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ID = 28725
Great Britain (UK)
Brand: BTH (B.T.H.), British Thomson-Houston Co, Ltd.
Developer: BTH (B.T.H.), British Thomson-Houston Co, Ltd. 
Tube type:  Triode, vacuum   Universal 
  <1925 very rare. ***
Identical to R4_BTH
Similar Tubes
Normally replaceable-slightly different:
Successor Tubes R4A  

Base Edison (candelabra) 12 mm
Was used by Military and/or Government
Filament Vf 3.5 Volts / If 1.1 Ampere / Direct / Battery =
Description Improved R type, able to oscillate. Satisfied naval service requirements - naval version NR4. Tube has an E12 for the filament, with wire leads from the top of the tube for plate and grid connections. It had a spherical bulb and was first made early in 1917 
Information source Saga of the Vacuum Tube, Tyne   page 219, 220
British Radio Valves - The Vintage Years: 1904-1925   Pages 26, 27, 40 & 86


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