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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.
The second laboratory prototype is represented in an ad page from the GEC Archives. It was presumably operated on the test bench around 1 August 1940. As reported by Clifford Paterson in his diary it was operated until 6 August, when the eight-cavity Megaw's design was approved by a high-level commission, including Oliphant, Randall and Ellis, in visit at GEC.
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 first one with 6-segment filamentary cathode and the successive revision 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 prototype No. 2 is advertised in the page coming from GEC Archives at this link.
The development of E1198 at GEC is summarized here.
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