Okay, so I see lots and lots of threads about people wanting power tube distortion at lower volumes. I also see a lot of amps with a pre- and post-gain control. You use the pre to control the amount of distortion essentially, and then the post to control the volume going to the power amp. Then you also have a master. Why can't you just have a pre-master and post-master to crank the power tubes, but then turn down the volume?
Okay, so I see lots and lots of threads about people wanting power tube distortion at lower volumes. I also see a lot of amps with a pre- and post-gain control. You use the pre to control the amount of distortion essentially, and then the post to control the volume going to the power amp.
pretty much yeah. you strategically place a knob in the signal chain to control the amount of signal going to particular gain stages.
Then you also have a master. Why can't you just have a pre-master and post-master to crank the power tubes, but then turn down the volume?
a master is usually placed before the power amp (in class A/B amps this would be before or after the phase inverter, i think many makers are putting it after the PI now-a-days).
now your question is "Why not just but another knob/pot after the power amp?"
because things aren't that simple. i am no EE, so i don't have a full grasp on the exact problems but i am sure the amplitude/voltage of the signal after the signal goes through the power amp has much to do with it.
lets look at three ways people do implement 'post master' volume control:
1) power scaling - actually decreasing values of the electricity in the power tubes (i wanna say B+ voltages for example). this decrease in electricity to the tubes makes the tubes produce less output
2) attenuation - the tubes are still driven at spec ratings, but a 'dummy load' (think of it as a fake speaker that doesn't make noise and turns the amp's output into heat energy instead) is inserted between the power amp and the speaker. basically there is a knob or settings that shunt a certain amount of power that would normally go to the speaker to the dummy load instead.
3) speaker efficiency - this would run the tubes full out, and doesn't shunt any signal. there are some speaker makers (eminence comes to mind) that have a mechanism in their speakers the increases the distance of the permanent magnet from the voice coil, making the speaker less efficient/quieter.
all of these techniques have tonal consequences. they all work fairly well when used in moderation, but if you lower the volume a lot then they usually aren't considered 'a preferred tone'. usually they get muddy, less defined, brittle, something along those lines.
so if you want a little less volume, these techniques work OK, but most people want A LOT less volume from these techniques and tone suffers.
I'm not an expert either, but I'm with Gumbi; the signal after the power amp has been ramped up to power the speaker so bleeding it off with a potentiometer (which is what a master volume does) would probably cause problems.
A master volume after the power amp is called an attenuator. It is a big beasty designed to withstand the full power of the output. You can't just use a pot, it would fry.
So basically what's being said is, after the level gets that high, a simple pot can't handle the power coming through it.
Also, people that claim they want Power Tube saturation are think of a tone in their head that is made up of much more than just PT Distortion. There's much more in the equation of "that" tone than just the Power Tubes. Check out this to get the full equation: