SQUARE WAVE OSCILLATORS 



necessary. The question that then arises is to what extent the boundaries 

 of the working region may be transgressed. For example, under what cir- 

 cumstance is a 'conducting hard' point hke Figure 15.6 permissible, where 

 both the anode dissipation and the maximum cathode current are being 

 exceeded ? 



Conducting 

 hard 



Cut-off 



Figure 15.5 



The answer is that the anode dissipation may be instantaneously exceeded, 

 provided it is not exceeded on the average, the average being taken over a 

 period short compared with the thermal time-constant of the anode structure. 

 The maximum cathode current rating can apparently also be exceeded 

 instantaneously provided it is observed on the average. What is happening 



HT+ 



Figure 15.6 



here is that the space charge is being depleted to supply the anode current, a 

 practice which should be indulged in moderately; it should be remembered 

 that the space charge gives the cathode a measure of protection from ionic 

 bombardment should the valve possess some residual gas. It is probably 

 wise to consult the valve makers in cases where appreciable instantaneous 

 excess cathode currents are contemplated. 



228 



