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RE: Another question about the cathode bypass capacitor

Lew

DC fixed negative grid bias is not steady during music reproduction , it is modulated with AC signal from driver(s) cathodes , hi constantly change the value depending from form & level of AC signal present on the Amp input , hi have steady state value only in - no input signal- condition . ,
but in the same time across the R/C member connected in catode(s) circuit that small DC voltage (automatic bias or self bias ) value ideally have to be constant .
Only what you need is to determine R/C time constant for lowest freq. that you want to amplify , and usually that is 20 Hz .
My point is actually next : for example , if accidetally input signal is become in subsonic range ( 5Hz or so ) , and is it with relative high amplitude than output power tubes un-necesery sufer relative high internal disipation without any final audible gain , further R/C member durring that sub-sonic ` fly back ` automatically raise up the self bias value causing efect of power tubes self protection regardless to grid subsonic high level input signal .



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