thanks PRR! makes sense now. my electronics class was before pentode tubes came out,
Rp is just that delta V vs delta I along the flat part of the tube graph, kind of like a Collector graph for a transistor,
for max pwr, it looks like we should pick a voltage in the middle of what the tube can handle to get max swing, find the bias voltage along the max pwr line, strike a tangent which will be our loadline, the slope representing out transformer Z, this tangent slope will go up as we use lower B+ and go down for higher B+, xfmr Z will go down for low volts, and up for hi volts, ok then,
we want to go vertical and change dynamic plate resistance by changing bias, 38K Rp is just what happens to current when we change supply voltage, don't really care about that too much,
have to use a tangent so we do not cross over max pwr line,
oh wow, Inside the Vacuum Tube, 1945, should have read this first>
https://ia600702.us.archive.org/30/items/InsideTheVacuumTube/InsideTheVacuumTube.pdf