ELECTRON TUBES FOR A COAXIAL SYSTEM 1113 



and for small signals, the ratio of the fundamental signal current to the 

 second harmonic component for the case of a very small load impedance is 



7^ = 12 — (see appendix for derivation) (11) 



i2p Ip 



This means that, for a given signal current amplitude Ip in the output, a 

 tube having the assumed characteristics will give a ratio which depends 

 only on the d-c. plate current, which in turn is very sensitive to changes in 

 grid-cathode spacing. 



A study of the variations in grid pitch and their effects on modulation 

 in a particular experiment showed that reducing the standard deviation 

 of the pitch distance from 16% to about 7% reduced the second-order 

 modulation by 4 db. Since the second-order modulation must be reduced 

 by feedback which is at a premium in the L3 system, this experiment 

 showed that control of the grid pitch was important, and that periodic 

 checks on this factor would be desirable in manufacture. 



The effect of cathode activity on modulation was studied in diodes so 

 as to eliminate the effects of grid variations. The variations in modulation 

 from tube to tube were found to be about the same as when grids were 

 present. The geometry of the diodes was so closely controlled that dimen- 

 sional variations could not account for the differences in the modulation 

 levels. This part of the investigation led to a recognition of the importance 

 of obtaining the best possible uniformity of cathode activity. It also became 

 apparent that the surface condition of the anode was a factor, and that it 

 is therefore desirable to maintain a high degree of cleanliness of the elec- 

 trodes to which positive potentials are applied. 



2.6 Life 



Long tube life is a very important requirement in the L3 system. The 

 most important consideration is the effect of the life on the reliability of 

 the system. There is also the obvious effect of the life on maintenance costs. 



Short life tends to reduce the reliability of a system which contains a 

 great number of tubes because the potential failures cannot be predicted so 

 accurately as when the life is long, without a prohibitively costly amount 

 of testing. , Even with the most frequent and accurate testing procedure 

 which might be considered, it would be amazing if more than 90% of the 

 potential failures were replaced before causing transmission trouble. To 

 illustrate the effect of short life, consider a 100-mile section of L3 line. 

 There will be five tubes in each of 24 amplifiers. If the performance of any 

 one of the total of 120 tubes becomes poor enough to make the circuit un- 

 commercial, that section must be taken out of service until the defective 



