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Valves: Cosmos Short Path AC Green Spot (1927)

This valve was released in September 1927 by Metro-Vick Supplies Ltd. A revolutionary dull-emitter valve that used indirect heating by AC mains - thereby reducing the possible hum induced into the mechanism by heating the cathode directly.  The filament of the cathode heater was dipped in liquid porcelain clay and fired to produce a hard, very thin insulation.  The tubular cathode was a ground-breaking 1 mm in diameter.
It would be five years before this technique was beaten for rapid warm-up time, efficiency and low operating temperature.

This particular example is an early valve from the AC green spot history, identified by its pip-top and odd five pin configuration - two pins shorter than the rest.  Later valves from 1928 had the standard B5 pin configuration. Special valve holders were needed which were supplied by Metro-Vick.


There were two valves in the set; the "Green Spot" (or AC/G) for use as a detector and the "Red Spot" (or AC/R) as a LF amplifier.  The glass bulb does actually have a coloured spot painted on it.  A bit odd really as the most common form of colour blindness is red/green insensitivity! 

In 1928 the series was supplemented by the AC/S (Screened Grid).
By 1933 this bulb-shape (reminding us that a lot of valve manufacturers were electric light bulb factories) had all but disappeared as the stepped dome shape took over.


Height 150mm, maximum width 55mm.  Has BVA logo on back and Registered Design number 27059 on bakelite base.