This B4 exhibit is very similar to B4 it was marked as a B7 but after a long period of examination the etched markings could be seen as a B4.
The B4 was the first dull emitter valve from BTH (The British Thomson-Huston Company) and was first introduced in 1923. In 1924 the B6 was released that reduced the filament consumption but offered about the same performance.
The design of the B4 was for 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 nickel plated base shell with two part brass pins.
An enhansed image showing the BTH logo and the word Type. The letter B can be seen and to its right is the feint figure 4.
The moulded base within the outer shell. The BTH logo is in the centre.
The lower end of the anode showing the grid. The latter BBC stamp on the envelope places production of this specimen to about 1924 - 1927.
The filament can be seen passing up the grid on the left of the picture.
The balloon envelope is 51 mm in diameter and, excluding the B4 base pins, is 93 mm tall.
References: Adverts, boxes & 1003. Type B4 was first introduced in 1923.