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.
The top of the envelope is painted green and inside the glass the Cossor parabolic anode can be clearly seen. Our other W3 is unpainted.
The W3 came in the original box. The image on the box shows that the electrodes followed the same pattern as the earlier valves but with the larger anode for the filament to work in a central support was added.
The box also carries the basic operating parameters of the W3.
The wide glass tube envelope is 28 mm in diameter, and excluding the B4 base pins is 84 mm tall.
References: 1003 & 1004. Type W3 was first introduced in 1925.