64 BELL SYSTEM TECHNICAL JOURNAL 



to the output. Mechanical feed-back can also occur in the case of 

 tubes whose parts can easily be set into vibration and a cure is usually 

 found in some form of vibration-proof mounting. The coupling 

 which is responsible for feed-back may be difficult to locate, but when 

 found can usually be removed. Both retard coils and transformers 

 may afford an easy method of coupling due to stray fields. If the 

 coupling induces voltages which are in phase with the input voltages, 

 it may cause singing, and if out of phase, the amplification may be 

 seriously reduced. Closed core coils and magnetic shields are the 

 usual remedies for this condition, although a rearrangement of the 

 circuit parts may be necessary. 



Certain kinds of electrostatic feed-back may be removed by en- 

 closing each stage in a separate grounded metal cage or box. The 

 electrostatic coupling due to tube capacities (Sec. 13) cannot be 

 eliminated but it is possible to so design circuits that trouble from 

 this source will not present itself. Thus an inductive impedance in 

 the output circuit may prove troublesome because it induces a negative 

 resistance back in the input circuit; a non-inductive output can never 

 do this. Feed-back through tubes increases with frequency, and in 

 the case of high frequencies, it may sometimes be necessary to use 

 resistance coupled rather than reactance coupled circuits. 



32. Blocking. Two entirely different types of blocking may occur 

 in an amplifier. They both result from the grid of one or more tubes 

 having been carried to a positive potential by the input voltage. 

 While positive, the grid picks up a negative charge of electrons which 

 is removed more or less rapidly by the grid leak. In case the leak 

 resistance is high, a residual charge may remain upon the grid for an 

 appreciable length of time, depressing its mean potential to so low 

 a value that the output of the tube is cut to zero or very nearly zero. 

 The remedy is obviously to reduce the input voltage or to increase 

 the voltage of the negative grid battery. In certain cases, a read- 

 justment of the resistance of the grid leak may be desirable. 



The second type of blocking involves secondary emission from the 

 grid as discussed in Sec. 9. It can occur only when the input is 

 sufficient to force the grid potential of some tube positive by as much 

 as 10 or 15 volts, and then if the grid leak resistance is large enough, 

 secondary emission will hold the grid at about this positive potential 

 and entirely prevent proper functioning of the amplifier. In elimi- 

 nating this type of blocking, the first step should be to note the effect 

 of increasing the filament currents as secondary emission is less likely 

 to occur when the filament yields a copious supply of electrons. If 

 this does not remove the trouble, the negative grid batteries in the 



