1112 THE BELL SYSTEM TECHNICAL JOURNAL, OCTOBER 1951 



with these tubes than that which will cause singing. With this situation, 

 it can be seen that any substantial factor of increase in the excess phase 

 introduced by the tubes, or any other components, could begin to reduce 

 the phase margin seriously. 



2.4 Noise 



Fluctuation noise is an important factor in the W.E. 435 A used in the 

 first stage of the input amplifier and in the W.E. 436A used in the first stage 

 of the output amplifier. There is adequate margin against the effect of low 

 frequency noise components such as microphonics, power frequency hum, 

 and "sputter noise" if reasonable precautions in tube and circuit design 

 are taken. From a design standpoint, the fluctuation noise is min"mized by 

 adopting a combination of cathode temperature and current density drawn 

 such that, with a normally active cathode, the space current is substan- 

 tially space charge limited, with ample margin for some loss of cathode 

 activity in service before the temperature limited condition is approached. 

 When the temperature limited region is reached, the noise is substantially 

 higher than for the space charge limited condition. The temperature and 

 the cathode current density ratings for these tubes have been set at values 

 which take these considerations into account. 



2.5 Modulation 



Since a major purpose of using feedback is to reduce the modulation 

 products arising in the amplifiers, the more nearly an ideal linear transfer 

 characteristic can be approached in the tube design the better, because 

 less feedback is required to obtain a given grade of system performance. 

 Unfortunately, however, a conventional triode or tetrode type of vacuum 

 tube operating under normal space charge limited conditions necessarily 

 has a transfer characteristic which is non-linear. Several possible special 

 structures which might give less modulation were explored, but none were 

 found which would provide the required figure of merit and be sufficiently 

 stable and reproducible. 



Considerable emphasis was placed on the problem of controlling the 

 variation in modulation from tube to tube. The most important factors are 

 grid-cathode spacing, uniformity of grid pitch, and cathode activity. Al- 

 though these factors must be well controlled for other reasons also, the 

 special requirements on modulation necessitated a thorough investigation. 



The effect of the grid-cathode spacing can be expressed in terms of the 

 d-c. plate current and the signal level. For a triode having an idealized 



dii 

 three-halves-power transfer characteristic with ttt" = as in Section 2.2, 



