[Splattle101]

Well, not quite.

The first valve is the preamp valve. The clean 'channel' uses only the first half of it (remeber, these 12A?7 valves are all TWO valves in a single bottle). Then one half of the second valve drives the tone stack and adds a little more gain.

The dirty channel uses both sides of the first valve, thus adding a lot more gain.

The third valve is the phase inverter, which is really not part of the preamp at all. It splits the signal into two out of phase halves, and sends one half to each side of the push-pull power section. It may also add a tiny amount of gain, depending on the design.