The best way to assure the amplifier is not working/oscillating is to remove the 6J7G. That would isolate the thyratron, that should not fire without signal from the amplifier.
I wonder if the 2A3G is bad... suppose you donīt have another to test it.
Donīt worry to measure the grid voltage. Remember that the thyratron starts conducting if the instantaneous relationship between the grid and anode voltages falls on the "conducting zone". And the grid instanteneous voltage is the sum of the RF coming from the amplifier, superimposed on the AC coming thru the 4000 ohm resistor, the filament xfrmr secondary and the AC line. The plate voltage comes thru the relay coils from the opposite end of the AC line. (See the schematic on
http://philcorepairbench.com/mystery/operation.htm) The 2A3G does not uses a filtered B+ supply, nor its grid voltage is from a steady DC level.
Is there a possibility that sometime in the past the two cables between the filament transformer and the power line were swapped, or maybe the filament transformer replaced? This would invert the phase relationship between plate and grid voltages, and so the thyratron circuit would not work even with a good tube.
I canīt say that the connections were reversed in the past without looking at the chassis, but maybe you can make the following test.
Lets say that the upper power line (the one with the On-Off Sw) is "A", and the lower one is "B". Could you unsolder JUST the two wires to the primary of the filament transformer and reverse them between "A" and "B"? (but keep undisturbed the conductor from the "A" line to the tap on the xfrmr secondary, and the one between the "B" line to one end of the 150 ohm R and the relay coil) If after that the thyratron does not fire, you can leave the cables reversed and put back the 6J7G to check if the amplifier works.