Just when you think you might be running out of audio valves for the Valve of the Month spot, along comes another interesting one. We found this example inside a McMichael compact radiogram (model 545) manufactured in the 1950s. The fifties were an interesting decade of the valve era with several manufacturers trying to influence design. This KT81 was Marconi's equivalent to the American 7C5. It is a B8B or loctal based audio output valve with a pressed glass foot. This is electrically identical to the 6V6 which can give 4.5W single ended or up to 14W as a pair in push-pull configuration. These loctals along with the miniature rimlock devices first appeared in 1947 but became obsolescent by 1951 when manufacturers decided that the B9A form was a better option, probably due to the increased production of tv receivers with higher valve counts. Incidentally, the 6V6 device was available in 9 pin miniature form as the 6BW6. The Chinese 6P1 has a very similar performance but with a different pin-out.
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