Since 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 {TAIL ~ variations} in the latter on focusing will be negated 
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Since 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 { TAIL ~ variations } in the latter on focusing will be negated
OTH OTH OTH OTH 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 REL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH