Guitar and bass amplifiers

A guitar amp is not a neutral hi-fi: it shapes the tone, distorts when pushed, and its speaker colors the sound. Pedals, effects loops and stand-alone modelers are covered in the guide to pedals and the signal chain; this page covers the amp itself.

What an amp does

• The preamp raises the pickup signal and shapes it. The gain control sets how hard it is driven: low settings stay clean, high settings clip into distortion. The bass, middle and treble controls sit here.
• The power amp supplies the current that moves the speaker. Its wattage sets how loud the amp goes before it runs out of clean power.
• The speaker and cabinet shape the sound more than any knob. Most guitar speakers reproduce little above roughly 5 kHz, which softens the fizz of distortion, and the same amp through two different cabinets can sound like two different amps.

Tube, solid-state and modeling

A tube amp's power section sounds its best when worked hard, which in a 50-watt amp is far too loud for a house; hence low-wattage tube amps. Power tubes wear and are replaced every so often, sooner with loud playing; preamp tubes last much longer. New power tubes often need their bias checked by a technician, because a tube amp holds dangerous voltages even when unplugged.

Solid-state amps are reliable, and their clean channel stays clean at high volume, which suits jazz, funk and clean pop. Their drive channels range from good to fizzy, and many players put an overdrive pedal in front.

A modeling amp suits a student who wants many styles, headphone practice and recording into a computer. A cheap unit can sound convincing on headphones and thin through its small speaker. Hybrids also exist, most often a tube preamp with a solid-state power amp.

Watts and loudness

Perceived loudness grows slowly with power. Doubling the wattage adds about 3 dB, a modest step up. Sounding roughly twice as loud takes about 10 dB, which is about ten times the power. A 100-watt amp is therefore not twice as loud as a 50-watt one.

A speaker that is 3 dB more sensitive gives the same gain as doubling the amp's power. A tube amp usually sounds louder than a solid-state amp of the same rating: tube power sections distort gradually and stay usable past their rated output, while solid-state clipping sounds harsh, so players stay below it.

Rough ranges, which vary with the drummer, the room, the speakers and how clean the sound must stay:

Speakers and cabinets

Guitar amps use 8-inch speakers in small practice combos, 10-inch speakers in some brighter combos, and 12-inch speakers most often, in 1×12, 2×12 and 4×12 layouts. Bass cabinets use 10-, 12- and 15-inch speakers; tens are generally punchier and quicker, fifteens rounder and deeper.

An open-back cabinet radiates sound from the rear as well, so it spreads across a room and sounds open, with less low end. A closed-back cabinet gives a tighter, heavier low end and projects more directly forward. Bass cabinets are almost always closed or ported.

Combo, or head and cabinet

A combo puts the amp and speaker in one box, one item to carry, and suits most students. A head and cabinet are separate, so the player can change or stack cabinets. Higher-powered rigs and most bass rigs take this form, and modern bass heads are often small, light class D designs.

Impedance, briefly

Speakers and cabinets are rated in ohms, commonly 4, 8 or 16. A tube head has an output selector or separate jacks for these values, set to match the cabinet's total impedance; two 8-ohm cabinets wired in parallel make 4 ohms. A tube amp must never be switched on and played without a speaker or a proper load connected, because the output transformer and power tubes can be damaged. A solid-state amp states a minimum impedance that must not be undercut; it delivers more power into a lower load, so its rating is quoted at a stated impedance.

Channels, master volume and attenuators

Many amps have two or three channels, typically clean and drive, switched by footswitch. On an amp with a master volume, gain sets the preamp distortion and master sets the overall level, so a distorted sound is available at low volume. A tube amp's power-tube distortion still needs the master turned up.

An attenuator sits between a tube amp and its speaker and absorbs part of the power as heat, so the power tubes can work hard while the speaker plays quietly. It must match the amp's wattage and impedance, since it becomes the amp's load. Some tube amps build in a power-reduction switch instead.

Quiet practice and recording

• A headphone amp plugs straight into the guitar's output jack, with built-in amp sounds and often an input for backing tracks.
• Many practice amps have a headphone output, often speaker-emulated so the sound is not harsh. A speaker output is never a headphone jack.
• An audio interface, a box that connects an instrument to a computer, takes the guitar into its instrument (Hi-Z) input while amp simulation software models the amp and cabinet. A small buffer setting keeps latency, the delay between picking and hearing, low. The same setup records.

Bass amps

The ear is less sensitive to low frequencies, and producing them takes much more speaker movement and power. In the same band, a bass amp needs several times the guitarist's power and more speaker area.

Bass is usually played clean, so headroom matters most: the amp should run well below its limit. A clipping bass amp turns the low end to mud and puts the speakers at risk. Many bass amps include a compressor, which evens out soft and hard notes so the line stays steady in the mix.

Most bass heads have an XLR DI output, a balanced line that sends the bass straight to the PA mixer. A pre/post switch chooses whether the engineer receives the raw bass or the amp's EQ, and a ground-lift switch can cure hum. On many stages the PA carries the bass to the audience and the amp serves as the band's monitor.

For rehearsal with a drummer, a bass player needs power in the range given above, into a 2×10, 1×15, 2×12 or 4×10 cabinet. A small practice amp distorts before it keeps up. The bass itself, from pickups to strings, is covered in kinds of bass.

Getting advice

Before buying an amp, ask what your music and your room actually need, at the free intro visit or in a lesson. Bill plays bass in a working band, so the question of how loud a rehearsal really gets is one he answers from experience. Lessons are described on the bass and guitar pages.

Questions