Thanks for your presentation. If you look at the circuit closely you will see that the two heater/thermistor pairs are connected back to back in the feedback loop of the op-amp - this is not for improving sensitivity as you state. This connection accomplishes the AC to DC conversion and nulls out any common mode errors due to ambient temperature changes etc. Jim Williams was a big fan of this instrument, its elegant operating principle was used in an IC he had a hand in designing.
Nice explanation. Just a note, later versions of the HP 3400A did use a JFET in the Impedance Converter stage and not the Nuvistor. Nuvistor A2 assembly P/N 03400-66502 and later JFET A2 Assembly P/N 03400-66514.
These are great old instruments, I also have one of these meters, it is mostly used to measure power supply rail noise and receiver sensitivity testing. Mine also uses a Nuvisor.
I don’t have the actual paper copy of the manual for this instrument, which would have the schematic. The block diagram came from the Keysight website, but their scanned image of the schematic was of very poor quality, and I didn’t think it would be helpful.
