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In Reply to: RE: Another question about the cathode bypass capacitor posted by Lew on February 20, 2014 at 06:53:11
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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Enlightened Evolution-Astral Projection
Follow Ups:
what you say here about protecting the output tubes from subsonic input. And both you and Chris have corrected my assumption that the DC current remains relatively stable whereas there is lots of variation in AC current under dynamic conditions. Apparently that is not the case. So I've learned something from you guys.
If you aim for a cutoff of 20Hz, you will introduce phase shift up to about 200Hz.
That is why we set our cutoff frequency at about 2Hz, so there is no phase shift at 20Hz.
If you have subsonic input (just IMO mind you) its not the job of the amp to process that out, rather that of the source to the amplifier to see to it that its not there.
(Of course we go for wide bandwidth in our preamps too, so that puts the onus on the actual sources, analog or digital.)
I mentioned that in a post below, and I calculated that for a 30-ohm cathode resistor (per cathode of a 7241), I would need about 3000uF to bypass audio with -3db at 2Hz. (With 3 cathodes in a 7241, that would give me a net of 10 ohms to ground.) "3000uF" was calculated "in my head". When I later did it with an actual calculator, I get ~2600uF. But since the voltage is very low across the resistor, one can get away with very high quality electrolytics as used in filament supplies (16V ought to do it at practicable values of R). Then bypass the lytic with a good film cap, which can also be small in size because low in voltage.
Having said all that, I am not at all certain that I will try it.
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