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 into a black ebonite base and having the electrode lead outs wire wrapped and soldered to the pin tops just proud of the ebonite. They were also prone to poor connections and transit damage, so much so that the valves were shipped ‘pins up’ in a box that had a circular peek hole in the lid. The idea was that the valve could be tested at the retailer without unpacking and prior to sale thus allowing the retailer to conduct Mullard quality control on prevent their product’s reliability being sullied by disgruntled consumers finding they had been sold unserviceable new valves!