.. AC current is transformed down to 88.4 uA RMS. I measured about 64 mV on the secondary. From these two values one can calculate the load represented by the cathode input impedance. It gives 724 ohms. Since I connected 1.5 k parallel with the cathodes, the true cathode input impedance is 1.4 kohm.
Well, I think I was ambiguous. The input impedance of a SRPP cathode (upper or lower) alone would be 1.4k each, the resulting input impedance including the parallel load resistor is 724 ohms.
