Tube amps can sometimes seem to be mysterious beasts. Matching output pairs, ECC83 or 12AX7, “death caps,” carbon comp resistors, standby switches, whether you can touch the glass with your bare fingers (you can, really. It’s ok, so long as they’re not too hot). There is an incredible amount of lore, some mythology, and a whole lot of what I generally call “voodoo.” One area which seems to generate quite a bit of confusion is what is referred to as “bias.”
The definition of bias is the application of voltage or current to a component to produce a desired mode of operation. For a vacuum tube, we use the term to refer to controlling the amount of DC current flowing to the plate (or anode) through the grid from the cathode, while the tube is “idling.” The tube is idling when there is no signal present.
There are generally two different ways this takes place. Which method is used depends on the tube type. Higher power tubes, such as most of the 8-pin type power tubes (EL34, 6V6, 6L6) are adjusted by controlling how much of a negative DC voltage is applied to the grid of the tube so as to reduce the current flowing from the cathode to the plate at idle. Usually this is done by means of a little knob, called a “trimpot” that is usually inside the amp. Here’s the confusing part: this adjustable kind of biasing is referred to as “fixed bias.” I’m not sure why, but one explanation I’ve seen is that the bias point is fixed by means of the control trimpot. I guess that makes sense. If all of that technical talk is mumbo-jumbo to you, don’t despair. You are not alone. Here’s the nutshell: to keep your power tubes running at an optimal level, there is a control knob in your amp. Too much and your tubes will burn up. Too little, and your amp will sound crappy. All of this can be measured and compared to specs, and there is some math involved, but, it boils down to this: either your tubes burn up, or you sound bad. You want to be in the Goldilocks Zone: just right.
Before I knew anything about how amps work, I actually used to adjust the bias on my amps “by ear.” I would play through the amp, and adjust the trimpot until things sounded right and felt good to me. I did this for years, on several different amps with no problems. I cannot recommend this method, because it is not technically responsible. But it worked. One note of caution: if your tubes are glowing red, this is a condition known as “red-plating,” and it means there is too much current, and your tubes are going to blow up. Best-case scenario is that you need new tubes. Worst case scenario is that you dump current back into the amp, burning up some other components, maybe even your output transformer. Repairing this can be costly.
Adjusting fixed-bias power tubes is a matter of finding out how much plate current is flowing while the amp is idling. There are a few methods of doing this, each with its own particular advantages or disadvantages. If you want to adjust your own power tube bias, I would recommend that you do some research and really understand what you’re doing before you try it. The information is readily available. Or, if you’re not confident, or don’t want to be bothered, take it to a reputable amp tech. Ask the amp tech which method he uses to measure and adjust bias, and his answer will tell you if he’s capable.
For tubes that dissipate less that 15 watts (including preamp tubes) the plate current is kept in check through a very clever method using a resistor on the cathode. These tubes are said to be cathode-biased. Cathode biasing is a self-regulating method that works very well with preamp tubes, and with some power tubes (such as the EL84s in The People’s Amp). Often, cathode-biased tubes are erroneously described as not needing to be biased. This is technically not correct. Plate current can be measured via the same methods that can be used to measure those in fixed-bias circuits, however, to to adjust a cathode-biased circuit means changing the value of the cathode resistor. Fortunately, this is not usually necessary. Sometimes old amps need to be checked.
So, what does this all mean? First of all, you should figure out whether the power tubes in your amp are fixed-bias (adjustable via trimpot) or cathode-biased. If your amp is more than 15 watts output, chances are it’s fixed-bias. Whenever you change power tubes, you, or your trusty amp technician, should measure the plate current and adjust the bias control so that the tubes are operating optimally. If you use an amp with cathode-biased power tubes (like The People’s Amp) you don’t need to worry about it, unless you suspect that something is wrong – either the tubes are redplating, or it sounds “cold” and feels unresponsive. In any case, beware the voodoo, and enjoy your tubey goodness!
2 thoughts on “What’s All the Fuss About Bias?”
[…] wrote another blog post about this issue here. The biggest thing is this: don’t worry! It’s not a big huge deal. If you’re […]
[…] has a slight effect on distortion and tone shaping. The resistor helps set the bias of the tube (click here for more on bias). This type of design, used in all preamp stages, as well as in lower-wattage […]