Since {TAIL ~ the bias voltage} is derived from the cathode to ground voltage it will always track or remain proportional to the cathode voltage so {HEAD ~ the effect} of variations in the latter on focusing will be negated 
Since { TAIL ~ the bias voltage } is derived from the cathode to ground voltage it will always track or remain proportional to the cathode voltage so { HEAD ~ the effect } of variations in the latter on focusing will be negated
OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH