So, what does a phase inverter do? Don't get too technical, you know how that confuses me!
Just think of it as a component that splits the signal from the preamp into two, sending one half of it to
one pair of output tubes and one half to the other pair, making the output stage more efficient.
It´s also known as the driver tube; it just a component that drives the output stage.
It´s not a correct way of explaining what actually happens to the signal, but I´m sure
you can find out more about push-pull, phase inverters and so on in other threads here.
It usually gets pretty boring when technicians try to explain things, the short short version
is rarely used!