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(2a3)+1
Yes, 2A4G. My mistake.
While waiting for the new (NOS) tube, lets keep thinking. The thyratron grid needs a substantial negative voltage to keep the tube from conducting. Returning it to the cathode would not cut off the tube, because you can see in the curve that at 0 grid volts, the tube conducts with about 20 plate volts. Thatīs why the lower end of the grid coil goes thru the 51K/0,5MFD and the 4K res to one end of the sec of the filament xfmer.
The reactance of the 0,5MFD at 60 Hz is about 5K, so the total impedance between the grid and the filament transformer is less than 9Kohms.
The schematic does not show where the filament voltage of the amplifier tubes comes from, so I will assume that from the main receiver, that also supplies +B and -C.
So, you have part of the filament transformerīsecondary supplying 2,5V to the 2A4G filament, and the other part of the secondary, that is longer in the drawing (but I canīt know its voltage), goes to the grid circuit. This is the voltage that keeps the 2A4G from firing, but it has to be in correct phase relationship with the plate voltage (that comes from the AC line at the points I arbitrarily called "A" and "B" in a earlier post).
For the circuit to work well, at the same instant the plate voltage is rising, the grid must be receiving a rising negative voltage from the transformer. So the tube wonīt fire. Only the RF peaks from the amplifier, superimposed on the negative grid voltage, will make it less negative so the tube may fire.
That is why I was wondering about an hypothetical inversion of the conductors that would supply the wrong (positive) half-cycle to the grid while the plate was also being made positive.
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