Although off-subject there seems to be a lot of interest in the EF50.
Attached is a text-only version of a recent article written by
Keith Thrower, an ex-Director of Racal and an electronics historian.
It comes from the journal of the "Centre for the History of Defence
Electronics"
at Bournemouth University - http://chide.bournemouth.a.c.uk
Walter G3JKV.
The following article was published in the December 2000 issue of "Transmission
Lines", the
journal of the "Centre for the History of Defence Electronics", based at
Bournemouth
University.
Illustrations have been omitted in this text-only copy to save space.
If you'd like to see the full article ask CHiDE for a copy of their journal -
[email protected].
The Famous EF50 Valve of WWII
by Keith Thrower OBE
By the early 1930s screen grid and pentode valves were available for RF
amplification for
frequencies up to about 30MHz, which was adequate for both broadcast and
commercial
purposes at the time, when radio usage had not extended into the UHF band. At
frequencies
above 30MHz the gain available from valves fell very sharply; there were two
principal
problems: the first was caused by the inductance and capacitance of the
internal leads that
connected the valve electrodes to the terminating pins; the second was due to
the finite transit
time that the electrons took to travel between the valve electrodes.
The first problem arose through the valve design and manufacturing techniques
which had
evolved from those used in the electric lamp industry. One particular
constructional feature of
the valve, copied directly from the lamp industry, was the use of an internal
glass stem and
pinch that held the support wires to the electrode assembly, and also provided
a vacuum seal
for the lead-out wires. The problem that arose from this method of construction
was that the
total length of the connections from the electrodes to their terminating pins
was quite long,
resulting in significant self-inductance of the wires as well as excessive
self-capacitance
between them. At frequencies below 30MHz, these parasitic inductive and
capacitive
components did not seriously affect the performance of the valve, but their
effects became
increasingly more serious at frequencies above this.
The second problem - the finite transit time for the electrons to move between
the electrodes -
was very serious for valve circuits operating at frequencies above 30MHz. For a
typical RF
valve of conventional construction, the transit time for the electrons to move
between the
cathode and control grid was about one nanosecond (one thousand-millionth of a
second). At
frequencies of a few megahertz, this transit time was insignificant compared
with the time for
one cycle of the signal frequency. At 100MHz, however, the time was about 10%
of one cycle
and this was very significant. The phase lag caused by this time delay resulted
in a low input
resistance at high frequencies, which significantly reduced the amplification
available.
A great deal of experimental research work was carried out at the RCA
laboratories during the
early 1930s to investigate the behaviour of radio frequency amplifier valves,
where it was
found that improved circuit performance could be achieved if the valve
dimensions were
reduced. With a linear reduction, the mutual conductance and other valve
parameters remained
almost unchanged, but the lead inductance, interelectrode capacitance and
electron transit time
all fell in direct proportion to the reduction of dimensions. In fact, such a
linear reduction was
not practical; however, the tiny 'Acorn' valves that resulted from this work
were capable of
providing amplification at frequencies up to about 400MHz.
The first of these valves to go into production was the type 955 triode which
was introduced
in 1934. This was followed by the 954 pentode in 1935 and a variable-mu
pentode, the type
956, in 1936 (see Figure 1). They all had indirectly heated cathodes, operating
at 6.3V, 0.l5A.
The diameter of the heater-cathode assembly was comparable with that of a common
household pin and the overall length was less than one half. The capacitance
between the
control grid and anode for both the triode and pentode was about half that of
conventional
valves, and all other internal capacitances were also significantly reduced.
Before long, acorn valves, based on the RCA design, were introduced in Britain
by Mazda,
Marconi-Osram and Mullard. Initially, all the British acorn valves had 4V
heaters, but 6.3V
versions were introduced in 1940.
Figure 2 shows how the input resistance of a valve is affected by transit time,
where a
comparison is made between an acorn pentode and an equivalent, conventional
pentode. At
30MHz, the conventional pentode (B) has an input resistance of 17k which falls
to only 1.5k
at 100MHz. The equivalent figures for the acorn pentode (A) are 220k at 30MHz
and 20k at
100MHz. This fall of input resistance, which has a critical effect on the
amplification that the
valve can provide is inversely proportional to the square of the frequency: if
the frequency is
doubled, the resistance falls by a factor of four and a ten-fold increase in
frequency results in a
hundred-fold decrease of input resistance. It is not difficult to see,
therefore, that conventional
valves were unsuitable for operation in the UHF band, whereas the acorn or
similar miniature
valves were better suited.
