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This SP18 without dot or suffix (B, G, R) is definitely existing (see photo).
Shortpath is a Cosmos brand, see Keith R. Thrower, British Radio Valves, page 48. Probably an anglicised version of the WD11 from the USA.
According to Tyne (pp.373, 374) there are three variants:
- SP18/R (1925)
- SP18/G (1925)
- SP18/B (1926)
in the Benjamin Catalog all SP18 are tubular, although Tyne shows an SP18/G having a globe shape. Error?
The SP18/G or Green spot was selected as a general purpose hf and af tube for RC coupling. The SP18R or red spot was selected with a lower Ra for output operation and had a gm of 0.9 to 1.0 mA/V.
The advertised price in 1925 for red and green spots was 12/6 = 0.63GBP
Story of Cosmos valves see here
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