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Widget Of The Week

Mullard OC Series Transistors 1952 – 1978

In a brief departure from dustygizmos’ usual format we’re looking at a single innovative and crucially important electronic component, used in the vast majority of British and European made radios, amplifiers, tape recorders and so on, from the mid to late nineteen fifties until the early seventies. It’s the transistor, specifically the legendary ‘OC’ series of Germanium junction transistors, the vast majority of them manufactured by Mullard, a UK subsidiary of the Dutch electronics giant Philips.

 

Mullard produced its first prototypes in 1952, barely four years after Bell Laboratories in the US invented the transistor. They were the OC50 and 51, which used the same ‘point contact’ construction as the original Bell transistors. However, they proved to be difficult and expensive, not to mention very fragile, so they never went into large scale production, A year later Mullard introduced the OC10, 11 and 12 which used artificially grown Germanium crystals, doped with various chemicals. These altered the crystals electrical conductivity to produce layers with differing characteristics. The crystals with ‘semiconducting ‘ properties were sliced into tiny slabs and connected to fine wires. The meeting points between the layers are known as ‘junctions’, led to the name ‘junction transistor’.

 

Skip this bit if you’re not a fan how-it-works stuff.  Essentially junction transistors have three wires or connections, known as the Emitter, Base and Collector. Think of a transistor as a super-quick tap controlling the flow of gas or a liquid. The Emitter and Collector represent the inlet and outlet of the tap, and the Base is the twisty bit on top controlling the flow. In a transistor a large voltage or current passes between the Emitter and Collector and the Base i.e. the twisty bit, fed by a much smaller signal current that controls the flow. Transistors can therefore act as switches, by turning the flow on and off really quickly. This makes them useful controlling things and as we all know, computers are mostly composed of switches, billions of them, each one basically a transistor. However, transistors really came into their own as amplifying devices, turning small signals – applied to the Base into much larger signals by controlling the current flowing between the Emitter and Collector.     

 

And we’re back, to the life and times of the OC series transistors. It only took a couple of years for Mullard (and other transistor manufacturers around the world) to iron out the kinks and wrinkles and figure out how to mass-produce these devices. They had become a commercial product and the fun could begin. Mullard got off to a flying start in 1954 by working out how to encapsulate their transistors in tiny round ended glass tubes. This eliminated at a stroke problems other makers were experiencing with other types of housing, which over time allowed air and moisture to get in. This could rapidly damage sensitive Germanium crystals and cause thermal instability.

 

Mullard's first commercial venture became one of the most popular, versatile and long-lived ranges of audio frequency transistors. They were the OC70, OC71 and OC72, easy to spot in their black painted glass housings and metal casings The three transistors were generally manufactured from single crystals then tested and selected for their specific properties. Transistors with low noise characteristics became OC70s, the OC71 was a standard all-purpose type and the OC72 could handle more power and fitted inside an aluminium cover to further improved heat dissipation.

 

Here’s a nerdy tidbit. Germanium crystals are light sensitive, hence the black paint on the glass encapsulation. Mullard exploited this effect by introducing a variant called the OCP71, also known as a Phototransistor. They were extensively used as light sensors and the electronic enthusiast magazines of the day were full of circuits and plans for light activated burglar alarms, electronic door openers, even light-based ‘walkie-talkies. Unfortunately OCP71s were horribly expensive, compared with standard OC71s, but it was discovered that you could scrape off the black paint on a bog standard OC71 to make a cheap OCP71. Both types were filled with a clear silicon gel. The story goes that Mullard got wise to the practice and changed the filling of OC71s to a lightproof paste. It turns out this was something of an urban myth. Allegedly the switch to an opaque paste was to improve heat dissipation and OC71s with the clear gel were actually rejected OCP71s so they either didn’t work as phototransistors, or the light-sensing performance was poor. For the record I recall using this trick to make a rudimentary light activated switch, and it worked very well indeed.

 

Throughout the 1950s Mullard continued to develop more advanced versions of the OC series transistor. The OC44 and OC45, which appeared in 1956, were designed to handle higher frequencies, which led to them becoming a familiar sight in early British made transistor radios. However, the original manufacturing process imposed limits on the frequencies junction transistors could handle. It wasn’t until 1958, when Mullard introduced the OC169 and OC171 – using a refined manufacturing technique -- that transistors finally found their way into high quality, British made radio receivers. At around this time Mullard were also making low noise transistors that could handle significantly higher currents. They were meant to be used in multiple stages, the smaller ones, known as ‘drivers’ controlling larger ones, housed inside ‘diamond’ shaped metal cases. These led to the first all-transistor ‘power amplifiers’ resulting in a new generation of high-quality audio components and ultimately the birth of the world-renowned British Hi-Fi industry

 

What Happened To Them?

As soon as Germanium transistors were developed that were capable of handling high currents and frequencies valves (or vacuum tubes as they call them across the pond) became virtually obsolete except for the most specialised applications. Transistors changed the world but low power Germanium types tended to be electrically fragile and easily damaged by high voltages and static discharges. Fortunately a more robust type of transistor was already in development by the early 1960s and highly purified silicon proved to be a vastly superior semiconductor replacement. Nevertheless Germanium transistors remained in production until the late 1970s and silicon was even used in OC series types like the OC201 and 207. However, to avoid confusion Mullard changed its component prefixes to AC for Germanium and BC for silicon.

 

There is a small community of collectors of vintage transistors and electronic components and very early Germanium types can sell for three and four figure sums. The big bucks are generally reserved for extremely rare prototypes, pre-production samples and short–lived or experimental devices. Don’t get too excited, though, they are not the sort of thing you’re likely to stumble across at a car boot sale, unless you are extremely lucky and know what you are looking at. Tens of millions of Germanium transistors were made so there is no shortage of them on ebay, either in NOS (new old stock) form, or inside old radios etc., but there are a few sought after types that can sell for several pounds. These include unused vintage transistors, especially if they come with their original packaging and more specifically OC44s and OC45s, which, thanks to their often-poor noise characteristics found their way into guitar ‘Fuzz’ boxes. The result is a very particular sound that can get some serious musicians quite excited…      


DUSTY DATA

First Seen:          1952

Original Price:     £5.00 – “0

Value Today:       £50 – £1000 (0120)

Features:             Low power audio, intermediate and radio frequency germanium junction transistors, glass and metal encapsulations

Power req:                   n/a

Dimensions:                 14.4 x 4.9mm (30mm lead length)

Weight:                         0.7g

Made (assembled) in:    Netherlands, England, Germany & France

Hen's Teeth (10 rarest):  7



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