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ID = 30520
Great Britain (UK)
Brand: GEC, General Electric Co.; London
Tube type:  Half-Wave Mercury-Vapor Rectifier   Power-supply 
Identical to GU21 = GU20
Similar Tubes
Normally replaceable-slightly different:

Base Edison giant, E40 = 40 mm-Goliat Top contact with a cap.
Was used by Industrial
Filament Vf 4 Volts / If 11 Ampere / Direct / Parallel, (AC/DC)

Va<13kV; Ik<1.25A mean.
Also manufactured labelled GU20/21 when the characteristics of the types were merged.

Text in other languages (may differ)
Dimensions (WHD)
incl. pins / tip
60 x 258 x mm / 2.36 x 10.16 x inch
Information source Taschenbuch zum Röhren-Codex 1948/49   
- - Manufacturers Literature   
GU21: Photo of tube from my collection - TS
Tomasz Szczesniak

GU21: Manufacturer's Literature
Emilio Ciardiello

GU21: GEC's catalogue supplement
Anonymous 413 Collector

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