JessJackson said:
Mate don't get ahead of yourself... I said the c12 is fixed bias and you said its self biased, therefore I'm discredited from the technical discussion?... I'll allow my friend the late Oliver Orchut put you straight.
( http://repforums.prosoundweb.com/index.php?topic=34443.msg508369#msg508369 )
I encourage you to fully read your quotes before putting them forth as evidence of absence.
In the above Oliver doesn't even mention any bias terms, only that the negative voltage is delivered from the PSU to the grid via a resistor. Nobody is debating that: there is much more to biasing than simple circuit topology.
And since he's not here to participate in the discussion (and never can), I won't ponder what else he might have meant.
JessJackson said:
My friend Klaus Heine also refers to the c12 as being fixed bias.
( http://prorecordingworkshop.lefora.com/reply/46982651/AKG-C12-Questions#reply-46982651 )
Indeed he does use that term: perhaps he can come here to this thread to debate this himself? Or do you speak for him?
What do
you think is correct, and
why?
JessJackson said:
and Mr Bock can further explain to you why its refereed to as fixed bias
( http://prorecordingworkshop.lefora.com/reply/46982702/AKG-C12-Questions#reply-46982702 )
A strange thing to quote: if I understand David correctly, he is arguing that "it must be fixed bias, because otherwise why would AKG put in an extra connector on the PSU, and use an extra 30M resistor otherwise?" David hangs around here occasionally maybe he can clarify what he meant.
So, again, do you have anything substantive yourself to add to this discussion?
In case it helps readers of this thread (and to try desperately to extract something meaningful from this discussion), I'll again put forth
my definition,
my reasoning, in
my words:
Fixed bias is any biasing method where the bias voltage is
independent of a) collector current (for a transistor), b) drain/source current (for a FET), or c) plate current (for a tube).
Self bias is any biasing method where the bias voltage is
dependent on the same factors.
I don't think my definition is out of line from any of the common texts on the subject: from the 1975 RCA Receiving tube manual, page 84 (emphasis mine):
Grid voltage may be obtained from a fixed source such as a separate C-battery or a tap on the voltage divider of the high-voltage supply, from the voltage drop across a resistor in the cathode circuit, or from the voltage drop across a resistor in the grid circuit. The first method is called "fixed bias"; the second is called "cathode bias" or "self bias";
Similar wording appears in the 1930 Radiotron Designers Handbook (probably on someone's table at AKG when the C12 was designed):
It is sometimes desired to operate the valve with fixed bias, either from a separate bias supply (battery or rectifier/filter combination) or from a voltage divider across the plate supply.
In either case, external batteries or separate supplies cannot be modulated by the current of the active elements (unless the supplies are poorly designed).
Further, from Tom Wheeler's book The Soul of Tone, page 90 (emphasis mine):
Fixed Bias: Grid voltage obtained from a fixed source, such as a tap on the voltage divider of the power supply. By far the most common method of biasing power tubes, fixed biasing entails an independent supply of negative voltage connected to the tube's grid. It sets, or "fixes", the control voltage at a consistent level.
There is an absurdly easy test that follows from these definitions: pop in a different tube that runs at a different quiescent current point, and see what happens to the bias voltage. If it changes based on the tube, it isn't fixed bias.
In the C12, tube return current sets the bias, which flows across R3 (R3 in my schematic, R8 is AKG's original schematic): the higher the plate current, the more drop there is across R3, which means the more negative the bias becomes, which means plate current tries to decrease. It's is the exact same negative feedback mechanism as an un-bypassed cathode resistor, and it if you asked the tube what biasing conditions
it sees, it wouldn't be able to tell the difference between the C12 method and an unbypassed cathode resistor method. Hence from my reasoning, the C12 isn't fixed bias.
Do the exact same test in a Fender AB763 "Super Reverb" amp (or one of it's many derivatives): set the bias for -40V, then start subbing in different output tubes. The -40V never changes (provided those tubes aren't driven into positive grid conduction, at which point all bets are off), regardless of the actual tube in the socket. Hence, it is fixed bias.
Jess, do you have anything to refute any of this? Or can I expect some more "proof" in the form of JPG's of album covers?
I'll say it again: I get it! You like your original C12! I hope you continue to do great things with it.
