The W3 from Cossor is a dull emitter audio output valve from 1925 and features an oxide-coated filament. The base is 32 mm in diameter. Unlike our other W3 this example has lost the paint at the top and the paper label.
The clear envelope allows the 'tin-hat' anode to be clearly seen. The etching is faint but the Cossor name can be made out on the right and the filament voltage of 1.8 is visible on the left.
Face view showing the electrodes. It is possible that the coil wound round the central support is a tensioning spring for the filament. The grid is a wire wound n notched supports and quite unlike the standard form of the 'tin-hat' valves. SeeW2 for original construction style.
A little more of the grid is visible here.
The wide glass tube envelope is 28 mm in diameter, and excluding the B4 base pins is 80 mm tall.
References: 1003 & 1004. Type W3 was first introduced in 1925. See also1925 adverts.