Effects and Amps
Tubes vs. Transistors
Guitar amplifiers come in two distinct (and often mixed) flavours, Vacuum Tube driven (otherwise known simply as Tube amplifiers [US] or Valve amplifiers [UK]) or transistor based Solid State Amplifiers. Traditionally, Tube amplifiers have always been the more desired incarnation (aside from a brief period surrounding the introduction of solid-state amplification as a cutting-edge solution), primarily because of the musical and aurally pleasing way in which Vacuum Tubes shape the sound of a guitar when overdriven, or pushed into saturation to quote the electronic term. When overdriven, Tubes introduce even-order harmonic overtones into the sound, predominantly in the 2nd order. These harmonics are referred to by audiophiles as "nice", "warm", or even "organic" in tonality. Conversely Solid-state amplifiers introduce more odd-order harmonics to the sound when overdrive, with a peak at the 3rd order. These harmonics sound slightly harsh to the human ear and as a result do not provide such a smooth and pleasing overdrive sound. However that is not to say that solid-state amplifiers do not have a vast base of users. In fact some styles of music lend themselves more to the aggressive bite of a transistor based amplifier - such as a hardcore punk or metal.
Preamps and Poweramps
All amplifiers are split into two amplification stages, known as the preamp and poweramp. The pre-amp stage takes the guitar signal and amplifies it to a stage where it can be worked on by the following power stage, Equalization (if the amplifier features Treble, Middle or Bass controls) is applied at this stage. The poweramp then follows the preamp stage to amplify the signal to a volume that can be fed to drive a speaker. In a solid-state amplifier, the preamp and poweramp are both transistor based audio-amplifier circuits, whereas in a Tube/Valve amplifier, these circuits are based around tubes and feature specifically designed preamp and power amp tubes. Popular preamp tubes include the 12AX7, 12AU7 and 12AT7. Power amp tubes include the ubiquitous EL34 (typically Marshall), 6l6GC (Fender standard) and EL84 (Vox-style) tubes.
In the 1990's Marshall introduced a line of hybrid tube/transistor amplifiers known as the Valvestate series. These amps were aimed at the beginner to amateur gigging musician and were a huge success story; blending a tube in the preamp to a solid-state power section the amps were priced competitively and to many people's ears represented an improvement on regular solid-state overdrive tone. The 8080 model became Marshall's top-selling amplifier at the time and these still command a reasonable resale value on ebay. The valvestate line paved the way for the future of hybrid and modelling amplifiers; most amplifier manufacturers have a series of hybrid or modelling amplifiers in their line today either using a tube in the preamp or power amp section of the amplifier or digital modelling technology to mimic the sound of a series of far more expensive analogue tube amplifiers.
Heads, Cabinets & Combos
Guitar amplifiers typically come in two forms, as a Combo or a separate head and cabinet. A Combo is a "combination" of the amplifier and speaker(s) in a single enclosure. Typically amplifiers suitable for practise at home are of the combo form (such as the Vox modelling amp pictured above) since they occupy much less space and have a volume output more suitable to practise. However combo amplifiers are produced from low wattage ratings through to 100W + by various companies.
In a head and cabinet configuration the Head contains the amplifier proper (all electronics, controls and connections) while the separate speaker cabinet contains speaker(s) wired in either series (combined impedance is the sum of all individual speaker impedances) or parallel (1/R = 1/R1 + 1/R2 + 1/R3 + ...). The speaker cabinet itself contains no active electronics and does not need power. A head and cabinet amplifier can be seen above in the photo of the Marshall half stack. The speaker cabinet in this case contains four 12" speakers and is commonly known as a 4x12" (or "four-by-twelve").
When driving a speaker cabinet from a tube amplifier, one question that always arises is what the impedance on the back of the amplifier should be set to. In the case where a single cabinet is being driven, this is simple - the impedance should be set to the same value quoted on the cabinet for the mode of operation in use (mono/stereo). However when two cabinets are in use this becomes more complicated, as the combined impedance the amp is to drive must be calculated from the impedance of the two cabinets. If the impedance of both the cabinets is the same, then the combined impedance is just half the impedance of one cabinet by ohms law. For example two 16 ohm cabinets:
1/Rtotal = 1/16 + 1/16 = 1/8
therefore Rtotal = 8 ohms.
However if the cabinets are mismatched then this is not so simple; take the case of one 8 ohm cabinet and one 16 ohm cabinet.
