The base of an ARDE
In 1923 Ediswan introduced its safety cap for receiving valves. The base was a standard B4 with a metal shell and split pins that had the filament pins reduced in length and made of unequal length. This arrangement and the geometry of the base made the accidental connection of the filament across the HT almost impossible.
The standard B4 base pin length was about 18 mm and this was retained for the anode and grid pins. The filament pins (pins 3 and 4) were reduced to 14.4 mm and 12 mm.
Valve holders of the day had the socket connections either completely open or with the metal brought to the surface of the Bakelite moulding. Ediswan were the only manufacturer to use the safety cap and it was not long before the redesign of the valveholder made the safety cap redundant. The redesign simply recessed the connection into the moulding. The geometry of the pins then completed the task.
The Safety Cap explained.
See R-Type and ARDE