Question about OPT impedance in RDH4

[jazbo8]

I was questioning the way RDH4 drew the composite characteristic, why don't they want to follow the straightest part in Class A region instead of the two points where the amp cross into Class B. That's why I want to confirm with you about that. Is the slope of the line suppose to be the plate resistance of the "composite tube" which is about 1/2 of the plate resistance of the individual tube?

The composite characteristic lines may appear to be straight on the page, but they aren't really perfectly straight if you can really zoom in on them. Drawing a line with a slope of 1/2 ra seems like an easier way for this rather tedious exercise. Fig. 13.32 does provide a better graphical representation, where you can actually see a bit of curvature pivoted around point "B". For the triodes, the top half of the curve bends upward, and for the pentodes, it bends downward, which BTW, was what Kiebert tried to describe in his paper, I think...