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BELL SYSTEM TECHNICAL JOURNAL 



small tubes. When the IF frequency is as high as 30 mc or 60 mc, the 

 circuit elements are physically small, so that the tube size is relatively 

 important. The power level is low enough so that miniature tubes are 

 applicable. The photograph shown in Fig. 4 illustrates the reduction in 

 the size of the IF amplifier obtained by using 6AK5 tubes in place of 6AC7 

 tubes which were widely used previously. The larger amplifier weighs 2 

 lbs. 4 oz. while the smaller one weighs only 9 oz. Each of these amplifiers 

 provides an over-all amplification of about 95 db at a mid-band frequency 

 of 60 mc. The bandwidths of the two amplifiers are comparable. The 

 amplifier using 6AC7 tubes requires 31.3 watts of power, while the 6AK5 



■jrr 



Fig. 4—60 Megac3xle IF amplifiers {M full size). 



amplifier requires 14.4 watts. The power savings possible with the 6AK5 

 tubes are not particularly important from the power economy standpoint, 

 but rather because of the easier heat dissipation problem. In compact 

 equipment such as airborne radar, the problem of keeping the operating 

 temperatures of the various components within safe limits is formidable. 



In the later years of the war the 6AK5 tube became the standard IF 

 amplifier tube for radar systems and because of its superior properties was 

 used for other applications in many other radio equipments. 



2. Amplification' and Bandwidth 



The amplification that can be obtained with a given number of stages, 

 and the useful bandwidth, are closely related. Within certain limits, one 

 can be increased at the expense of the other. In fact, the product of the 

 amplification per stage and the bandwidth is one important measure of the 

 goodness of a particular tube and circuit design. A simple case will illustrate 

 how this comes about. Assume the band-pass interstage shown in Fig. 5. 

 L is the inductance of the coil, R is the shunt resistance equivalent to the 



