Probably the largest manufacturer of early semiconductors in Europe was Philips of the Netherlands. As well as their own laboratory and manufacturing facilities in Eindhoven, Philips had three overseas subsidiaries that manufactured semiconductor devices:

Of these, Mullard was probably the largest in volume of semiconductors produced, although Valvo was not far behind. Devices from La Radiotechnique are much rarer than those of the other two. In the USA, Philips devices were marketed through another subsidiary: Amperex, although they are almost all marked 'MADE IN HOLLAND'.

Philips seems to have operated as quite a loose federation of companies, with all of the above potentially doing research that resulted in new devices. However, with only a few early exceptions, all devices followed a common naming convention and all the subsidiaries marketed and sold the full Philips range.

Another vintage semiconductor enthusiast, Mark Burgess, has created a site documenting the history of Philips’ semiconductors in the 1950s.

mullard logo

Mullard Limited was founded by Captain S.R. Mullard in 1920 to manufacturer high-power transmitting valves for the British Admiralty. (For American readers, I had better explain that in Britain an electronic 'valve' is what you call a 'vacuum tube'). His business prospered and in 1924 he raised capital by selling half the company to Philips, who bought the rest in 1927. Wholly owned by Philips, Mullard Ltd. went on to become a major manufacturer of valves and associated components.

When the transistor was announced by Bell Labs in 1948, two UK companies were not far behind in research terms, STC and GEC. Mullard had no semiconductor research laboratory as far as I know, and I suspect that Philips also had done little work in the field. However, a former Mullard employee told me that Philips made the decision in about 1950 to start manufacturing transistors, with the goal of capturing 95% of the European market. I do not have statistics, but the Mullard brand certainly became dominant very quickly, and remained so for nearly twenty years.

Mullard's original manufacturing facilities were in Mitcham, and their head office remained in London, but their semiconductor production moved to a purpose-built plant in Southampton in 1957. A description of the site's history is available on the Web (14.8 MB pdf file!).

Philips group's original semiconductor product lines were the OC series of germanium transistors and the OA series of germanium diodes. They moved on to use many Pro Electron prefixes such as AA, AC, AD, AF, ASY, ASZ, and AUY. Eventually they manufactured silicon types with prefixes BA, BC etc. The OC series was very widely used in Europe for a period of almost twenty years and many millions were made. Other companies such as Tungsram, Intermetall and Telefunken made their own OC types.

True Mullard devices usually bear the name MULLARD in capitals, the type number e.g. OC71, and an indication of the place of manufacture, usually either 'GREAT BRITAIN', which signifies made in a British factory, or 'BRITISH MADE', which means made elsewhere in the then British Empire. This information was sent by a correspondent who also included a long list of Mullard factories in many countries. Occasional examples can be found that are marked 'MADE IN HOLLAND', which I believe indicates a Philips factory, and very rarely 'MADE IN FRANCE'. Examples can also be found that bear no manufacturer's name or place of origin, I believe these to be made by Philips in the Netherlands. They also usually have a code printed on them, composed of a small number of mixed-case letters and numbers. I do not know how to decode this, or even whether it is a date code or something else. If you can help, please

You may come across vintage GEC (UK) transistors and diodes bearing the Mullard name. This because, in 1962, GEC merged its semiconductor interests with those of Mullard Ltd., to form a joint venture called Associated Semiconductor Manufacturers (ASM). Devices were marketed through Mullard, and so GEC devices can be found bearing the Mullard name. In fact, ex-GEC employees have told me that the 'joint venture' was a takeover by Mullard in all but name.

I do not possess all the transistors below, in fact I am seeking many of them. It is noted in the text where I am seeking examples of any particular type. If you have some for sale or exchange, please

Conversely, I am happy to help anyone looking for information on these devices. I have an extensive collection of original data sheets and books, and can provide characteristic data on almost all OC series types.

OA55 diode OA55 diode OA61 diodes

Mark Burgess' history of Philips semiconductors states that the first they made were germanium point-contact diodes:

I have a Philips publication 'Germanium Dioden' by Dr.S.D.Boon dated March 1954 that describes

However, an old Mullard Maintenance Manual, undated but believed to be from about 1955, contains only

The leftmost image shows an OA50. The body has an incomplete white stripe with CATH printed in the gap and OA50 underneath it. There is no manufacturer name or logo.

