REFLEX OSCILLATORS 



579 



struction of the bow is to reduce the internal stresses produced by bending 

 during the tuning action. 



A cathode, control grid and a pair of focussing wires are supported by 

 micas in a position facing the open side of the U channel. The channel 

 serves as an anode for the cathode current, which is controlled by the grid. 

 The focussing wires beam the cathode current into the anode. The grid is 

 proportioned so that under all operating conditions it remains negative, 

 and the control system need supply no power to it. 



The heating of the channel by the electron bombardment causes it to 

 expand with a large differential with respect to the bow. As a result the 

 bow flattens out and its center moves toward the channel. The purpose 

 of this construction is to provide a magnification of the expansion of the 



Fig. 75. — An external view of the W.E. 2K45 — an early thermally tuned reflex oscillator 

 designed by the Bell Telephone Laboratories. 



channel which by itself would provide insufficient motion. The cross 

 member welded to the center of the bow and the vertical struts transmit the 

 motion of the bow to the diaphragm which supports one of the cavity grids. 

 The action is illustrated in Fig. 78 which shows a series of X-ray views of an 

 operating tube. Thus, the first view shows the conditions for no power 

 applied to the tube, the second, for the tube operating but with the tuner 

 grid biased to cut-ofT. The following views show the behavior with progres- 

 sive increases in the power into the tuner channel. 



The ramifications in the design of a thermal tuner are many and the possi- 

 ble configurations of the mechanism will depend greatly on the individual 

 requirements of the application. It is possible, however, to lay down some 

 basic principles. These are concerned with positiveness of action and speed 

 of tuning. With regard to the first, it may seem anomalous to speak of 



