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The SP13, SP13C and SP2 are typical Mullard-made 'straight' screened pentodes of the mid-to-late 1930s. Note that the -C suffix on a Mullard type indicates a variant of the normal type. The SP13 was fitted with a Continental side-contact base cap because it was copied from a Philips original. The SP13C, manufactured only in Britain where some setmakers insisted on the B7 base, was considered as a derivative.
All are products of a by then mature oxide cathode process and use a mica-based mechanised electrode assembly. Additionally, the two indirectly-heated AC/DC types (SP13 and SP13C) have alumina-based heater insulation (a development of the slip-coating process pioneered by Mazda some 10 years before). These features made them reliable valves, relatively cheap to manufacture, and probably still in good working order today.
The weakness of such valves was not in quality but in concept. By the time these valves were manufactured American-influenced modern superhets, highly selective and making full use of AVC, were already dominating the British domestic market but such sets had no place for 'straight' screened pentodes. Moreover, again under American influence, other manufacturers' valves were beginning to look smaller and sets now had to fit into compact Bakelite cases. Mullard, by this time wholly controlled by Philips, was the British outlet for valves designed on the Continent where cheap, basic, local-station sets (typified by the German 'Peoples' Radio) were the norm.
Technical salvation came in 1939/40, in the nick of time. German pressure for a State Television Service forced Philips to invest in really modern valve design and manufacturing technology in order to achieve sufficient gain-bandwidth product in compact, mass-produced video amplifiers. The result was the famous Type EF50. Initially these were made only in Holland but early in 1940, just ahead of the Nazi invasion, Philips sent its production team and equipment to the UK.
In short, since Mullard was constrained to follow Philips' policy, its valve catalogue was a year or two out of date compared to MOV's wholesale adoption of American (RCA) technology around 1937, and Mazda's concurrent design initiative of super-efficient home-brewed types.
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