320 



BELL SYSTEM TECHMCAL JOURNAL 



Whether P equals Q or not, the unwanted products are 



= LV) log p^ 



20 log /^ = 20 log ^ 



+ 



20 log /^ = 20 log ^^-^" + 6 



20 log 1^ = 20 log ^ - 6 



20 log /^ = 201ogf -6 



ip_3 -L 



20 log ^' = 0. 



TYPE 6AK5 

 PLATE VOLTS 150 

 SCREEN VOLTS 150 

 GRID-BIAS VOLTS 3.6 



GRID VOLTAGE, 6 

 Fig. 2 — g-e characteristic exhibiting third order modulation. 



Third Order Modul.a.tion 

 When we do take into consideration the fourth term of equation (3), 



that IS - — ^ t)^ the g-e characteristic will no longer be a straight line but a 

 de- 



parabola. It turns out that all of the computations of the second-order 



effects as shown are so little affected that no corrections are necessary. The 



6 56^ 



add new types of modulation products to the output. These are usually 

 objectionable. 



A typical g-e characteristic with third-order modulation present is shown 

 on Fig. 2. The curvature is usually concave upward. Instead of measuring 



the derivative ~ needed for the computations, it is more practical to scale 

 ae'' 



presence of third-order modulation caused by the term - j^ i^ will, however. 