British companies, such as MOV and Mullard, found the acorn valves very
difficult to
manufacture because of the highly skilled labour required. As a result,
considerable quantities
of the valves were imported from the US for use in military radar equipment
during World
War 2. Because of the manufacturing problems and the eventual availability of
alternative
valves, the acorn types were blacklisted by the Inter-Service Technical Valve
Committee in
June 1941.
With the commencement of high definition television in 1936 there was a need
for a new type
of valve capable of providing wideband RF amplification. The frequencies
required for the
Alexandra Palace transmission were 41.5MHz for the sound channel and 45MHz for
the vision
channel, the latter requiring a bandwidth of 3MHz in order to accommodate the
full picture
information. In order to achieve satisfactory amplification of the video
signal, valves were
required with a high value of mutual conductance, and if this amplification was
to be achieved
at RF the valves must have low values of internal capacitance and
self-inductance, in addition
to a short electron transit time. The early valves produced for this role were
far from
satisfactory.
By the mid-1930s top-secret work was in progress at Bawdsey Manor in Essex on
radio
direction finding (RDF)later to be re-named radar. For this, once again, valves
capable of
providing wideband UHF amplification were required. At this time Tom Goldup, a
senior
director of the Mullard Valve Company, was liaising with the British government
and was
made aware of this requirement. Mullard was wholly owned by the Dutch Philips
Company
and all the valve R&D work was carried out at the Philips Eindhoven plant.
Goldup
approached Philips asking if there was a valve with the required specification.
(Because RDF
could not be mentioned I suspect he referred to television applications.) He
was told that a
suitable valve was being developed for the Dutch government; samples,
therefore, could not
be supplied to Mullard. It would appear that the UK government approached the
Dutch
government and samples were then supplied. The valve in question was the EF50,
which
became available for television use in 1939. At this time all the valves were
being
manufactured in Holland.
The construction of the Mullard FF50 is of interest because it marked a
significant departure
from the conventional types used in Britain at the time. The usual Bakelite
base and internal
glass pinch were replaced by an all-glass base. Elimination of the stem and
pinch resulted in a
considerable reduction in length of the internal wires. The valve had nine
chromium-iron pins,
which were sealed into the glass base and arranged uniformly around a central
metallic spigot,
which was keyed in order to facilitate insertion into the valveholder. The
spigot was joined to
an external metal screen that covered the whole base, with small holes to allow
the pins
through. Because of the screening provided, it was possible to bring all the
connections out to
the base, avoiding the need for a top cap connection.
With the outbreak of war it was realized that the supply of EF50 valves would
dry up and
Mullard did not have the capability of manufacturing the special glass base
with sealed-in pins.
Consequently, just before Germany invaded Holland, a truck came from Holland
with one
million of these glass bases. Later, huge numbers of the valves were
manufactured by Sylvania
in the US.
Figure 4 shows a selection of EF50 valves from various suppliers available
during the War.
The original Air Force type number was VR91, the Army type ARP35 and the CV
number
1091. From left to right these are:
(i) and (ii) Sylvania manufactured VR91, front and back view.
(iii) Silver UK version of VR91 for RAF use.
(iv) Silver UK version ARP35 for Army use.
(v) Mullard EF50 red.
(vi) Mullard EF50 silver
(vii) Cossor 63SPT
(viii) Osram Z90.
A typical use of the EF5O was in the Pye 45MHz IF strip - Receiving Unit Type
153 - which
was used universally in British radar equipment during the war. A picture of
this can be seen in
Figure 5. It had six EF50s (VR91s) and one EA50 (VR92) miniature signal diode.
A variant of the EF50 was the RL7 (VR136). This had aligned grids to reduce
partition noise,
and the cathode was connected by leads to two separate pins to reduce still
further the
selfinductance. The valve was capable of proving RF amplification at 200MHz and
could thus
displace the acorn valve. In post war years it was re-designated as EF54.
Acknowledgements
I am grateful to Dr Graham Winbolt who provided the photographs of the EF5O
valves in
Figure 4 and the Receiving Unit Type 153 in Figure 5. The information on the
RL7 was
obtained from Brian Callick's excellent book Metres to Microwaves (published by
Peter
Peregrinus Ltd and obtainable from the IEE).
For more information on early British valves see History of the British Radio
Valve to 1940 by
K Thrower and published by Speedwell.
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