CATHODE RAY TUBE 



However, if the time-base generator is a Miller valve, the amplitude of the 

 output is already appreciable and no extra amplification is required beyond 

 the provision of a second output, in opposite phase to the Miller output and 

 equal to it, so that the X plates are fed in push-pull. To do this we use a 

 'paraphase' amplifier {Figure 32.20). If R^ and R2 are equal, we have 

 negative feedback round the paraphase valve in which B = 1, so the gain is 

 Al(l + A), where A is the gain of the latter without feedback. Al(\ + A) is, 

 of course, just less than unity, and the paraphase amplifier gain may be 



A,. A, 



Astig 



Paraphase 

 amplifier 

 valve 



HT- 

 Figure 32.20 



made exactly equal to unity by making R^ rather greater than R^. i?i and i?2 

 may reasonably be about 5 times the values of the anode loads of the valves, 

 and the value of the A'-shift control is, of course, determined by the HT 

 negative supply available and the bias required by the paraphase valve. 

 Screen stabilization of the latter is only necessary in the highest class of 

 apparatus and it is often possible to omit the stabilizer tube, or even to 

 replace the pentode by a triode, here. The performance of the circuit at 

 high sweep speeds is improved by shunting R^ and R2 by equal small capaci- 

 tors, say of 25 pF, to compensate for the input capacitance of the amplifier 

 valve. 



Post deflection acceleration (P.D.A.) — This is a recent development with 

 electrostatic cathode ray tubes by which high deflection sensitivity is com- 

 bined with good brightness and focus. The scheme is to use rather low ^j, 

 A 2 and A^ potentials so that the electrons pass slowly between the deflector 

 plates, and then to accelerate them in a further electrostatic field. P.D.A. 

 electrodes may be formed by one or more annuli of deposited graphite on 

 the inside of the flare of the tube envelope (Figure 32.21a). These are 

 connected to potentials considerably positive with respect to ^3 and the 

 deflector plates. For example, the G.E.C. type 1601 BCCA(F) cathode ray 

 tube has V^^ = 3-5 kV, V^^ = 7-5 kV and K^, -- 10 kV. Another, 

 extremely elegant, P.D.A. scheme is to use a single electrode in the form of 

 a deposited graphite spiral {Figure 32.21b) along which there is a potential 

 gradient, so that the accelerating field is continuous rather than stepped. 



459 



