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ID = 2787
Great Britain (UK)
Brand: Marconi-Osram - (Valves/Tubes); GB
Developer: Marconi-Osram - (Valves/Tubes); GB 
Tube type:  Triode, vacuum   Power/Output 
  Ca. 1925 up to 1945 rare.
Identical to PX4 = ACPX4a = LP4_Ferranti = VT131_GPO
Similar Tubes
Normally replaceable-slightly different:
  CV1168 ; F5 ; HV4/2 ; P10 ; P12/250 ; S30C
Other base and data slightly different:
Multiple differences or of other kind:
First Source (s)
1929 : The Saga of Marconi Osram Valve

Base Europe 4-Pin B4 (Eu A, E, 4A) 1914 (Codex=Eo)
Was used by Radio/TV-reception etc.
Filament Vf 4 Volts / If 1 Ampere / Direct / Specified voltage AC/DC

Initially mainly used in battery sets, it was later widely used in mains equipment and amplifiers. It went through several stages of development over 3 decades from the starting date of 1929 and is found in balloon, ST and straight envelopes.

The early versions had a maximum anode rating of 200 volts and 40mA with a filament current of 0.6A. The bulbs of these were blackened by the azide process used for the filaments.

Some rare balloon versions from the first three years of manufacture also exist with angled structures. These were favoured for use in telephone repeater amplifiers where the angled structure was thought to prolong the life of the tube.

From about 1933 as it became used less in battery sets, the anode rating was increased to 300 volts and 50mA enabling a 3.5W power output instead of the 1W of the 1929 version; the filament current being increased to 1A to accommodate this increased power. The transconductance quoted was raised from 5mA/V to 6mA/V.

Dimensions (WHD)
incl. pins / tip
61 x 150 x mm / 2.40 x 5.91 x inch
Tube prices 3 Tube prices (visible for members only)
Information source - - Manufacturers Literature   
Taschenbuch zum Röhren-Codex 1948/49   
  PX4: GEC's catalogue supplement
Anonymous 413 Collector
PX4: Photo of tube from my collection - TS
Tomasz Szczesniak

More ...
PX4: Telefunken Werkstattbuch
Wolfgang Bauer

PX4: osram wireless guide 1931
Giuseppe Antonio Tusini

More ...

from Just Qvigstad

Usage in Models 1= 1929 ; 4= 1930 ; 1= 1931? ; 6= 1931 ; 1= 1932? ; 11= 1932 ; 1= 1933? ; 3= 1933 ; 14= 1934 ; 2= 1935 ; 2= 1937 ; 1= 1938 ; 4= 1945? ; 1= 1946 ; 1= 1947 ; 1= 1948 ; 1= 1949

Quantity of Models at with this tube (valve, valves, valvola, valvole, válvula, lampe):55

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