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ID = 28726
Great Britain (UK)
Developer: BTH (B.T.H.), British Thomson-Houston Co, Ltd. 
Tube type:  Triode, vacuum   Universal 
  <1925 very rare. ***
Identical to R4A
Predecessor Tubes R4_BTH   R4_Osram  
Successor Tubes 1922   R4B_BTH   R4B_MO  

Base Edison (candelabra) 12 mm
Was used by Military and/or Government
Filament Vf 2.5 Volts / If 0.46 Ampere / Direct / Battery =
Description Thrower states a filament current of 0.65 A. Tyne says 0.46 A. The early BTH R4 tube had a spherical bulb and was first made in 1916. 
Information source Saga of the Vacuum Tube, Tyne   Pages 219 & 220
British Radio Valves - The Vintage Years, Thrower   Pages 26,27,40 & 86


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