

5Y3GT: Directly heated full-wave rectifier - 3 active elements: filament and 2 anodes.

5Z4G: Indirectly heated full-wave rectifier - 4 active elements: heater, cathode and 2 anodes.

6SL7GT: Indirectly heated twin triode with separate cathodes - 7 active elements: heater, 2 cathodes, 2 control grids, and 2 anodes.

6K8: Triode hexode frequency changer - 8 active elements: heater, common cathode, 2 control grids, hexode g2/g4 (commoned), hexode g3, and 2 anodes. This is a glass valve, if there is a metal version, it will not have a second figure group no 8.

6K7: Vari-mu indirectly heated RF pentode in metal outer shell - 7 active elements: shell, heater, cathode, g1, g2, g3, anode.

It is not a myth, cherished or otherwise and it always works out!

Military Codes

A number of Military codes are used, but these are unlikely to be encountered in domestic receivers.  If one is encountered the best course of action would probably be to find out the commercial equivalent from a suitable data book or web site - or from my Valve Data CD-ROM!

Military codes cannot decoded in any way - they are generally a fixed prefix (such as CV) followed by a serial number.

The information on this page has been gathered and assembled from various sources including "Radio Bygones" magazine No. 9 (February/March 1991). Corrections and additions welcomed!

Repair and Restoration Menu

Home