Shunt regulators offer many sonic advantages over series regulators; they usually find a shorter and cleaner path to the ground reference, and they absorb noise rather than transmit it, but at the cost of decreased efficiency. Inefficiency, however, is something with which tube audio designers have learned to accept and some believe to be virtue: "the less efficient, the better the sound."
The principle of operation of a shunt regulator in AC terms is simple enough: the noise signal on the power supply rail is compared to ground, then the current through the shunt regulator is varied in response to that signal, and then this current develops a varying voltage across a resistor or choke that cancels this same signal. In DC terms: the power supply rail voltage is compared to a voltage reference and the idle current through the shunt regulator is varied to bring the power supply voltage in line with the reference.
Like the series regulator, the shunt regulator needs three elements to work: a shunting device with a variable conductance, a voltage reference that is AC bypassed to ground, and a feedback mechanism to control the flow of current through the shunt regulator. Amazingly enough, the triode comprises all three elements at once. The triode's current can be varied by varying grid-to-cathode voltage, which is why the triode is called a "valve" in England. The same grid-to-cathode voltage creates a soft voltage reference, as the triode's plate will buck or fight any change in plate voltage. This occurs because a triode, unlike a pentode or a transistor or MOSFET, exhibits plate resistance (rp). For example, a 4k resistor attached to a power supply of 400 volts will experience a current flow of 100 mA. Moving the power supply voltage up and down will increase and decrease the current flow through the resistor, as
DI = DV / R.
If we replace the resistor with a triode with the correct grid-to-cathode voltage, the same current would flow as did with the resistor. But here is the important difference: if we move the power supply voltage up and down, current will increase and decrease much more severely than it did through the resistor, because the rp of the triode might only 400 ohms,
DI = DV / rp.
Before going on to the third element, we can see that a triode gives us all we need to make a simple shunt regulator.
These two examples of a simple shunt regulator show just