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The Mullard 20CG is a gas filled photocell with a caesium/oxidised silver cathode. The gas filling increases the sensitivity by amplifying the current by an order of magnitude.
On the data-sheet the base is described as B8G to fit the European designations. The American designation originally was Loctal and today we know the base as B8B.
The semi-circular cathode with rod anode in front.
Face view of the cathode. On the base is the locating pip for the base.
The base originally had four pins fitted but pin four has been cut off.
The wide glass tube envelope is 29 mm in diameter and, excluding the B8B base pins, is 61 mm tall.
Reference: Data-sheet. Type 20CG was first introduced in 1949. See also 1949 adverts.


Absolute Maximum Operating Conditions
Light Control
Thanks to Frank Philipse for supplying the above PDF datasheet.
Updated December18, 2019.
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