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BELL SYSTEM TECHNICAL JOURNAL 



setup is remarkably stable and satisfactory to operate. Resistance 

 measurements on a given tube at specified operating points can be 

 repeated with a precision of two or three per cent even when weeks 

 elapse between measurements. 



In addition to being a function of frequency, the input resistance of 

 one of these tubes is also a function of all of the operating conditions, 

 that is, applied voltages, plate circuit tuning, and load. In Table II 

 are shown values of this input resistance for a typical tube at several 

 frequencies and over a considerable range of operating conditions. 

 Because of the large number of variables which affect this input resist- 

 ance it is difficult to devise any way of plotting up these data so as to 

 give a comprehensive picture of tube performance. 



The variation of input resistance with plate circuit tuning has, for 

 this design, consistently been of the form illustrated in Fig. 15. How- 



0.93 0.94 0.95 0.96 0.97 



0.98 0.99 1.00 1.01 



TUNING VARIATION 



1.02 1.03 



1.04 1.05 1.06 1.07 1.08 



Fig. 15 — Reaction curve. 



ever, the relations between maximum, minimum, and "in-tune" values 

 vary somewhat with frequency, operating conditions, and plate load. 

 Also, as may be expected, they vary somewhat in different tubes which 

 have been made up with various grid and screen spacing, etc. A con- 

 venient numerical measure of the magnitude of this reaction is ob- 

 tained by taking the difference between the maximum and minimum 

 values at any specified operating point and dividing this difference by 

 the "in-tune" value. This reaction ratio will also be found listed in 

 Table II for various operating conditions. 



Gain Measurements 

 The measurement of the voltage gain of an amplifier stage con- 

 taining one of these tubes is a relatively simple matter. As stated in 

 the description of the circuit, provision is made for connecting a peak 

 voltmeter directly to each tuning condenser plate in both plate and 



