From MetroAmp Wiki
The most important part of an amplifier is the power supply. What does it do? It supplies power of course!
Let's first ask ourselves...just what is an amplifier? An amplifier is an adjustable power inverter. A power inverter takes DC and converts it to AC through the use of a signal generator. In this case, the signal generator is your guitar, which creates the frequency of the AC output. The amplifier takes that signal and uses it to modulate the DC power from the power supply and send it to the powered device. In this case, the powered device is the speaker.
If an amplifier could supply 2 amps @ 120VAC at its output and you connected a 60Hz signal generator to the input, you could power a 120VAC appliance rated to consume 240 watts of power. Of course, due to the low impedance output of guitar amplifiers, a 16 ohm speaker pulls 2.5 amps with only a 40 volt drop across the speaker at 100 watts. But the theory is exactly the same.
The power consumed by your speakers ultimately comes from the power supply. In order for the speaker to consume that power, the power has to be there to consume in the first place. In and of itself, an amplifier is just an adjustable AC power inverter (again, a device that converts DC to AC through the use of a signal generator). But it has to get that power from somewhere, and this is the job of the power supply.
This category contains everything you ever wanted to know about tube amp power supplies. Everything from the different rectifier circuits to filtering all the way down to the MetroAmp exclusive dual voltage operation...it's all right here.