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BELL SYSTEM TECHNICAL JOURNAL 



^ de 6dt^ 



To compute fourth-order effects a tangent is drawn to the curve in the 

 middle of the range Ae (Fig. 3). It will represent a parabolic pentode over 

 the range Ac and Ag. The departure D of the actual curve from the para- 

 bolic pentode at the extremes of the range Ae is measured. The g-e curve 

 becomes 



Ae (Ae)3 



TYPE 6AC7 

 PLATE VOLTS 150 

 SCREEN VOLTS 150 



GRID VOLTAGE, e 



Fig. 3 — g-e characteristic exhibiting fourth order modulation. 



As a rule D is negative. It decreases the slope of the curve. The plate 

 current is 



. , ^ , 1 Ag 2 , 2D 4 



2 Ae (Ae)3 



From this expression the fourth-order effects may be computed. 



(5) 



Single Frequency Input 



Fourth-order modulation has no effect on the gain of the amplifier. It 

 affects the second-order products, the rectified current and the second har- 

 monic and adds the fourth harmonic. 



In an amplifier it is unimportant to correct the value of Ih for fourth- 

 order modulation. The fourth harmonic is given by 



H^ = 20 log ^ -1- 60 log y + 12. 



