This BTH B4 came complete with original box as shown below.
The box is 70 mm square and 137 mm long.
The top contains operating details for the valve.
The main picture shows the envelope complete with lettering. Our other B4 is without lettering and the box is larger and shorter.
When seen from the other side the BBC logo is visible as is the electrode structure.
The B4 was the first dull emitter valve from BTH (The British Thomson-Huston Company) and was first introduced in 1923.
Our exhibit comes complete with a 7th edition of the B4 leaflet that was dated February 1925. One passage advised that when using the B4 as an amplifier an anode voltage of up to 120 Volts can be applied but with anode Voltages above 40 Volts a negative grid bias should be used. The bias should be -3 Volts for anode voltages in the range 40-60 Volts and above 60 Volts anode voltage the bias should be -5 Volts.
The design purpose was as an AF amplifier to drive small loudspeakers. These would not be the moving coil devices of today but probably a horn device powered by an oversize earpiece and being wound with a high impedance coil that was wired directly into the anode circuit of the valve.
The balloon envelope is 48 mm in diameter, and excluding the B4 base pins, is 92 mm tall.
References: Adverts, boxes & 1003. Type B4 was first introduced in 1923.