A guitar amp has two main parts. The pre amp and the power amp. The pre amp controls the input single from the guitar (volume,and tone functions. this is powered by the12ax7 tubes)).The power amp, the large tubes are driven by the pre amp and are only as loud as the volume in the pre amp.
To get the best rock sound the pre amp section has to be driven hard to get the power tubes to produce overdrive and natural harmonics.
But that is not always the best because it is as loud as the amp can go.
A master volume is a volume control in front of the power amp section. this lets you turn the pre amp up to 7 or 8 and the master down to 4 or 5.
You get the tone and drive of the amp but not the loud volume.Go the opposite for a clean sound.

All this is just to say you will need a overdrive box to get the best rock sound with your Tranor amp.