THE GREAT MULLARD MAGIC BLOG — MULLARD PM VALVES

The unreliable 'split-pin' PM series were swiftly replaced by valves having a new improved base. The base was characterized by having the same ebonite base but the inset pins were now tubular, having two slots in orthogonal configuration with electrode lead arrangements..outs being passed up the pin centre and connected by soldering at the tips. The bases now had a paint filled groove also labeled ‘A’ to denote the anode connection adjacent to the anode pin – This paint was applied by deft fingered young ladies but the process was deemed too labour intensive and so was discontinued in 1928,...

Well, using the recently developed Philips Azide technology for coating and preparing cathodes with barium oxide, the age of the dull emitter had dawned. Following technology transfer from Eindhoven to the Balham works, the production of the PM series valves commenced ( – PM for Philips – Mullard – geddit????.......) with the PM 3 and PM4 in 1925.     Original valve box images  by kind courtesy of Alan Wyatt of The National Valve Museum = see more at www.r-type.org  These early PM valves were very distinctive in having four brass pins, each with a single slot cut into them, set...