The AC signal voltages on schematics start with an input that is about fifty times smaller than what your humbucker is going to input. You need to look at the gain along the signal path. The good 'ole ab763 is a good example. The gain is about 11 at the grid of V1B, so a 200mV signal from your humbucker will put 2.2V on that grid. The gain at the grid of V4B is around 42, so you will have 8.4V on that grid. The Hot Rod DeLuxe has a gain of 145 at the grid of V2B, so a 200mV input will put 29V on that grid.