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.