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The EF50

EF9 showing connection lengthsConstruction of valves followed techniques used to produce light bulbs, with a glass pinch used to lead the wire connections out of the glass envelope to the base contacts.

This method of construction resulted in the connection from electrode to pin being quite long. Although not a problem at lower frequencies, it became a significant issue as the frequency rose. In some valves the control grid or the anode was led out the top of the valve to get past interactions caused by the close proximity of leads at the base, but these could still not cope with frequencies above 30MHz.

An example is shown in the diagram - this being of an EF9.

Even the bakelite bases of valves caused some problems as its dielectric properties altered with temperature.

Acorn and doorknob construction dispensed with pinch by sealing connecting pins directly thru glass. 955 was first, then 954 and 956. UK made some but hard to make and eventually blacklisted by TVC in 1941. 1938 Philips made EF50, then given over to subsiduary Mullard for production. EF50 pulled out of Germany as worries that production in Holland would be stopped by the war.

See the EF50 document at the National Valve Museum


Callick: 1938 Mullard introduced EF50 which amplified at up to 100MHz. Acorns imported but when made here proved difficult, needed highly skilled workers to assemble each unit individually before sealing. Training took weeks and difficult to maintain standards. Mullard RL4 solved this in 1940. ISTVC blacklisted acorns in June 1941.

Thrower: RCA and GE introduced metal valves in 1935 - these had pins sealed into a hard glass base, then welded into metal shell. Forerunner of loctal types? EF50 made in Holland by Philips for TV.

Stokes: similar to American loctal but with metal shell and 9 pins. The original was a prototype with a metallised bulb and unorthodox hooked pins. Quickly discontinued in favour of straight pins, hooked pins and the need to rotate the valve to lock could have caused glass fractures.

Type approved as VR91, the EF50 replaced the VR65A in British radar RF and IF amps. Sylvania made loads of EF50s in the USA.

Hooked pin base EF50 with hooked pins EF50 with hooked pins EE50 with hooked pins
The initial EF50 was made with hooked pins which were rotated to lock into the valve base, and the valve was spray shielded rather than the later metal shell and loktal base..
Spray shielded hook pin EF50 Left: spray shielded original hooked pin base EF50; right: the later, and well known, EF50 EF50


This file was last modified 10:10:58, Sunday July 24, 2016