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Valves: Unidyne K4

This is the Unidyne K4 from 1924, the very early days of valves (note that the first generic mass-produced valve, the "R" valve was available in 1922).  It is classed as a bi-grid space-charged tetrode and was the first five-pin British valve.  It was not, however, the same format as the eventually adopted "B5" standard layout.


In May 1924 Practical Wireless started publishing details of the "Unidyne" (one-force) circuit, which used a single power source from low tension power rather than LT and HT batteries.  The circuit idea was invented by G. V. Dowding and K. D. Rogers and this valve was specially produced for that circuit by  Bower Electric Ltd, of Shaftesbury Avenue, London WC1.


Bower offered a complete constructor's kit for a two-valve receiver for 6 6s 0d  (6 guineas) for loudspeaker reception in June 1924 - it didn't last long, so these valves are quite rare.


Notice the top-pip, which in this decade was the sign that valves were sealed at the top of the valve - a feature that became obsolete by 1927.  Subsequent valves had their vacuum seal made at the base until the all-glass miniature valves of the late 1940's.


The lack of gettering residue makes it wonderfully easy to see the tetrode structure.  The envelope still bears the "Unidyne K4 Valve" markings (on two I own) and the pins are the "banana" plug type to ensure a good connection in the valve socket. 


Height 100mm, maximum width 45mm.