The middle image shows an OA55, identifiable by the green-green stripes on the end pin indicating five-five in the normal electronics colour code. The body is printed PHILIPS OA55 HOLLAND. I have some that omit the HOLLAND. This has a noticeably longer body than the OA50, and shorter end pins.

The rightmost image shows two OA61, identifiable by the paint stripes on the cathode pin : blue plus brown, which are six and one in the colour code. These have a slightly shorter body than the OA55 and little printed on them, just OA61 on one of them and an indistinct blue mark on the other. It's puzzling that all these diodes have different markings and even slightly different physical sizes and exit pin lengths. The 1954 Boon book has diagrams showing the OA5n and OA6n types having coloured bands on the cathode-end pin, but the OA70 and OA73 in a different outline with no indication of markings.

I am seeking any other OA diode in this large glass encapsulation. If you can help me find any, please

OA70 diode

The Mullard Maintenance Manual shows OA70 having two 'coloured bands' on the body. This example could be one of those, it is in a Mullard carton which suggests it is an early example, and it has the expected purple-black bands as well as a rather faint red one at the cathode. I have many other OA70 but they are painted black with no bands and printed MULLARD OA70 BRITISH MADE; I believe these to be later.

OC51 transistor

The first transistors made by Mullard were the OC50 and OC51 point-contact types from 1952. As this image of the OC51 shows, they looked just like a typical Bell Labs 'outline #1' case, except that the metal is shinier, and the plastic plug in which the leads are fixed is pale brown. I have a few of them and only one has any markings on it, suggesting that the white printing rubs off the black band very easily. These transistors were probably not commercially available: some were probably given to a few research institutes, then Mullard rapidly moved on to junction types.

I am seeking examples of the OC50, and indeed of the OC51 in identifiable form, for example in an original box. I also wish to buy original data sheets for these devices. If you have any of these, please

OC10/11/12 transistor

The first (grown?) junction types made by Mullard are the OC10, OC11, and OC12 (all shown in the compound image) from 1953. These are all low-power audio-frequency amplifiers, the first two differing in noise level, the OC12 having a higher gain. Unfortunately, it was soon found that the plastic encapsulation was not hermetic, and moisture crept along the leads and ruined the transistors.

I am seeking examples of the OC10, OC11 and OC12. I also wish to buy original data sheets for these devices. If you have any of these, please

OC72 ASM transistor OC70 OC71 OC72 transistors

Mullard then developed a unique glass encapsulation, and in 1954 issued three new alloy-junction audio-frequency transistors, the OC70, OC71 and OC72. The image shows the OC70 and OC71 in the glass case, painted black, plus the OC72, a higher power type, achieved by slipping an aluminium can over the glass tube.

This black glass encapsulation, and the version with the aluminium sleeve, both referred to as SO-2, are used by many of the transistor types in the OC series, although a few types use the TO-7 metal outline, and power types use the TO-3 'diamond' outline. A correspondent told me that in the early days of the industry, an apprentice at his company was sacked because he had amused himself by crunching the glass capsules under his boot. At the time each transistor cost a week's wages!

Examples of transistors branded ASM (for the joint venture with GEC: Associated Semiconductor Manufacturers) are rare. The OC72 shown is the only such type I have found.

OCP71 transistor

Like other manufacturers, Mullard produced a phototransistor, the OCP71 shown on the left. This is a very early type, available commercially from 1956. It is well-known that semiconductor junctions can be light-sensitive, and there is a widely repeated story that the OCP71 is just an ordinary OC71 without the black paint (so-called 'dope'). Because the OCP71 was more expensive, some people just scraped the paint off an OC71, to create an equivalent to the OCP71. The story then claims that Mullard changed the filler in the capsules from clear to opaque, to prevent this practice.

Thanks to correspondence with an ex-Mullard employee from the Mitcham works, I now know that this story is entirely wrong. At the time, the behaviour of manufactured transistors varied considerably, so devices from one production line were sorted into either OC70, OC71, or OC75 using a Transistor Test Board to measure their performance. These transistors were dipped in a light blue silicon 'bouncing' putty before insertion into the glass capsule which was then filled with Alundum paste: an aluminium oxide and silicon oil mix that was lightproof and also conducted heat away from the transistor junctions. The exterior was painted with a black 'dope'. Scraping off the black dope therefore reveals a blue tinged whitish interior. However, the OCP71 capsules were filled instead with a clear silicon grease to allow light through. They were tested as phototransistors and those that were rejected were re-tested to see if they met the normal OC7x series parameters. Those that did were sent off to be painted with the black dope and be printed as OC7x devices before being packaged as normal. Therefore, if scraping the paint off an OC71 reveals a transparent interior, this is NOT an early example before Mullard switched the filler, but is in fact a rejected OCP71. Using it as a phototransistor is not a good idea!

