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In Reply to: Re: just a thought... posted by Try-ode on March 17, 2007 at 18:55:33:
"If one takes the U-L as applying NFB to a pentode, the arguement that a triode is a pentode with NFB becomes a whole lot more believable."Good point!
Follow Ups:
What's going on with the usual NFB idea is the output is fed back to *THE* input. In the pentode, the output is fed back to *AN* input. In the triode/pentode case the pentode( taken as a beam tube with beam plates attached to the cathode for simplification ) degenerates from a 4 element device to a 3 element device when the g2 reaches the anode.
Not sure I fully understand your point of "the" vs "an". In a typical global feedback scheme, the signal is returned to the input stage but not the actual input "port" e.g. The input signal is applied to g1 of input amplifier but FB signal is applied to the cathode of same amplifier. Really not conceptually unlike UL is it? The only common topology that comes immediately to my mind where "THE" output is fed back to "THE" input would be a "plate follower".
The cathode or grid input has the same effect. The same resistor that is being used in the cathode circuit to develop the voltage could be moved to the grid circuit. I was considerng it to be the same...perhaps a bit of a simplification on my part.
scratch some of that. writing w/o morning stimulant dose.but still, moving the cathode is just the same as moving the grid. In the circuit it is a whole lot easier to move the cathode( to effect grid-cathode ) than to try changing the grid-ground relationship.
Changing the voltage at the cathode not only changes Vgk, but also Vak. It might not mean much, especially in the case of a tube with high plate resistance, but something with very low ra and mu? I dunno, the cathode (even ignoring input impedance) is not *exactly* the same input as the grid and won't have *exactly* the same distortion characteristics. The difference might be genuinely negligible compared with all the other realities involved, but...
You have it right. I do think it's a small thing. As implemented with a pentode, Va-k change is trivial. When it's done with a small resistor underneath( between ground and the cathode bias and bypass ), I think I can ignore the cathode's input Z. For a triode at the front of the amp, the FB signal might amount to a few % of Va-k. I agree with your assesment that this is also likely a trivial effect. It is a good thing to realize when you're 'neglecting air resistance', yes?
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