Metro-Vick using the Cosmos trade name introduced a number of short-path valves in the 1920s. The SP-18 Red is a dull emitter AF amplifier valve from 1926-1928 and has a thoriated tungsten filament.
The essential construction employed in the short-path valves was a flattened tube anode and a grid attached to a single stout support. The filament is a single strand threaded through the centre of the grid.
From the top of the envelope the spacing of the concentric electrodes can be clearly seen. The grid is a much tighter spiral than the early R-Type.
The wide glass tube envelope is 25 mm in diameter and, excluding the B4 base pins, is 87 mm tall. At the base of the composition base moulding the diameter is 30 mm.
Reference: 1043. Type SP-18 Red was first introduced in 1925. See also1925 adverts.
