What is Vacuum Tube Biasing?
Biasing a vacuum tube may seem like black magic and can be confusing. For example, you need to set the bias on some amps, while on others, you don't. You hear about biasing power tubes, but what about preamp tubes? And people talk about biasing hotter and colder and how it affects tone.
In the simplest term, biasing a tube sets the idle current going through the tube. And all tubes need to be biased.
But wait, you never adjust the bias on tubes like 12AX7! You might say. That is because there are two types of biasing.
One is where a tube bias is adjusted automatically. In this type of circuit, a biasing resistor is connected to the cathode, raising the cathode potential from the control grid. The voltage across this cathode resistor is the bias voltage. This type of circuit is called "cathode bias" or "self-bias."
Another way to bias a tube is to supply a constant negative bias voltage to the control grid. Supplying fixed bias voltage (fixed unless you adjust it) is called "fixed bias."
Practically all preamp tube circuits use cathode bias. Tubes like 12AX7 use cathode bias and do not need bias adjustment. Additionally, single-ended power amplifiers are mostly cathode biased also. This is why you do not see bias adjustment on single-ended amps like SET audio amps or Fender Champ.
Cathode bias not only automatically sets bias, but it also works as negative feedback. When more current passes through the tube, the voltage across the cathode resistor increases, reducing the current flowing through the amp. Tubes never go into a thermal runaway with cathode bias.
Cathode bias is automatic and works great, but there is one flaw. By its nature, cathode biasing reduces the gain and output. This is not a problem for small signal circuits where you can bypass the cathode resistor with a capacitor. However, this is not the case with output power tubes. You don't want a biasing scheme that takes power away from the output stage. You want full power from the output tubes, and throwing power away is not a good idea.
Fixed bias, as the name implies, supplies fixed voltage to the control grid of the output powers. With fixed bias, no power is lost to the cathode resistor, and power tubes can realize full power.
The downside of the fixed bias is that it needs adjusting. Biasing can vary for each tube, and the bias current, which is how much plate current passes through for the same bias voltage, can vary quite a bit. A fixed bias circuit has an adjustable bias voltage to address tube variability.
Class A, AB, B
Output tubes in push-pull pair work by alternately conducting current. Bias controls how much current goes through each tube and changes the operation class of the output stage. We will go through classes B, AB, then A.
Class B operation is when you turn down the bias to where the push-pull tubes turn on and off alternately. The tubes are handing off current from one to the other in this class. The handoff is abrupt and creates cross-over distortion. Class B is more efficient than class AB or A, however.
When you increase the bias, you enter class AB operation. The tubes stay conducting a little in class AB instead of turning off completely. There is less cross-over distortion, but less efficient than class B. There are different degrees of class AB operation, class AB1, and AB2. Most tube amps work in class AB.
Class A is where you increase the bias further, so the push-pull pair is always drawing the full current. Class A has no cross-over distortion. On the other hand, class A is the least efficient since you are always running full power, even at idle. This means that a 150W tube amp is always using 150W of power even when it is not making any sound. If you leave a class A amp turned on, it wears out the power tubes whether you play full volume or not. An example of a class A amp is a Harmon Kardon Citation II.
What happens if bias is wrong?
One of three things happens if output tubes aren't biased correctly.
In the first case, bias may not be optimal, but it is in the ballpark, and the amp will work fine. If the biasing isn't extreme for the tubes, this will be the case if you swap power tubes without bias adjustment.
In the second case, if the bias current is very low (cold), the tubes will work closer to class B. The amp won't sound good, but nothing terrible will happen.
Lastly, if the bias is extremely hot for the tubes, the tubes may eventually overheat. When power tubes overheat, it conducts more current, making overheating worse. This vicious loop could eventually blow a fuse. If you are less lucky, power tubes and a resistor or two may burn out.
If you replace the output tubes and don't adjust the bias, you most likely will end up with one of the first two scenarios. There is no guarantee, however, that you don't end up with the last case where the bias is too hot for the tubes and the amp, and something goes up in smoke.
It can be inconvenient to adjust bias, but we recommend that you always re-bias when replacing the output tubes to avoid the last scenario.