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
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.
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
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
Type approved as VR91, the EF50 replaced the VR65A in British radar
RF and IF amps. Sylvania made loads of EF50s in the USA.
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..
||Left: spray shielded original hooked pin base EF50; right: the later, and well known, EF50
This file was last modified
10:10:58, Sunday July 24, 2016
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