I'm probably going to come off like some kind of a thinks-he-knows-everything ass by commenting here, which I'm not, but I do happen to know what Master Volume controls do elecronically and why amps have them. B4 I even start, I'll remind everybody that there's a small grey rectangle somewhere on the right side of the screen that will take you expeditiously to the next post if you find me a little too windy....
Each stage in an amp takes the signal up in level. In every tube amp circuit I've ever studied, the actual gain of each stage is fixed. The output of the stage is varied by a variable resistor at the output that lets you feed anything from 100% to near 0% of the signal on to the next stage. In the olden days when these amps were first designed, the idea in the designers mind was that distortion was a really bad thing. So they would design the gain stages such that each one (typically two sometimes three) had a little more head room than the next so that as the signal built up you were always maximizing the tradeoff between distortion and signal to noise for that stage (that isn't actually technically accurate, but the idea is right). Usually there would be a single volume control after the first or second gain stage so that you could regulate how much of the signal flowed on down the chain and thereby adjust the output volume. Somewhere along the way, somebody figured out that if you cranked that volume all the way up and fed the amp a signal from a hotter than normal pickup, it sounded really cool... but it was really loud!!! So somebody got the idea to let the hot input signal overdrive one particular tube in the chain, but then to put an additional attenuation control after that and before the signal enters the power stage so you could keep the volume in line. You distort the signal by turning it up in the early stage and then regulate the volume by turning this distorted signal down with a second volume control. This extra volume control is what came to be known as the Master Volume. Many of the old amps were configured effectively as mixers, with multiple input stages in parallel that summed together. The master is usually located after the combining stage, exactly like a master fader on a mixing board. A few more modern amps will have multiple masters for more flexibility, but that's really a different thing. As I've talked about in a couple of different threads in the last couple of days, the reason a lot of tube amp snobs aren't keen on master controls is that they do what they do by starving the power tubes of signal. Yes, that brings the volume in check, but it takes away the sound of those tubes being overdriven, which in the most famous amps around is the first thing that happens as it starts to get loud. So, with the master control down, and the gain up, you still get the sound of overdriven tubes, but they're overdriven preamp tubes rather than overdriven power tubes. Which is different from what you got from overdriving early designs back in the day. You also lose the sound of saturating the output transformer, which is also a key part of the sound of overdriven classic amps.
None of this means that overdriven preamp tube are a bad thing or that they sound bad. most of what we think of as "modern" distortion, when it isn't coming from a silicon circuit is coming from overdrive preamp tubes, sometimes several in a row. So, please don't misunderstand me. That is not a bad sound, it is just a different sound from what can be had from overdriving the output stage.
J
volume controls for TWO channels. When I am faced with a MV, I usually use it to turn both channels down equally, rather than to tune my OD sound, as I prefer to get power-tube OD with an attenuator.