VISUAL INDICATORS 



Ion burn may be combated in tetrode tubes by an elementary application 

 of mass spectrometry. The electron 'gun' — consisting of grid, cathode and 

 first anode — is inclined to the axis of the tube {Figure 32.36). Under the 

 influence of a lateral field — from a small assembly of permanent magnet and 

 pole pieces, which slips over the tube neck — the electrons are diverted into 

 their proper path along the axis. The ions are not ; they hit the side of the 



Permanent magnet 



GunJ 



Pole pieces 



Tube neck 

 Figure 32.36 



Ions 

 — Electrons 



tube and are eventually collected by A^. Adjustment of the ion-trap magnet 

 is a simple matter of searching for the position which gives the brightest trace. 

 Shift magnets. Rather similar small assemblies may be bought which 

 provide a transverse field of variable strength and direction to provide beam 

 'shift' facilities. From Figure 32.37 it is clear that the device is similar to the 

 ion-trap, but the permanent magnet may be turned as shown between the 

 pole pieces so that the strength of the field between them follows a cosine 



Adjust 



Final anode 

 connection 



Ion 

 trap 



Focus 

 magnet 



Shift 

 magnet 



Figure 32.37 



Deflector 

 coil assembly 



Figure 32.38 



law. By rotating the device as a whole round the tube neck the direction of 

 shift is controlled. The magnitude of the shift is determined by the position 

 of the rotatable permanent magnet within the device itself. 



The order in which the various ancillaries are assembled is shown in 

 Figure 32.38. 



Deflection amplifiers 



The direct-coupled deflection main-amplifier of the author's demonstration 

 oscilloscope is shown in Figure 32.39. A long-tailed pair is used as a phase- 

 splitter to feed a push-pull power output stage in class AB. The frequency 

 response is 6 dB's down at 10 kc/s. Using a 17 in. diagonal rectangular screen 



466 



