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Michael Koster explains it this way

" The Zout of the mu-output is a function of the gfs of the MOSFET you are using, the value of the MOSFET source resistor (Rset), and the internal plate resistance of the tube. For a "large" value source resistor the value approaches that of a simple source follower of 1/gfs and is less dependent on the tube anode resistance. For finite small Rset values, the anode current is a fraction of output current defined by 1/(gfs * Rset). The Zout thus also includes Rp / (gfs * Rset) in series.

The gfs increases, therefore Zout decreases, with increasing current. The plate resistance also decreases with increasing current, resulting also in decreased Zout at higher current. Unfortunately, higher current means a smaller value Rset, which tends to increase Zout counteracting some of the improvement due to higher gfs and lower Rp. "

"Anything you do to increase the resistance between bottom and top devices will decrease the output impedance at the "follower" output."

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"Still Working the Problem"

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