A gem beyond price

As one of my duties as Store Keeper/Recorder at the Museum’s reserve storage facility I have been cataloguing previously unlisted items onto the Museum’s Collection Management System, one of my recent projects was to catalogue all the unaccessioned electronic valves in the Museum’s collection. Whilst this may not sound too exciting, there were, I discovered some fascinating stories behind these small objects.  The EF 50 is one such valve, developed to meet the demands of the nascent pre-war television industry it played an important role in the development of early radar which gave Britain and her Allies an advantage over their enemies in airborne interception radar that would be maintained for the duration of the war.

Mullard EF 50 valve, RAF designation VR 61 (79R/R/823)

Television broadcasts in Britain began in 1936 from a single transmitter at Alexandra Palace in North London on a wavelength of 45 MHz, however, the maximum range at which the television signal could be received was 25km from the Alexandra Palace transmitter. A British electronics company, Pye of Cambridge were keen to increase their market share of television sales by developing a new model television with double the range at which the signal could be received.   Pye undertook extensive research and development in designing their model 915 television which was to be revealed at the Olympia radio exhibition in August 1939.  

Pye logo

As part of the Model 915’s development Pye approached British valve manufacturers Mullard to see if they could provide a valve with the performance required to double the receivers range.  Mullard, a wholly owned subsidiary of the Dutch electronics company Philips, referred the request to the parent company, who like many other electronic component manufactures had been working on developing valves specifically for the television market. 

The valve that Philips had developed was the EF 50 which was a high gain, low-capacitance pentode (radio) valve whilst not an invention in the true sense, it was notably different from valves then in production as being of all glass construction ( as opposed to glass and Bakelite) and employing a new method of manufacturing,  the new design and manufacturing method ensured that the EF 50 was far more powerful than its competitors and that it could be mass produced cheaply.  Philips revealed the valve at the Zurich Television Conference of 1938. Mullard incorporated six EF 50s into a chassis creating a 45 MHz tuned radio receiver sensitive enough to pick up the Alexandra Palace signal at a range of up to 50 km for Model 915 television. 

Advances in radio technology had also caught the attention of the British Government who had  asked Committee for the Scientific Study of Air Defence otherwise known as the Tizard Committee after its Chairman Henry Tizard, to investigate the feasibility of a “death ray” strong enough to bring down an aircraft. When this proved impossible the Superintendent of the Radio Research Laboratory, Robert Watson-Watt in a paper to the Air Ministry titled “The detection and location of aircraft by Radio methods” suggested that it had been observed that aircraft interfered with radio waves being transmitted from the GPO transmitter at Daventry and that this phenomenon should be explored further with respect detection of aircraft.  The Tizard Committee met with AVM Hugh Dowding in early 1935 to discuss Watson-Watt’s paper. Despite initial scepticism Dowding in his role as the Air Council’s Member for Research & Development agreed to make £10,000 available for research into radio detection. 

From this research Radio Direction Finding (now known as radar) developed and the building of Chain Home the world’s first air defence radar.  As advanced as Chain Home was it had some major drawbacks one of which it was only able to look out across the sea, once aircraft crossed the coast they could only be tracked by ground observers which on clear days was acceptable but in poor weather or at night virtually impossible.  Watson-Watt realised that an enemy would quickly turn to night bombing if daylight raids suffered heavy losses and that Chain Home would become compromised. To counter this he tasked one of the scientists who had worked on Chain Home, Dr Edward “Taffy” Bowen, with investigating airborne radar for airborne interception of aircraft (AI) and surface ship interception, air-to-surface-vessel (ASV) radar.  

In 1935 Bowen and his small team at Bawdsey Manor set to work on developing airborne radar. An early piece of luck for the team was the acquisition from an unknown source of a 45 MHz Tuned Radio Frequency (TRF) designed by EMI for the television service at Alexandra Palace, Bowen described the acquisition as a “gem beyond price... it was far and away better than anything which had been achieved in Britain up to that time”[1].  The receiver comprised of seven or eight valves mounted on a chassis about 15 inches long, this was then combined with CRT (Cathode Ray Tube) indicator which formed the basis of all radar development in the next few years.  Bowen  and Watson-Watt were unable to source anymore of the TRF chassis, the one chassis became the sole component for all radar development work and was taken out of one experimental set and placed in another when the need arose. 

AI Mk.I receiver chassis with EF 50 valves, 3 November 1939 (PC98/131/9)

By early 1939 development was at such a state that limited introduction to service of ASV and AI was being considered, the one major stumbling block was the 45 MHz TRF strip. The Bawdsey  team were still using the original strip they had acquired in 1936 sourcing more from EMI had not proven fruitful,  however by another stroke of good luck Taffy Bowen’s former Professor at Cambridge contacted Bowen and informed him that Pye were undertaking production of a 45 MHz TRF strip for their new television set.  Bowen rushed to visit Pye and was “rewarded with the remarkable sight- scores of TRF (Tuned Radio Frequency) chassis of just the type we were looking for"[2]. Not only had Bowen found a source of supply for a 45 MHz TRF strip it was also found that the Pye strip, which incorporated the EF50, was smaller, lighter and notably better than the EMI strip, which lead to a marked improvement of the operation of the AI receiver then under development. Pye were awarded a contract to produce thirty AI receivers for the Air Ministry.

