The self bias on the 1H4 is not 50mV but it should be -600mV

This is because the free grid potential was measured connecting the 1GΩ voltmeter between negative filament and grid.


Along the grid there are all the potentials between 0 and 1300mV, the voltage of the heater battery.


So the average potemtial of the cathode is 1300/2mV or 650mV.


The potemtial difference between 50mV and 650mV is -600mV.


Connecting the 9V does increase the grid bias.


Between negative filament and grid when no plate voltage is applied the pd is -300mV. Ie the grid bias should be -950mV.


When the potential at the plate is made 9V the bias should become -600mV.


It has to verified by measuring the grid potential with respect to  the positive filament and taking the average.