There exist also OCP70 phototransistors in the same style, an unpainted capsule in a plastic holder. However my experience is that they are either unbranded or made by Valvo, unlike the OCP71 which is usually marked 'MULLARD'. I suspect the unbranded ones are Philips devices for OEM use. If anyone can confirm this, or has an OCP70 marked Mullard (not an OCP71), please

OC15 transistor

The first Philips group prototype power transistor is exceedingly rare: the OC15. My image of it is rather unclear because it was taken through a perspex sheet. The transistor is part of a demonstration transistor amplifier in a perspex and metal case, probably made for a trade show in the 1950s. It contains two OC71's, two hefty interstage transformers, and a single OC15. The OC15 should have a paper label but that has fallen off and is trapped in the wiring. I have not yet attempted to open the perspex casing.

Mark Burgess states that this device was developed by Valvo in Germany, who originally numbered it 100.O.C. Amperex in the USA numbered it 2N115, although they also used the same number on later OC16 devices. Images of these variants can be seen on Joe Knight's pages on Jack Ward's web site.

OC16 transistors

The OC15 was not commercialised, but Philips/Mullard/Valvo did produce the first commercial high-power transistor in Europe: the OC16, in about 1956. The prototype version on the left in the image handles 24 Watts. The commercial version to its right uses a different stud outline in which two leads emerge through a hole in the centre of the fixing screw. This outline was only used for the OC16, apart from two exceptions that I know: I have some OC24s in this outline (shown below), and I possess an undocumented Mullard type CXT6b, shown further down. The OC16 can be found with a number of suffix letters: according to the Mullard 'Industrial Newsletter' dated September 1958, suffix R is specially selected for use in DC converters and suffix T is a high-gain version for use in industrial control circuits. However I also have examples with suffix G and W : I don't know what those values signify.

OC24 transistor OC24 transistor OC23 transistor OC22 transistor

Mullard soon abandoned the proprietary stud encapsulation for high-power transistors, and changed to using the TO-3 'diamond' outline which became the standard for power transistors until plastic encapsulation was developed much later. The Industrial Newsletter for 1958 lists OC22, OC23 and OC24 in addition to OC16. The rightmost image shows an OC24 in the old OC16 outline, presumably an early one. This is most unusual, and I wonder if there exist OC22 and OC23 in this shape also?

Incidentally, I don't know what the three-character codes under the part number signify.

If you know about any such early OC22 and OC23, or can decode the three-character codes, please

OC20 transistor OC19 transistor

There are lower-numbered power types than that OC22/23/24 triplet; the OC19 and OC20. There are apparently no OC17, OC18 or OC21. However, the German book 'Kristalldioden and Transistoren Taschen-Tabelle' 1969 lists OC18 but with almost no data. Was this number used for a prototype that was not commercialised, possibly by Valvo? If you know anything about type OC18, please

The OC19 is relatively uncommon, I have two in Mullard valve-style cartons. Very unusually, they are also marked 2N350, which is an American power transistor type made by Motorola that first appears in the 1958 edition of the General Electric Transistor Manual. A correspondent, Gert in Germany, has pointed out to me that OC19 has exactly the same electrical specification (including low gain and low maximum voltages) as the OC16, suggesting that it is the same die in a different outline. This could explain why OC19 has a higher central dome than the other Mullard TO-3 types. However it seems to have been developed later than OC22, OC23 and OC24: the oldest reference for OC19 that I have found is the 1961 Mullard 'Maintenance Manual'. The oldest reference that I have for OC20 is the 1963 Mullard 'Power Transistors' booklet.

OC29 transistor OC28 transistor OC27 transistor OC26 transistor OC25 transistor

The OC2n power series continues with OC25, OC26, OC27, OC28 and OC29. The type OC27 is rare, and is not listed in any of my Mullard data books, although it can be found in the German publication 'Kristalldioden und Transistoren Taschen-Tabelle 1969'. My example is unusual in not bearing the three-character code under the part number. Why?

OC36 transistor OC35 transistor OC30 transistor

The OC power series then changes somewhat: type OC30 is unique in using the smaller SO-55 outline. These are usually also printed 'HOLLAND' which suggests that they were really manufactured in a Philips plant.

