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A short history of A.C. Cossor Ltd.

In 1896 a business was established in Clerkenwell, London, by Mr Alfred Charles Cossor, specialising in blowing scientific glassware.

In the late 1890s Cossor made the first British example of the Braun-type cathode ray tube and the earliest X-Ray tubes for Crookes and Lodge. In 1904 Cossor made some of the first valves for Fleming and experimental apparatus for Marconi. This was the origin of Cossor's claim to be the oldest valve manufacturer in the world. Cossor made many innovations in the field of vacuum-tube and thermionic technology, including the twin-beam cathode ray tube. Cossor also invented the 'Mica bridge' construction method. This was the use of a mica plate to hold the electrode elements in position and is still used in almost all valves up to the present. Cossor were the first manufacturer in Britain to produce an R.F. pentode, some three years before others. They also patented the multi point filament suspension method. Cossor was formed as a private company, A.C. Cossor Ltd., in 1908.

In 1914, the beginning of the First World War led to increased military requirements and this encouraged Cossor to diversify and expand its interests. Apart from valves and scientific glassware, Cossor also manufactured electric lamps. At some point the company moved from Clerkenwell to Highbury Grove, Highbury, London N.5 .

After the war the company still produced valves for military use for instance the AT40 and AT50 transmitting triodes from 1920 and 21. They also moved into production of valves for domestic use. In 1922 Cossor introduced its first post-war designs, the P1 and P2.

These were followed in 1924 by the P3 audio amplifier and P4 HF triode, both with thoriated tungsten filaments, and in 1925 by the oxide cathode Wuncell series WR, WR1, WR2, WR3. Wuncells marked W1 to W3 were also produced. These Wuncell valves had an oxide coating applied directly to the filament and were not very robust or long lived, presumably due to the difficulty of maintaining adhesion between the barium/strontium oxide mix and the red hot filament wire.

All these valves had the classic Cossor tubular envelope on B4 bases, variously of nickel plated brass or black ebonite, and the unusual flattened hemispherical or paraboloid 'cup' anode. Some sources suggest that this was a means of avoiding a Marconi patent on cylindrical anodes but there were many manufacturers both in the U.K. and in Europe and the U.S. who did make such valves, so I feel this may not be the correct explanation. Cossor themselves said when describing the distinctive features of these valves:-

"The curved filament glowing inside the hood-shaped Grid and Anode is an exclusive Cossor feature. This design is responsible for the use of practically the whole of the electron stream, and results in greatly improved rectifying properties with a higher amplification factor. Cossor Valves are particularly noted for their production of undistorted speech with a total absence of microphonic noises."

The envelope of each valve was engraved with the Cossor name in script slanting upwards. To the left of this was the type number, below which was the filament voltage and below that the anode voltage. To the right of the 'Cossor' name was usually a patent number and below a batch code or serial number.

By late 1925 Cossor were pumping the valves from the base and had introduced a new balloon shaped envelope, similar to the P3 but without the 'pip' top. This Stentor 2 power output valve dates from this period.

In 1926 Cossor introduced its new 'Point One' valves, which used only one tenth of an amp of filament current at two or four volts. The familiar Cossor anode continued to be used, at least for this early example. The envelope engraving was replaced with paper labels. This Point One LF (right) is a late 1926 or early 1927 production example with a secondary BVA label (the BVA was formed in 1926). By 1927 Cossor was using a numeric prefix to the type letters, which indicated the filament voltage and current. E.g. this LF would be designated 410LF - 4 volts 0.10 amps Low Frequency. A different oxide coating process was also introduced at this time. This one was in a box bearing an attached gummed label with the type number 410LF, which probably indicates that this was a transitional valve of older manufacture but sold after the new type designation was introduced. See also Cossor HF.

On the right is a later (Ca. 1930) Cossor 210DET, manufactured by the new process. The anode of this valve is no longer the classic Cossor pattern but a plain rectangular box. I believe that the introduction of the new process was the end of the cup type anode.

Other sources on the Internet suggest that Cossor stopped electric lamp production in 1926 and thereafter concentrated on valves, receiving sets and other electronic equipment, but this is not the case, as they still had an electric lamp works at Aberdeen Lane, N5 as late as 1935. In 1930 Cossor introduced the first British RF pentode, the MS/PenA. The anode impedance was low and it was not widely used, but it predated the main introduction of the RF pentode by three years.

To the left is a 1930s S130 cold cathode voltage stabiliser, designed to regulate the anode supply for a receiver. On the right is a later still SU3150 EHT rectifier with a late 30s style brown Bakelite base.

A.C. Cossor Ltd. became a public company in 1938.

After WWII Cossor concentrated its valve and tube production in one unit and then formed Electronic Tubes Ltd. (Electronic Tubes) as a subsidiary to undertake its valve business, In 1949, by an arrangement with E.M.I., all the ordinary shares of Electronic Tubes were converted into preference shares and a small issue of ordinary shares was subscribed by E.M.I., which from that time has controlled and financed Electronic Tubes. Thus control of Cossor's business in valves passed to a subsidiary of E.M.I., although Cossor still had a financial interest as a preference shareholder. Cossor still obtained much of its supply of valves and tubes for set production from Electronic Tubes. It continued to supply valves under the Cossor brand, although they were made by other BVA members.