The BTH B5 is a triode that was first released in 1923. It is a general purpose valve and has an μ of 7. The filament rating of 2.8 Volts was chosen so that the valve could be heated by two dry cells in series.
This exhibit has a portion of the envelope free of the silvering formed by the getter. The vertical anode is of small diameter with the single strand filament at its centre.
The base is marked with the BTH logo. The wire from the pinch passes through the base and is soldered to the pins.
This experimental picture was taken in sunlight with the Nikon D80.
The balloon envelope is 37 mm in diameter and, excluding the B4 base pins, is 87 mm tall.
Reference: 1003. Type B5 was first introduced in 1923.