Step 2.
The amplifier is now powered down and the load resistance RL is
measured across the input terminals of the resonant pi-network tank
circuit (from plate to ground) with an HP-4815 Vector Impedance Meter.
The resistance is found to be approximately 1400 ohms. Because the
amplifier was adjusted to deliver the maximum available power of 100
watts prior to the resistance measurement, resistance RLP looking into
the plate (upstream from the network terminals) is also approximately
1400 ohms. Accordingly, a non-reactive 1400-ohm resistor is now
connected across the input terminals of the pi-network tank circuit
and source resistance ROS is measured looking rearward into the output
terminals of the network. Resistance ROS was found to be 50 ohms.

Points in response:
1.This is actually two steps: one measurement made from the plate
looking towards the load; one measurement made from the output jack
looking towards plate.
2.The measurement at the plate without a powered tube unloads the Q of
the plate tuning. Does this unloaded Q present a false reading of the
transformed load resistance?
3.The load line with 800 v / 0.26 a / 2 = 1539 Ohms.
4.“Maximum Available Power” has not been established, only arrived at
by inference. It could be more, it could be less. Being less is
unlikely given the subsequent demonstrated power delivered to the load
being 100W; but it is suspect that exactly 100W is all that could be
obtained. The point, however, is that such attention to “Available
Power” is an unnecessary elaboration to the point of taking these
steps.
5.The measurement at the output towards the plate similarly unloads
the plate tuning Q when the 50 Ohms is removed to make the
measurement. Does this unloaded Q present a false reading of the
transformed plate resistance?
6.The measurements made in both directions confirm the capacity of an
unloaded circuit to perform the transformations in a symmetric fashion
(a good test), but you have not tested if the issue of loaded Q is a
factor or not. If you were to replace the 1400 Ohm resistor at the
plate tuning input (at the plate connection) with a 1000 Ohm resistor,
would the 50 Ohms follow that shift? By how much?

73's
Richard Clark, KB7QHC