I thought that it might be the 5y3gt tube, so I switched it with another one, and nothing happened. I left the black wires connected to power, then removed the green, yellow, and red wires from the board and measured them. They all read about 13 volts. I then removed the transformer from the chasis. There was a yellow stain below the transformer on the chasis, and it smells like burnt electronics, which can't be good. It seems the most likely culprit. Especially because I didn't have the fuse wired in at the time to protect it. It's expensive, but I guess I'll have to order a new one.