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If you talk to technicians who are not familiar with guitar amplification, you will find two common misconceptions:
Both of these are quite incorrect, although there are good reasons to play any amplifier around its maximum capability ... read on ...
For nearly all styles of electric guitar amplification they are used as an integral part of the sound creation process. The guitar itself, the pre-amplification (which includes equalisation, as well as sound effects), the use of the power amplifier in varying amounts of overdrive, the speakers and the cabinet each add their own essential elements to the final sound.
This sound modification and creation process does not even end at the speaker! In fact, it is a loop, where sound is fed back to the source. Performances are often played and recorded at medium to loud volume levels, where the acoustic energy from the speakers drives the already vibrating string(s) to modify the natural decay characteristics, and/or drive the string to produce acoustic feedback with the addition of many controlled techniques.
This type of feedback is nothing like the undesirable feedback experienced with a microphone and PA system. For example, a guitarist might play a single note, and with sufficient gain, cause the note to sustain indefinitely. Then by touching the string at any of various points along its vibration, cause it to change pitch to a harmonic of the sustained note. These techniques are not accidental or experimental, and not necessarily performed for theatrics; they are rehearsed part of electric guitar technique.
The need for high gain to perform these techniques smoothly does not imply excessive volume levels. Guitar power amplifiers and/or preamplifiers are commonly driven with very high gain into overload for a variety of reasons. One benefit of this practice is to mask the difference in levels between a plucked string, tapped notes on the fret-board, and various techniques to produce string harmonics.
It may seem strange to non-players that you would actually want to overdrive an amplifier. After all, everyone knows it produces distortion, which sounds bad. On the contrary, the evolution of guitar amplifiers since the mid 1960's has been aimed almost entirely at producing desirable and musical types of overdrive, to the point where it is now a technical art form. It should also be acknowledged that there are various musical styles in which the more vulgar the distortion, the better, however, there are more players and styles seeking a purer type of overdrive distortion.
The sound of a valve power amplifier moving into and out of its clipping region is the sound that many guitarists crave. Even though it can be emulated at lower volume levels (with overdrive preamps, digital modelling and even power attenuators) it is not as good as "the real thing". The following pages describe this effect in more detail, but this is essentially the reason guitarists prefer to use an amplifier at its maximum power level.
Of course, if guitarists just wanted to play as loud as possible, they would just buy a solid state amplifier. At current prices, I can buy a 1600 watt solid sate amp for the same price as my 30 watt valve amp! The solid state amp would be more reliable as well, but I want tone, not volume. The trick is to use an amp that you can play at maximum power that still allows you to mix volume levels properly in your live playing situations.
If you find it difficult to imagine what this type of overdrive might sound like, it is probably best described as having some similarities to the wave-form of a harmonica, while other overdrive sounds may be more like a saxophone or violin. Of course, these instruments have other characteristics such as resonance, timbre and volume envelopes which clearly distinguish them from an overdriven electric guitar sound.
As more overdrive is used, the guitar's dynamic range is reduced. A clean (no overdrive) sound has the volume envelope of a plucked string, like a piano note. At the other extreme, large amounts of overdrive produce almost constant volume, from when he note is struck until it decays. This effectively increases sustain of the volume envelope, allowing a combination of many different techniques and styles not possible with a clean guitar sound.
It is also important to understand that the type of guitar used affects the type of overdrive produced. Single coil pickups (such as those on the popular Fender Stratocaster guitar) provide a brighter and chunkier sound than humbucking pickups (such as those on the Gibson Les Paul guitar), which produce a fuller, creamier sound. These differences are very obvious with clean amplifier settings, but are still quite apparent at heavy overdrive settings.
Just like guitar amplifiers, guitar speakers are also designed to colour the sound. Hi-Fi speakers on the other hand, are designed to reproduce sound accurately. There are many differences between instrument and hi-fi speakers. Here's a diagram showing what happens when you pump up to double the rated power into each speaker type.
WARNING Please don't interpret this as: "GM Arts says you can put more than the rated power into instrument speakers!" It is definitely not recommended!
What this means is that a hi-fi speaker is designed to reproduce sound accurately (the linear region) right up to its rated power capability. After that, you will start to damage the cone and quite likely burn out the coil.
An instrument speaker on the other hand will remain linear up to near its maximum power, then gradually restrict the cone excursion to provide a relatively clean, but somewhat compressed sound reproduction. When operating a speaker near its maximum power, there are still bursts of power such as drums, picking a guitar string, striking a piano note, etc. Instrument speaker are designed to handle these short bursts. Of course if you push it too hard, you will still cause cone and coil damage.
Instrument speakers achieve this by generally having a wider but shorter voice coil winding.
