The resistors on your pilot are your artificial center tap. You could also use a balance pot. With a balance pot you could find the lowest possible hum. On some amps I have worked on, the balance pots were all the way to one side which is equivalent to grounding one side. On the 5E3 schematic one side of the filament is grounded. That is not always a bad thing if that is your point of lowest hum. But remember just because a schematic shows one side being grounded, it doesnt mean you can ground either side with the same results. If grounding one side lowers hum then grounding the other will increase it. If you do use the artificial center tap method make sure you use 100 ohm resistors, not 100k. Many people make this mistake.