Guitar & Bass:
Amps 101: Knowing Your Head From Your Combo
With all the talk about amps, preamps, heads, stacks, tubes, and so on, a few readers have written to us with questions about the basics. Not everyone is a pro-level musician — many of you are just serious hobbyists or players aspiring to get to the pro level. So for those of you looking for a basic handle on all these amp parts and how they fit together, this tutorial’s for you!
Preamps for bass guitar often times include parametric EQs, on-board compressors, and graphic EQs. These features are almost entirely absent from guitar preamps, though a few include graphic EQs and other amp-specific tone-shaping circuits.
In tube amplifiers, 12AX7 and ECC83 are the most commonly used preamp tubes. They are basically the same thing sonically – ECC83 is just the European designation for the tube, while 12AX7 is the American designation. The irony is that most of these tubes are now made in Russia and China — go figure.
Preamp tubes are also used in buffer circuits for effect loops and for bringing line level signals in power amps up to hotter levels, so if you see an amp listed as having 5x12AX7, all five tubes are generally not used in the tone shaping circuit. One is probably dedicated to the effect loop and another to the reverb. But of course there are thousands of amp designs that all do things just a little bit differently from each other.
Solid-state amplifiers do not use vacuum tubes, and instead rely upon transistors or modeling technology to generate tone. Although most pro guitar players opt for tube amps, some pair solid-state or modelling preamplifiers with tube power amps, and many bass players opt for solid state rigs throughout.
Power Amplifier (Power Amp)
In tube amps, the most widely used tubes for high-wattage amps are the 6L6 (5881 equivalent in Europe) and EL-34 tubes. Amps with lower wattage often use smaller EL-84 or 6V6 tubes. Each of these tubes has different sonic characteristics, and depending on the amplifier design (Class A, A/B, tube or diode-based rectification), amps powered by these tubes can sound significantly different from one another.
Many bassists use digital solid-state (not tubes) power amps today, which provide large amounts of wattage and volume, with very little physical weight. A 300-Watt tube power amp might weigh nearly 100 pounds, while a comparable digital power amp may weigh less than 10 pounds!
Speaker(s) and Speaker Cabinet
Bass guitar amps are often times paired with a wider range of speakers including larger 15” speakers, 10” speakers are more common, and high-frequency drivers (tweeters) are often added into the mix to provide a wide spectrum of tones.
While there are hundreds of different amplifier manufacturers, the list of speaker manufacturers is much smaller, and the most widely-used speakers are made by Celestion, Eminence, Jensen, and Peavey.
Guitar players typically use speaker cabinets in the following configurations: 1x12, 2x12, and 4x12. A 1x12 cabinet is the (typically) smallest and most portable option, while 4x12 cabinets are staples among rock and metal players (bassists in the rock world love their 4x10 cabinets). 2x12 cabinets provide a great solution for guitar players who don’t want to carry large, heavy 4x12 cabinets around but still want some significant presence to their sound. With the popularity of lower-wattage tube amps, 1x10 combos have recently become more common in the market.
Once again, bassists have another advantage over guitarists when it comes to moving around large speaker cabinets. Many 10" speakers for bass cabinets use light-weight neodymium magnets. A typical 4x10 bass cabinet with these speakers can weigh 20 pounds less than a similar 4x12 guitar cabinet. But if you like to torture your roadies and your audience, you can also buy monstrous 8x10 bass cabinets!
Who needs all those speakers? More speakers move more air, which creates a fuller presence to the sound, even though a 1x12 cabinet may have the same relative volume. Many premium 2x12 cabinets have comparable presence to 4x12 cabinets, but we have yet to experience a 1x12 cabinet that provided the same kind of presence as either of the larger offerings.
Speaker cabinets can be wired for either mono or stereo operation — many cabinets are wired to accommodate either configuration. Of course a 1x12 cabinet is mono by default — you’d need two cabinets to run a stereo rig.
Additionally, speaker cabinets come in three basic designs: closed-back, open-back, and semi-open. Most 4x12 cabinets are closed-back, but with 2x12 and 1x12 cabs, all three options are pretty easy to find.
Most combo amps have open- or semi-open backs. It’s not just a convenient place to store your foot pedal and cables when in transport — the open back changes the sound of the cabinet (this is part of the characteristic sound of some classic Fender and Vox combo amps, for example). Some speaker cabinets have removable back covers, allowing you to experiment with changes in tone that this makes. It’s really just a matter of personal preference.
The combo amp format is all-inclusive. The front panel has controls for the preamp and power amp, though the user experience is just that you have tone and volume controls, plus perhaps reverb and some other effects, and level control over an effects loop.
Typical combo amps are either offered in 1x12 or 2x12 configurations, and some 2x12 combo amps like the famous Roland Jazz Chorus JC-120 operate in true stereo. Generally speaking, a 2x12 combo provides greater sound presence than you’ll feel from a 1x12 combo, though the 1x12 can still be more than loud enough to play gigs in a large club.
Most combo amps provide the option to connect external speaker cabinets for additional tone options. A 1x12 combo sitting on top of a 1x12 extension cabinet can provide a nice big sound, and you don’t have to take a larger, heavier 2x12 combo with you everywhere.
Heads and Stacks
Many bass heads are constructed in standard 19” rack-mountable configurations for installation in racks. Some come in their own enclosures from which they can be easily removed to facilitate racking at a later time. Although they look more like rack preamps (see Rack Separates below) than guitar heads, they are generally full amps containing both the preamp and power amp section in one head.
Rack Separates — Preamps and Power Amps (Guitar)
The advantage of this design is that it provides very flexible tone options — more than with almost any traditional amp or head. You can mix a preamp from one company with a power amp from another, or select from among multiple power amp options within a single company. For example, Mesa/Boogie has multiple power amps with 6L6 power tubes as well as one with EL-84 tubes. The 6L6 power amps offer vastly different power ratings as well as other differences in their design that all contribute to a significantly different guitar tone.
There are numerous options for rack-mounted preamps and power amps from multiple companies. We’ll look at some of these options in our rack gear feature story. In the meantime, our amp reviews section includes reviews of many rack preamps and power amps.
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