abl circuit automatic brightness limiting circuit not shown in fig3 is employed as is known in the art this abl circuit is a circuit for detecting a current called an 34 abl current 34 proportional to the brightness of the image receiving tube and for feeding back this abl current to a cathode voltage thereby limiting the brightness to a preselected value as indicated by the straight line l1 of fig4 this abl voltage is varied in a linear manner in accordance with the abl current in this drawing 34 c o 34 indicates a zero value of brightness namely the cut off point 34 h l 34 shows a maximum value of brightness namely the highlight point a point 34 p 34 represents such an operation point where the abl circuit is operated and the brightness of the highlight is limited to a constant value returning to fig3 the input terminal t4 is connected via a smoothing circuit constructed of the resistor r5 and the capacitor c2 and also via the zener diode d used as a constant zener diode voltage limiter to a base of the transistor tr1 an emitter of the transistor tr1 is connected via the resistor r6 to the power supply input terminal t3 also a collector of this transistor tr1 is connected via a resistor r7 to the ground with the above described connections the transistor tr1 may function as an inversion amplifier that is a voltage called an 34 inverted abl voltage 34 such as that indicated by the straight line l2 of fig5 appears between the collector and the ground.