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The Azide Process
    
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This interesting exhibit is almost certainly one of the little-used (and hence scarce) Mullard copies of early Philips pentodes. At a guess it is a Type PM24C but there were several other rather similar PM types.
The type number is almost certain to be etched on the glass and should be readable in a good light, but this was not observed during the photo session.
These large directly-heated pentodes date from around 1930 when the popular types of moving-coil loudspeaker demanded a watt or two of audio output power. M-OV introduced PX4 but Philips felt obliged to up-stage them with pentodes.
Unfortunately, to get the same power as a PX4 with 200V HT these early pentodes required 300V or even 400V, which was not popular with set designers.
Mazda, of course, capped the lot by introducing their indirectly-heated Type AC/Pen, requiring only 200V but Mazda, of course, had invested in oxide-coated cathode technology instead of the Philips azide process, which was not really suitable for indirectly-heated valves.
We have no operating details for this exhibit. Note the side screw contact.
The balloon envelope is 55 mm in diameter and, excluding the B4 base pins, is 121 mm tall.
References: Observation.

 

Updated December02, 2012.
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