There is then a gap: OC31 does not exist; OC32, OC33 and OC34 are very early low-power types made by Intermetall in Germany.

The last two Philips group OC power types are OC35 and OC36. The latter is sometimes found with a suffix T but I'm unaware of its significance.

Most of the above TO-3 power types can be found unbranded, which often means Philips production, and branded Valvo. Some types can be found branded La Radiotechnique and some were made by Tungsram and by Tesla. Most of them are listed in the 1972 Newmarket 'Product Portfolio' but I only have one: a Newmarket OC36 (although I do have more CV equivalents). OC19 and OC27 are the rarest and I only have Mullard examples. Many of them were available as matched pairs, denoted by the prefix '2-' e.g., 2-OC28 in the data books, but not on the devices themselves.

Mullard power transistors

There are later Philips group germanium power transistors in the standard pro-Electron numbering:

OC44 and OC45 transistors

The first radio frequency transistors produced by the Philips Group, and amongst the first in the world, were the OC45 and the OC44 both announced by Philips in October 1956. There is an ongoing market for the OC44 on eBay because it was used in some early guitar effects units, in particular the Dallas Rangemaster, and purists insist that only the original will do.

OC66 transistor OC57 transistor

Like other manufacturers, Mullard believed that the hearing aid provided an early application for the transistor, because of its small size, low voltage, and lack of heater circuit compared to valves/tubes. This was ultimately only a minor market, but Mullard created four sub-miniature types, OC57 to OC60, only a few millimetres long, and two miniature types OC65 and OC66, slightly larger. All are audio-frequency amplifiers for use in the output stages of hearing aids.

OC169 transistor

The first RF transistors used the alloy-junction fabrication process, which limited their upper-frequency limit. Mullard soon followed the American lead in improving upon this with a technique called alloy-drift, where controlled doping gradients were used. Their first examples were the OC169 (shown) to OC171 in 1959.

AF114 transistor

Later their AF114 to AF118 types from 1961 onwards, using the same TO-7 metal outline with shield lead, became standard in the RF stages of British transistor radios.

Unfortunately for vintage radio enthusiasts, many of these TO-7 RF types showed problems over time. Investigation by NASA Goddard has now shown that the problem is tin whiskers growing out from the interior walls of the can, which is connected to ground as a screen, ultimately touching the germanium die or internal leads and shorting it out. As such a whisker is very thin, a pulse of high current can melt it, and if lucky, may not damage the transistor!

OC122 and OC123 transistors

The Philips group also made the TO-7 types OC122 and OC123. My image shows Mullard examples marked 'BRITISH MADE', but I suspect that these were originally developed by Valvo. The Valvo 'Handbuch' for 1962 describes OC122 as 'germanium PNP HF transistor for switch applications and high-quality AF amplifiers' and OC123 as 'germanium PNP HF transistor for switch applications especially driver stages in magnetic storage'.

ASZ20 and ASZ23 transistors

Finally, the Philips group also made the TO-7 types:

However, the only examples of AFZ12 and ASZ21 that I have seen are unbranded. Type number ASZ22 was never used.

OC78D transistor

Mullard made a small number of devices that were intended as driver transistors for audio output stages. The part number has the suffix 'D': OC78D, OC81D, and OC82D. For reasons unknown to me, these types use the SO-2 glass capsule but often (not always) with a coloured plastic sleeve, rather than a metal sleeve (which is also found). This is occasionally seen on other types too.

If anyone can explain this, please

LFJ3 transistor package

Mullard sold groups of some of their early transistors as 'packages'. The Mullard Maintenance Manual 1961, together with its supplements lists:

Several of these are unique in listing M-suffix versions of normal Mullard types. These M versions use a metal encapsulation, usually TO-1, rather than SO-2 glass or metal-over-glass. I have not seen these M versions documented elsewhere. I have an LFH3 package in a glassine envelope and an LFJ3 package in a valve-type carton. If you know where I can get any of the others, please

OC41 transistor OC139 transistor

Mullard also produced switching transistors for use in computers and logic circuits. In 1959 they announced a number of these, including the OC41 and OC42 in the SO-2 metal outline, and the rather unusual OC139 to OC141 which are germanium NPN types in the SO-2 glass outline, usually marked 'MADE IN HOLLAND'. All of these devices found widespread use in the UK electronics industry, including computer manufacture.