Experimental instillations in Blenheims with AI Mk.I began in August 1939 and further development work was undertaken over the winter.  With small scale production underway it was assumed that the supply of the EF 50 was safe as it was believed that the valves were being manufactured by Mullard in Britain. In early 1940 it became apparent that Mullard was having difficulties manufacturing the EF 50 and that despite bearing Mullard’s name all EF 50s were being made by Philips in Holland. This news shocked Watson-Watt who realised that the supply of EF 50s was threatened by a potential German invasion of Holland. Watson-Watt hastily arranged a meeting with Philips’ head of development Theodore Tromp in early spring 1940 and impressed upon him that “the British Government was most anxious to set up a large scale production of EF50 in England and requested, if possible to, supply Mullard with all the production equipment and special tools for these valves”[3].


AI Mk.IV, Beaufort training aircraft radar operators station, 9 April 1943 (PC98/131/23)

Watson-Watt did not inform Tromp of the reason for the request, but it was obvious that the EF 50 must be of vital importance for the British Government to go to such lengths to secure its supply.  On his return to Holland Tromp ordered the production of as many EF 50 valves and their bases as could be produced and the production of duplicate equipment for Mullard to undertake production should Holland be invaded.  Philips’ employees worked night and day to fulfil the order; on the 9th May a lorry was loaded with machines, 25,000 EF 50s and 250,000 EF 50 bases. The lorry reached the docks at Vlissingen where it was loaded onto a vessel bound for England, at 5 O’clock the following morning the Germans invaded the Netherlands, the ship was hurriedly made ready and slipped her moorings to cross the Channel despite aerial attacks the ship safely birthed in London securing supply and the means of production of this most valuable valve.  

Illustration of the Pye strip installed in Receiver Unit R.3066 of AI Mk.IV  (AP2551A R016459)

The German invasion was no great surprise and Philips initiated their evacuation plan, but such was the speed of the German invasion that the convoys of lorries from the Philips factories were soon captured.    However most of the Philips family and the board of directors were evacuated along with the Dutch Government from the Hook of Holland on the evening of the 13th May aboard HMS Windsor. 

Illustration of the interconnexion of units of AI Mk.IV  (AP2551A R016459)

Such was the importance of AI Mk.IV with its Pye Strip that it was among a number of scientific instruments that was taken to America in September 1940 by the British Technical and Scientific Mission headed by Sir Henry Tizard more commonly known as the Tizard Mission. The Mission’s purpose was to display British technology in the hope of securing American funding and expertise in furthering research and development begun by the British but which they were unable to exploit due to other demands placed upon her wartime economy.  

The Americans were surprised by much of the technology that was displayed, amongst which were the radar developments.  The AI radar was a technology that the Americans had not even considered, successful trials were conducted with it being installed in a Douglas A-20 and the US Army Air Corps ordered it directly into service.

Illustration of the layout of AI Mk.IV equipment in a Beaufighter, the first aircraft to become operational with AI Radar (AP2551A R016459)

Taffy Bowen described the EF 50 "a valve which was destined to play almost as important a part in the radar war as the magnetron"[4], but the magnetron will have to wait for another blog.

There is a considerable literature on the development of radar,  to name some of the more prominent sources I used  
“Radar Days” by E G Bowen,
“The history of air intercept radar & the British Nighfighter 1935-1959” by Ian White,
“The signals history” Volume 5 by HMSO,
"Top secret exchange" by David Zimmerman
A particularly useful online source for the Philips story was “the EF 50, the tube that helped win the war” by Ronald Dekker (http://www.dos4ever.com/EF50/EF50.html)

[1] "Radar days" P32-33
[2] "Radar days" p77
[3] Letter from Theodore Tromp to Mr Bell, 16 January 1979, retrieved from http://www.dos4ever.com/EF50/letter_Tromp.htm .
[4] "Radar days" p77

Martin Ward : Storekeeper / Recorder
About the Author

Martin Ward : Storekeeper / Recorder

I joined the RAF Museum in 1999 to assist in the re-location of the reserve collection from Cardington to Stafford. Initially helping unload over 150 lorry loads of artefacts. Shortly after I started data entry on the Museum Collections Management System, locating items with an inventory level record. Currently I am working on the entire 3D collection, updating artefacts to a “Full Catalogue” record status which has helped the Museum in making selections for its WWI exhibition.

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