Although “all-tube” amps—i.e., those with vacuum tubes like the ones pictured here driving both their preamp and power-amp sections—are most popular among guitarists, the term “all-tube” is a loaded one that bears closer scrutiny and perhaps less militant adherence. Photo by Andy Ellis
Amplifiers“All-tube”: Once upon a time, most electronics—even computers—were “all tube,” meaning the electrical current in the circuits was regulated by various types of vacuum tubes. Today, a good definition for “all tube” would be an amp in which tubes are primarily responsible for shaping the sound. However, some amps fitting this description do use solid-state parts—for example, as rectifiers, to buffer effects loops, feed reverb circuits, and other tasks where they don’t really affect the core tone.
But you could say the line between “all-tube” and a “hybrid” of tube and solid-state circuitry has been fuzzy for a while. In the 1980s, Marshall added diodes to its otherwise all-tube JCM800, which is now regarded as an iconic hard-rock amp. Another example: the solid-state graphic EQ found in classic Mesa/Boogies.
Matchless amps such as the Lightning Reverb shown here are a well-known example of an amp with
a cathode-bias power section.
Bias: If a tube amp uses a fixed-bias setup (see below), biasing can refer to the process of setting the initial current draw of an amp’s output tubes (kind of like setting the idle on a car motor), or more generally to the type of biasing used in an amp with a tube power section (the portion that sends the signal to the speakers). In guitar amps, there are two bias types:
Cathode bias amps, sometimes referred to as “self-biasing,” let the tubes themselves set their own current draw via a resistor and capacitor connected between the cathode and ground. Examples of amps that use a cathode-biased output section include the Matchless Lightning, Carr Super Deluxe, tweed Fender Deluxes, and countless others, as well as all single-ended amps. See also Fixed bias.
Meanwhile, fixed biasamps have a set (or fixed) amount of negative voltage applied to the grid of a power tube. Ironically, “fixed bias” is actually adjustable (either with a potentiometer that adjusts the resistance from the bias supply, or by replacing one or more resistors in the circuit). In fact, when you replace power tubes in a fixed-bias amp, you should have a qualified amp tech adjust the bias for optimal current draw with the new tubes. Correct biasing not only affects the amp’s tone, it also prevents them from running too hot (which burns them out) or too cold (which can cause ugly distortion and other sonic yuck).
All other too-technical-for-this-article amp details being equal, cathode-bias amps generally produce less power and less capability for clean, undistorted volume (aka headroom), but a “warmer,” rounder, often more responsive tone. It’s also worth pointing out that fixed-bias amps can be reconfigured for cathode biasing and vice versa.
Class-A and class-AB refer to the operating conditions of an amp’s power section (be it tube or solid-state). There are other classes (B, C, and so on), but these aren’t typically used in guitar amplification. As in politics, class distinctions can get complicated but we’ll try to sum it up simply: With a tube amp operating in class A, the output tubes are set to remain halfway between cutoff (no sound) and saturation (distortion). They draw current at all times—even when there’s no signal feeding them. Single-ended amps operate in class A. Push-pull amps can also operate in class A (though there’s plenty of debate about whether all amps called “class A” actually are). Class-A amps tend to sound rawer, but other factors also contribute to this quality. In class AB, the bias point is set so that current flows are more than half but less than the full 360 degrees of the input cycle. This is more efficient and can produce more power from the same types of output tubes. Most push-pull amps operate in class AB. See also Push-pull and Single-ended.
Some amps come with an effects loop, which many players prefer to route delay and other time-based effect units through for a more pristine sound.
Effects loop:A set of 1/4" jacks that allow effects to be added somewhere after the amplifier’s input or preamp stage, thereby letting the amp’s gain and tone controls shape the sound before it hits the pedals or rack effects patched into the loop. There are two types of effects loops, and both tend to works best for time-based effects like reverb, delay, chorus, etc. Gain effects (overdrive, distortion, fuzz, and compression) generally work better in front of the amp’s input.
In a series effects loop, the signal is sent out of the amp, through the connected outboard effects, and then back into the amp, where it continues to the output section. In a parallel effects loop, the audio signal branches into two separate paths. One path passes the signal through the connected outboard effects, while the other path stays in the amp, unprocessed. The effected signal can then be added in with the uneffected (or dry) signal in varying amounts—from barely detectable to highly effected—using a wet-dry mix knob. Parallel loops are more complex to build and therefore tend to add to the cost of an amp. Further, they don’t always sound better than simpler (and more common) series loops.
Headroom: The amount of volume available in an amp before clipping (or distortion) sets in. In the simplest terms, the higher the headroom, the louder the “clean” sound.