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Beam Tetrodes and the 807

    
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The beam tetrode was invented by MOV during the early 1930s as a means to circumvent the Philips pentode patent and to improve on the rather mediocre power pentodes then available. The electron stream beam forming process involved turn-by-turn alignment of grid and screen grid wires and the maintenance of this alignment during warm-up and throughout life. This and other critical requirements were difficult to meet on a high-volume production line and so although MOV's research labs manufactured a few hundred successful samples the original design was not at that time put into production.

Some of the samples were sent to the RCA labs in the USA where the design was refined and special machinery was developed to wind and mount the electrode assembly with great accuracy. Other important improvements were made to cathode uniformity, grid coatings and grid cooling. The result was the famous type 6L6, its smaller brother the 6V6, both on RCA's IO base, and the transmitting version the type 807 on a UX5 base. The MOV versions of these were the KT66, the KT63 and the KT8. In later years the 807 design was repackaged on a B8B base by STC as the 5B/254M

The 6V6 was especially successful as a single ended audio output stage in domestic receivers. Not only was there the original metal 6V6 but the large glass type the 6V6G and later the 6V6GT glass tubular type. As the technology evolved to glass disc and then all-glass construction the 6V6 became the 7C5 on a B8B Loctal base and then the 6BW6 on a B9A base.

A special virtue of the type 807 was its low grid and screen grid interception currents, due to their close alignment and the manner in which the electron stream was focused away from these grids. Low interception current not only saved power and increased efficiency but also ensured that the grids did not overheat. This meant that the grids did not distort physically (thereby losing alignment) and were never hot enough for grid emission to become a serious problem. As a consequence the type 807 could be run at high ratings without overheating, which made it a suitable choice for compact, economical transmitters required to generate around 100 W at HF.

Very large numbers of 807's were required during WWII and these were supplied by several manufacturers in the US and in Britain. However, it soon became clear that not all 807's were equal. Some manufacturers' products curled up and died at ratings which the RCA originals laughed at. Tens of thousands of good valves were urgently required while tens of thousands of dubious valves filled store rooms and continued to roll off production lines. The problem was compounded because all, good and bad, carried the same Services' identity markings.

It was obviously necessary to distinguish between Sheep and Goats. Having built and equipped new production lines specifically for mass production of 807's, and with requirements being so urgent, it was not practical to accept 807's only from RCA and to abandon the rest. Instead, the products from different sources were tested to different standards and marked with different VT or CV (etc.) numbers. The very large range of Services' equivalent type numbers illustrates how complicated this was. The different types were then issued for different sets according to the ratings actually required and, of course, priority. Tens of thousands of dubious 807's were never issued for Service use but were released onto the Surplus market after the war. Amateurs, of course, shunned them and still do, although a real RCA 807 was worth its weight in gold.

Exhibit 807 appears to be a goat. In external appearance it is a Chinese copy of the original RCA design. However, it carries a post-war Tungsram label and by this time Tungsram no longer made their own valves but bought and relabelled other manufacturers' products. We guess that this specimen was made by Cossor for the war effort but sold for civil use. It would not necessarily be useless provided you used it gently and did not expect the sort of performance that amateurs used to boast about from their real 807's.

Exhibit 807 has been completely redesigned to suit manufacture by an Ediswan-Mazda production line. Mazda had better designers and more sophisticated production facilities than Cossor. They knew all about grid cooling in power valves from their pioneering work in output pentodes for AC mains sets and they had been making powerful (and successful) beam tetrodes for at least two years before the War. Their VT60A is probably less advanced than the 'real thing' but is probably a much better valve than 807.

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