Thanks Wayne; It was the term "deep into class A" that I wasn't sure of. I am aware of operating class distinctions. What throws me is how we distinguish the borders of conduction limits that set the classification of operating points. Did that make sense? I am trying to understand how the bias point of a tube dictates the resulting circuit parameters and how that impacts on the Z of the circuit. I am struggling with the way that these parameters affect the various AC impedance values and why that equals different effects on the loudspeaker. I have a circuit built on PC boards that allows you to use fixed or cathode(self) bias. I need to know why I am making a choice of bias schemes. So I can choose wisely.