Overview
The effort spent on setting and maintaining a tube's correct idle current yields sonic and reliability benefits for a tube amplifier. If the amplifier in use is a push-pull design, the current balance between tubes is critical, as an unbalanced DC current results in less power and increased distortion due to the output transformer saturating. And if the amplifier in use is a single-ended design, the total idle current can be just as critical, as an excess of current can also result in the saturating of the output transformer. Furthermore, as the efficiency is so poor, an SE amplifier output tube must by necessity be run at a high wattage dissipation, so any additional current flow is potentially damaging the output tube. 

Usually, setting the idle current for a tube involves determining the correct cathode resistor value or adjusting the DC voltage on the grid with a potentiometer. Each method has its disadvantages: large valued cathode resistors wastefully dissipate heat and require a large bypass capacitors; potentiometers invite twisting by those who shouldn't and, occasionally, potentiometers lose scraper contact, which can result in tube meltdown. Both suffer from the inability to automatically handle tube idle current drift that

occurs over the life of the tube and indeed can occur over any twenty minutes of use.
The cathode resistor usually does a better job of maintaining the desired bias current in the absence of user attention. However, when user attention is available, the potentiometer allows for the precise re-adjustment of the current.


Design Goals
Auto-bias means never having to worry about the correct current adjustment for an amplifier. As the output tubes age or are replaced with fresh new tubes, the bias current will remain constant. The main disadvantage to auto-biasing is that it can only be used when the amplifier is run in a strict Class A mode. (Of course many see running an amplifier in strict Class A mode as being a major advantage.) If the amplifier is run in Class AB or B mode, this auto bias circuit will strive to cutoff what it senses as being excess current flow,  i.e. any current flow beyond the specified idle amount. Advertising departments have blurred the definition of what constitutes Class A operation.  The test is not:  do the tubes dissipate more watts of heat at idle than they put out into the load?  Nor is it:  does the amplifier draw a huge amount of power from the wall?  The test is: do any of the output devices ever completely stop conducting current during any part of the output signal? If any do, the amplifier is not being run in Class A. While tubes are inefficient, and while Class A is the least efficient class of  operation,  inefficiency does not necessarily equal Class A. In a Class A amplifier that is SE and transformer loaded or push-pull in design, the idle current and the average of the current flow at peak output are the same. The key word in the previous sentence was "average." the output tube in a Class A amplifier will travel from doubling of the idle current down to nearly turning off altogether, but the average of this current swing will equal the idle value. Now in a Class AB amplifier the current might be as little as 30 mA at idle, and as great as 300 mA at full output.
   
Cathode Voltage Auto-Biasing
A cathode resistor is a simple form of auto-biasing, as it smoothes the variation of idle current with mismatched tubes. In other words, this form of biasing tightens the spread of idle current variation between tubes of the same type, whereas the fixed bias method of setting the grid voltage to a fixed