[Guest]

Thanks for your detailed responses, Ron.
I took some time to do some further measurements, and in the process found a couple of inconsequential wiring discrepancies from the diagram... taking cap #17 to the top end of resistor #21 instead of ground, for instance. Correcting them made no difference.
I double-checked resistors #23 and #34, along with the cathode resistor #21. They're all new, and within 1k of spec.
The B-C resistor, #38 is within a handful of the readings you gave for the model 90.
Using B- as the common, all the cathode voltages are very close, as you suggested they'd be.
The mystery is the detector plate voltage, and the only idea I have is that the voltage at point D might be too high. I get 290 volts there, but it seems the only way the detector plate voltage could be so high, is if it's not drawing enough current across #23 and #24.
Still scratchin' my head...
Nelson