VISUAL INDICATORS 



The procedure is to choose a 'bleed' current through the chain, large com- 

 pared with any of the tube electrode currents, say, 1 mA. The chain may 

 then be treated as a simple unloaded potential divider, and the component 

 values chosen to provide the necessary ranges of potential. In oscilloscope 

 practice it is the rule to earth the positive EHT terminal, otherwise the 

 coupling capacitors to the plates have to be rated at approximately the full 

 EHT. 



HT+andEHT + 



From 

 earlier 

 stages 



Cathode 



Hj. Brightness 



Figure 32.18 EHT- 



For general electrobiological work, direct coupling to the deflector plates 

 is more satisfactory, and in this case the deflection amplifiers and the tube 

 supply network have to be designed together. Figure 32.18 shows a possible 

 scheme in which a long-tailed pair forms the output stage of the Y deflection 

 amplifier. The output of the time-base generator may be fed to the X plates 



HT+ and EHT + 



From earlierl— ■ 

 stages in 

 time base 

 generator 



From earlier 

 stages in 

 X ^deflection 

 hift <, amplifier 



HT- 



Shift 



HT- 



Figure 32.19 



Brightness 

 HT- ?^ iSrid 



EHT- 



by a similar long-tailed pair, and if this is done a highly refined form of 

 astigmatism control becomes possible in which the first and third anodes are 

 fed from a fixed tap across the HT supply, and the long-tailed pairs are each 

 provided with tails of variable length {Figure 32.19). In this way the mean 

 potential of both pairs of deflector plates may be adjusted with respect to 

 the third anode to achieve optimum focusability. 



458 



