The Cosmos SP55R from Metrovick was introduced in the period 1926-1928. The filament is for 6.0 Volt operation and the SP55R was designated for audio output service in radio receivers. The amplification factor or μ is 6.
The SP or Short Path refers to the construction method that enabled the grid to be set closer to the filament than conventional cylindrical designs.
A close-up of the top of the electrode assembly shows the flattened nature of the construction and that the bright grid is wound on a single notched rod.
In profile the approx 2 mm thick anode cavity is clearly seen.
The valve came with a set of operating conditions and the four pages are reproduced here.
On the second page the operating points for various duties are given
A full graph is given.
The back page has the Metrovick logo.
The balloon envelope is 50 mm in diameter and, excluding the B4 base pins, is 99 mm tall.