There are a lot of mysterious components within any guitar amp, many of which remain puzzling even to hobbyists who have built a DIY project or two. In the eleventh part of Mojotone’s series What Does This Thing Do? we’re examining the output-tube bias circuit and its essential function within any amplifier.

Most guitarists who have played tube amps for any length of time will understand that their output tubes need to be biased to function properly, and all contain an output-tube bias circuit of one type or another to get the job done. It’s a lesson you tend to learn well before you decide to build an amp for yourself, and one that’s a necessity to understand the importance of—even if you don’t know how to do it yourself—any time you change tubes. Many amps on the market are cathode biased and don’t require any adjustment when you change your output tubes, but these still have a simple bias circuit (“simple” perhaps being an understatement, since it generally consists of just one large resistor). The rest are usually fixed bias, a system which uses a more involved but still relatively simple circuit to enable the user (or his or her amp tech) to tailor the bias to each individual set of output tubes. 

So, before looking at the circuit that achieves this in any amp and why we need it, what is bias in the first place? Put simply, bias is the means by which the output tubes are set to function optimally according to the DC voltage level that is being supplied to them by the power stage. Inevitable variables in tubes’ manufacturing process mean they operate at slightly different levels of efficiency, performing slightly differently according to the voltage delivered to them by different amplifiers. At the same time, different amplifiers’ power supplies can deliver considerably different voltage levels to the tubes used within them, further necessitating some means of adjusting how the output tubes handle that voltage. 

Reputable tube dealers will usually match these output tubes into pairs and quads that at least operate together at something very close to the same efficiency and performance levels, but the inevitable differences between different new sets of output tubes means that for fixed-bias, Class AB amps it’s impossible to set a one-time operating level that will work optimally with all possible tubes that might be used throughout its lifetime. 

For this reason, most such amps include an adjustable bias circuit that enables the user—or their tech—to dial in each new set of tubes after a swap to ensure they are functioning their best according to the high levels of DC voltage they are seeing. (Some vintage amps were indeed made with a set, non-adjustable bias network that gave a “good enough” performance with different sets of tubes, and the quality of tubes used in the 1950s when these amps proliferated meant they often didn’t vary too widely within acceptable parameters, either. Mesa/Boogie has also traditionally used non-adjustable fixed-bias circuits, preferring users purchase replacement tubes within a specific operating range to suit their amps.)

Fixed-Bias Circuits

The term “fixed bias” can be somewhat confusing, because this is the type of bias circuit that is actually adjustable in most cases. The “fixed” part indicates that the circuit is providing a set negative voltage to the grids of the output tubes, which determines their operating level relative to the much higher DC voltage that powers them and the incoming AC voltage—the guitar signal—which they will amplify. That is, it’s not something you constantly adjust like a volume or tone control; it’s more set-and-forget.

Most bias circuits of this type consist of a couple of resistors, an electrolytic (filter) capacitor or two, an adjustable potentiometer, and—crucially—a single diode (usually a 1N4007 in modern amps). Put in relatively simple terms:

• The resistor at the start of the circuit helps reduce the incoming voltage from the transformer to a level that the bias circuit can handle,

• the diode blocks the positive side of the AC voltage and passes only a negative DC voltage onward, 

• the small electrolytic capacitor smooths problematic ripple from the signal,

• and the potentiometer fine-tunes the voltage to achieve the level at which the tubes will best operate.

Fully understanding how to use this adjustment is a matter for another article. In many amps, it’s also a job that should be assigned to a qualified repairperson if you’re not experienced in working safely with the high voltages that are present within tube amp circuits, even when they are unplugged from the wall outlet. (This article also doesn’t intend to explain all the differences in sound between fixed-biased and cathode-biased circuits, either, which is another installment’s worth of amp exploring.)

Cathode-Bias Circuits

As simple as the fixed-bias circuit above is shown to be, the cathode-bias circuit is considerably simpler still. In essence, it consists of just one component: a large, high-powered resistor connected between the cathodes of the output tubes and ground. As the voltage flows through the tube during normal amplification duties, the way in which it is drawn through this resistor to ground via the cathode determines its bias voltage. As most readers with at least a little bit of experience with cathode-biased amps will realize, this is a less-efficient and somewhat less-stable biasing method than the fixed-bias circuit, and isn’t particularly suitable to very high-powered amps either. But it’s also a very easy means of biasing a set of output tubes, it requires no adjustment during tube replacement, and it enhances particular tonal characteristics that are a big part of several classic types of amplifiers—the Vox AC30 and AC15 and Fender tweed Deluxe among them, alongside many other classics and modern reproductions thereof.

The cathode-bias resistor is usually partnered with another component, a relatively small electrolytic (filter) capacitor that is wired parallel to it between cathode and ground. This is a cathode-bypass capacitor (discussed in detail in What Does This Thing Do? Part 6), and strictly speaking it isn’t participating in biasing the output tubes, but is instead helping to voice them while the resistor alongside it covers the task of biasing. 

Hopefully this installment of the series has helped you recognize the two major types of output-tube bias circuits in use in guitars amps today, and at the very least, enables you to point to the small network of components in the fixed-bias circuit or single large resistor with bypass capacitor in a cathode-bias circuit and say, “Hey, I know what that thing does!”