The Cosmos short path valves SP-18 were introduced in 1925 all had thoriated tungsten filaments and thus were dull emitters. The first valve in the series was the SP-18 as we have here. The following year they appeared with coloured dots to differentiate the different variants.
The plain SP-18 was designated for audio use and also as an output valve - albeit low power.
The anode end on.
The electrode assembly. The filament has disintegrated and all that is left are the top and bottom connection points.
The top of the anode showing the grid helix and the top filament support. Note the absence of a tension spring.
The grid helix is attached to the support rod by a thin wire wrapped round the support and trapping each turn of the grid.
Another view of the grid fixings to the support.
The etched lettering is very faint on this exhibit but under the right lighting can be seen clearly. With the colour coded SP18's the colour is etched after the SP18. The R by the MetroVick logo is not the variant code, there is a C on the other side - perhaps a reference to use in RC circuits. There is no BBC logo on this valve.
The wide glass tube envelope is 25 mm in diameter, and excluding the B4 base pins, is 85 mm tall.
References: Data & 1043. Type SP-18 was first introduced in 1925. See also1925 adverts.