MAGNETRON AS GENERATOR OF CENTIMETER WAVES 317 



coaxial output circuit of these magnetrons is terminated. It is possible by 

 a displacement of 0.040 in. of the whole end plate to effect about a 6 mc/s 

 change in pulling figure near the value of 15 mc/s. An end plate consisting 

 of a convoluted diaphragm was constructed which could be adjusted at final 

 test if necessary. By this means it was possible to make the adjustment 

 without disturbing the airtight seal needed in some pressurized installations. 

 The adjustable end plate is to be seen in the cutaway model of Fig. 66. 



Despite initial difficulties, it was possible in the first nine months of 

 production to increase the yield of shippable magnetrons meeting all the test 

 specifications from a very low initial figure to over 85 per cent of all those 

 completed and reaching final acceptance tests. Although subsequent 

 tightening of the test specifications, such as the increase of pulse length at 

 test from lyus at 1000 pps to 2 yus at 325 pps, caused temporary' setbacks in the 

 percentage yield, it was nevertheless increased until it ran consistently better 

 than 95 per cent. 



Although better magnetrons have since been built in the 3 cm. wavelength 

 range, the 725A occupies a special place in magnetron development at these 

 wavelengths. It represents the first high efficiency, x mode, strapped mag- 

 netron for 3 cm. operation, and as such has served as the prototype of most 

 of the subsequent 3 cm. magnetron developments in the United States and 

 Great Britain. 



The 725A was manufactured by the Western Electric Co. in this countr\' 

 and the Northern Electric Co. of Canada. The Raytheon Mfg. Co. used 

 Bell Laboratories' design information to produce a 725A magnetron but 

 redesigned the resonator system to use a vane type more adapted to their 

 manufacturing practice. The total number of 725A magnetrons produced in 

 these three plants was over 300,000, indicating the extent to which it was 

 used during the war. 



17.2 The 2J4S, 2J49, 2 J 50, and 2J53/725A Magnetrons: Following 

 completion of the 725 A magnetron at 3.2 cm., requests were made for three 

 similar magnetrons. These were to have the same characteristics as the 

 725A, differing only in their frequencies of operation. 



The 2J48 is an exact duplicate of the 725A but has a narrower spread in 

 frequency, ± 5 mc/s, around a mean value of 60 mc s lower than the 

 nominal 725A frequency of 9375 mc/s. The 2J48 was never manufactured 

 by the Western Electric Co. 



The 2J49 and 2J50 are 725A types operating at i.i cm. and 3.4 cm., re- 

 spectively. These new wavelengths were met by a new resonator design 

 involving larger resonator holes than in the 725A. The nominal wavelength 

 of the new design was 3.35 cm., tuned either into the i.2> cm. or the 3.4 cm. 

 band by strap manipulation. 



