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At the CVD meeting on 17 July 1940 Megaw was asked to change the design of his E1189 low profile magnetron to eight-cavity, in order to operate efficiently in the low field of a standard 6-lb magnet. He launched a rush production of four units. Two of them were used for internal test purposes, the remaining two being completed as E1189 S/N 12 and 13.
This sample, recently found, presumably coming from a GEC-Marconi collection, is believed to be the first unit of the Megaw's eight-cavity revsed design, hastily assembled without radiator and sealed with vacuum grease, while continuously pumped on the test bench. Likely it started to operate in the last days of July 1940. As reported by Clifford Paterson in his diary on 6 August, about one week later, the completed lot was approved by a high-level commission, including Oliphant, Randall and Ellis, in visit at GEC.
In the days after the delivery of the sample No 12 to Bowen, this unit was used to perform a life endurance test, ended with the opening of the heater after 210 hours of operation.
As said before, this prototype does not have the finned radiator. On the side of the anode copper block is punched the inscription '1198 C 328'. Likely C indicates the third revision, after the ones with 6-segment filamentary cathode and with 6-segment oxide-coated cathode. An additional flared copper tube, similar to the ones used for cathode/heater connections, is brazed to the anode block. Almost certainly used as peephole to monitor the cathode temperature under back-bombardment condition by pyrometric techniques.
The twin prototype No. 2 was advertised in the GEC Archives at this link.
The development of E1198 at GEC is summarized here.
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