The cone excursion is limited smoothly at its extremes, mainly for two reasons:
There are other differences - here is a diagram showing frequency response.
A hi-fi speaker and cabinet is designed to cover the entire audible spectrum without any colouration (variation) at all. This is achieved often with 3 or more speakers (woofers, mid-range, tweeters, etc), with other design features such as bass ports, crossovers and internal acoustic damping. They attempt to cover the range from 20Hz to 20KHz.
Guitar speaker cabinets are rarely acoustically padded, and crossovers with high frequency horns are not popular. Even bass ports are used only by players seeking the bass-heavy chugging styles. These speakers and cabinets have a strong, but broad middle emphasis. Bass response is typically rolled off around the lowest guitar note (80Hz); middle response is strong, with some peaks and troughs in the response around 2KHz to 5KHz , then a sharp roll-off of upper response above about 5KHz.
Bass response is even further reduced in open back cabinets, although this can be recovered (if you wish) by boosting the bass control. Provided the speaker can handle this without the damping provided by a sealed box, it should sound fine. Open back enclosures have a higher-middle sound and wider sound dispersion, while closed back (sealed) enclosures have a tighter and punchier bass response.
The peaks and troughs are caused by sound reflected from the inside of the box back out through the cone. Some frequencies add to the existing cone vibration, while others cancel to cause this effect. A single speaker will generally produce more pronounced peaks and troughs, so most players prefer sealed boxes with 2 or 4 speakers, so the effect is still evident without producing extreme resonances.
These different characteristics give rise to different box design and speaker combinations aimed at different musical styles. For example, heavier styles will benefit from a fuller bass, but notches a little lower in the audio spectrum than rock styles.
If you're building your own cabinet, you might need to experiment to some extent. Even better, borrow or hire different boxes and build something similar to the box & speaker combinations you like. The type of wood used makes a subtle difference. If the (internal) texture is smooth, it will enhance the frequency peaks and troughs more than a rough and porous finish. You should also consider the compromise between weight and rigidity. Bracing can be used to strengthen large, flat panels.
There are several speaker design programs on the Web, but a general rule of thumb is: the larger the internal volume, the better the bass response. If you have a box that is too "peaky", you can reduce this effect by acoustically padding one or more internal surfaces.
There is little doubt that early overdrive sounds were discovered in attempts to play as loud as possible, sometimes without logical reason, at other times in a need to be heard above noisy audiences and other band members. There is also no denying that this approach still exists today in several musical styles.
Nevertheless, mature players who rise to the top of their field are masters not only of their instrument, but also of their sound production. With guitar amplification an integral part of the sound creation, skilful players know how to use various levels of overdrive dynamically, in addition to their technical instrument skills, to express their musical art form.
Any solo instrument, to be heard as such, must be played at a level above the background sound level. In the case of a rock band, this usually means a drum kit and other band instrumentalists. To obtain a useful dynamic range above that sound level dictates the volume level used, and therefore the amplifier power required.
Playing music loud has many social implications also, such as "drowning out" other influences. There is also an element of power. Technically, the acoustic energy of the speakers is often used artistically for feedback with the vibrating strings to produce sustain and controlled feedback of harmonically related notes.
It is worth noting that the basic guitar and (variably-overdriven) valve amplifier has been used as a key musical instrument in popular music since the mid 1960s Its popularity has been unaffected to date by solid state designs, music synthesisers, and the computer/digital musical hardware and software revolution.
If guitarists just wanted to play loud, they would have opted for the super clean, high wattage solid state amplifiers which have become available. For example, solid state amplifiers of several hundred watts (used typically for PA systems) are available for about half the cost of a good valve amplifier of 50 to 100 watts.
Most venues now use full production, which means that the entire band and instrumentation is mixed through a single high powered PA system. This allows players to use a favourite amplifier in any venue, and a typical power requirement is about 50 Watts RMS. Without being re-amplified, a valve amplifier of about 50 Watts RMS suffices for small club work, while about 100 Watts is required for disco and outdoor work.
The usual way to build a reliable amplifier is to use components that can easily handle the highest load expected of them. For example, you would expect to see resistors rated at least twice their maximum possible power requirement. This requires careful thought in an overdriven amplifier, though. For example, the power output stage can be expected to deliver at least 1 and a half times it's rated power, so output components, such as transformers need to take this into account.
On the other hand, the power supply should not be capable of delivering unlimited power, because this would allow the output stage to ultimately burn out! Some designers use valve rectifiers (or otherwise limited power supplies) to ensure this does not happen. In fact, a limited power supply adds another desirable feature: more sustain as notes start with enough power to reproduce the pick attack, then power is restricted until the note decays with little volume loss with a return to full, clean power. Players often describe this effect as "bloom".