[hipofutura]

Basically your tube amp has two section - the preamp and the power amp. The preamp is the section that contours the signal from the guitar. It's the volume and tone controls. The preamp has its own set of tubes. The power amp is the section that takes the signal from the preamp and amplifies it. The power amp has it's own tubes. When you look in a tube amp you'll see small tubes which are for the preamp and you'll see large tubes which are for the power amp.

The master gain is an audio taper pot (volume control) that sits between the preamp and the power amp. With it you can crank up the regular volume knob (preamp) and then use the master gain to crank down the signal as it leaves the preamp and goes into the power amp for amplification. By turning the volume up to "10" you saturate the preamp tubes which creates the distortion. When the master gain is turned down the distorted preamp signal is not loud. This makes the amp sound like its running wide open, but it's at a bedroom level. Because the signal going into the power amp is reduced by the master gain you don't get the distortion that comes from the power amp being over driven (power tube saturation).