Mullard silicon transistors

The first silicon transistors were made in the USA by Texas Instruments. Probably the first in Europe were the group OC200 to OC207 by Philips/Mullard dating from about 1958. They used the same SO-2 metal-over-glass construction as Mullard's earlier devices, and so the OC200 types can often be found on eBay described incorrectly as germanium devices! To compound the confusion, they are all PNP polarity, unusual for silicon transistors. The group contains mostly AF amplifiers and general-purpose types. Mullard soon changed to using the pro-Electron BC prefix for silicon, and AC for germanium, which should have eliminated any confusion. However some ebay sellers seem to be unaware of this naming standard and advertise devices starting with B as germanium.

Mullard silicon transistors

A few BC types are found in the SO-2 metal packaging, before Philips changed to more modern TO- standard encapsulation. The anomalous types are:

I am seeking examples of these BCY and BCZ types branded Mullard or Valvo, except for Mullard BCZ11 which seems to be the most common, and Valvo BCZ10. If you know where I can find any, please

OA10 diodes

Of course, Mullard made diodes as well as transistors, and their OA series found widespread use. They made point-contact, gold-bonded, and junction types, in both germanium and silicon,. This included subminature axial types, miniature axial types such as the OA81 often used as the AF detector in radio circuits, and switching types like the OA10 in the SO-2 glass package. Most of these diodes are painted black, but some are yellow. I don't believe the colour is significant. Later types use the Pro Electron prefixes AA (germanium) and BA (silicon) in their part numbers.

OAZ200 diodes

Philips/Mullard's first series of Zener diodes was the OAZ200 series, made from 1959 in the SO-2 metal-over-glass encapsulation.

These are silicon, despite the 'A' in the part numbers; as far as I know, no germanium Zener diode was ever made by anyone. I have read two possible reasons for this: germanium has a higher reverse leakage than silicon, and a germanium diode has a less abrupt breakdown characteristic. Both make germanium less suitable than silicon for a Zener diode. I don't believe the colour of the aluminium is of any significance.

This was followed in about 1964 by a series in miniature axial glass outline:

and by a small series in SO-2 black-painted glass:

Intriguingly, I have two subminiature types, AAZ241 and AAZ244, that look like Mullard production but for which I cannot find data.

experimental transistor

This device is in the same primitive stud package with leads out of the top used for the prototype OC16 in about 1955. Disintegrating greased paper labels are marked 'Semiconductor Lab. Development Sample. No guarantee for subsequent delivery.' The label should bear an added written type number, but this is either missing or has faded to invisibility.

CXT6b transistor

This transistor CXT6b is a real puzzle. It is in the stud package used for the commercial OC16, the first Mullard power transistor, which suggests that it dates from about 1956. The only information that I have unearthed is that the 1963 CV Register of Electronic Valves mentions two GPO types, CXT1 and CXT2, with CV equivalents. This suggests that this CXT6b may be a GPO type, probably equivalent to OC16. However, GPO types are usually found in the PO series, itself rare.

If anyone knows about this device or the CXT series, please

MEI transistors

There are other unusual Mullard types that I possess, but for which I have no data despite possessing many Mullard books and datasheets spanning the whole germanium period. These include type MEI (or possibly ME1) in the SO2 black glass tube but with red and white rubber bands around it, type NC11 in the SO2 metal outline, type T1 in the SO2 black glass tube, type T3 in a TO-7 outline, type T4 in a TO-1 outline, a dual-typed OC16 also marked 2N115, and a type 2N583K in the typical GEC copper outline with blue sleeve.

If you know about any of these devices, please

GET116 transistors

This GET116 is an example of a vintage GEC (UK) transistor bearing the Mullard name. This means that it post-dates the creation of ASM, after which GEC branding gradually disappeared. This is borned out by the date code UE which stands for May 1963.

CV transistors

Like most UK semiconductor manufacturers, Mullard made CV equivalents of their devices. These are military-grade versions of commercial types, given a Common Valve type designation such as those shown, and often identifiable by the manufacturer code D for Mitcham or DG for Southampton. There are many such Mullard CV types, and I have a good number. A selection of those in the 1963 CV Register that I still would like to acquire is:

Mullard went on to manufacture many silicon transistors in the pro-Electron series BA, BC, BD etc., and ultimately integrated circuits. Some fairly early devices that I may add here at some time are Mullard's thyristors, or silicon controlled rectifiers. These came in several series: BRY, BT, BTY, BTX, and BTW. They are largely standardised and therefore less interesting, but oddly seem quite hard to find.