[Fwd: LF: The EF50]

[Andre' Kesteloot]

Walter Blanchard wrote:

> 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  -
> chide@bournemouth.ac.uk.
>
> 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.