1/Rtotal = 1/16 + 1/8 = 3/16
Rtotal = 16/3 = 5.333 ohms
Since this setting will not appear on the back of the amplifier, you must set the output impedance level to 4 ohms. The amplifier must be connected to a load of at least the same ohm rating is the output impedance is set to. A general rule of thumb when using mismatched cabinets is to set the amp to half the value of the lowest impedance cabinet. Playing through a tube amp with no load applied or insufficient load applied to the output will seriously damage the amplifier and most likely blow the (expensive) output transformer. Tube amplifiers are complex and fragile pieces of electronics that must be handled with care and respect to reap the rewards of ownership.
Information from the nice folks at EuroTubes about biasing tube amps (this must be done after replacing power tubes with a matched pair/quad).
All you need is a bias probe. A bias probe looks just like the base of a 6L6 tube and it fits in your tube socket and then your tube goes into the top of the probe, then you turn on the amp wait 30 seconds and switch off the standby switch and the probe will read the amount of plate current that the tube is drawing. You only need a single probe because the trim pot controls all the tubes and since your tubes are matched you only need to measure one tube. You can adjust the bias by turning the trim pot which is located on the inside of the amp. To calculate your bias you should measure the DC plate voltage from pin #3 of any power tube to ground using a multimeter set to DC volts. This is done with the amp on, speaker plugged in. From the bottom of the socket pin #1 is the first pin going clockwise from the timing key so pin #3 is the third pin going clockwise. For most amps it will be between 400 to 500 volts. If you purchase the Weber bias probe with the V1 option you can simply flip the switch and read the plate voltage.
The formula for biasing is the plate dissipation of the tube (25 watts for a 6L6 or an EL34 or KT77) divided by the plate voltage (lets use 480 for this example) times .7 will just get you out of crossover distortion and you can go as high as .9 percent.
25 divided by 480 = .0520 or 52 milliamps X .7 = .036 or 36 milliamps
25 divided by 480 = .0520 or 52 milliamps X .9 = .046 or 46 milliamps
25 divided by 500 = .050 or 50 milliamps X .7 = .035 or 35 milliamps so you can see that a variation of 20 volts only equate to 1mA.
A bias probe can be purchased from Weber for $105.00 (The BR-2).
Speaker Wiring Combinations
A speaker cabinet may contain 1, 2 or 4 speakers wired in one of three different ways to produce a desired impedance output to be matched to the driving amplifier, and speakers themselves generally are rated as 4 ohm, 8 ohm and 16 ohm individually. For a one speaker cabinet or combo the connection is trivial, but for 2 and 4 speaker configurations either series, parallel or series-parallel wiring can be adopted to produce different combined output impedances.
Series wiring involves connecting all the speakers in a daisy chain, i.e:
+ lead to + connection on Speaker1
- connection on Speaker1 to + connection on Speaker2
- connection on Speaker2 to - lead
A Series wiring as detailed above simply sums the impedances of the speakers in the chain. So two 8 ohm speakers in series have a combined impedance of 16 ohms.
Parallel wiring involves connecting "across" speaker terminals with common connections, i.e:
+ lead to + connection on Speaker1 and Speaker2
- lead to - connnection on Speaker1 and Speaker2
A parallel wiring as details above divides the individual speaker impedance by 2 if the two speakers have the same impedance. So two 8 ohm speakers in parallel have a combined impedance of 4 ohms.
Series-Parallel wiring involves connecting two pairs of speakers in series with each other and then connecting those pairs of speakers in parallel. For all 8 ohm speakers this gives a combined impedance of 8 ohms, For all 16 ohm speakers this gives a combined impedance of 16 ohms.
What impedance combinations are possible with four 8 ohm speakers?
- Series = 32 ohm
- Parallel = 2 ohm
- Series-Parallel = 8 ohm
What impedance combinations are possible with four 16 ohm speakers?
- Series = 64 ohm
- Parallel = 4 ohm
- Series-Parallel = 16 ohm
Marshall Amplifiers - British - Manufacturers of the Plexi, JCM800, JCM900, JCM2000 DSL/TSL and Valvestate series'
Fender Amplifiers - American - Manufacturers of the Twin Reverb, Deluxe Reverb, Pro Reverb, Bassman, Vibrolux, and Hot Rod series
Vox Amplifiers - British - Manufacturers of the AC30, AC15, Pathfinder and Valvetronix series
Amplifier Modification Links
Warning do not attempt unless you understand that any modification you perform on an amplifier will void your warranty and is also done at risk to yourself and the amplifier itself. No liability will be held for any damage that may be caused to your person or property as the result of these modifications.
-- add compression pedal detail here
Some popular compression pedals are:
Generally designed to emulate the pleasing, soft clipping of a vintage tube amplifier. Perhaps the most popular Overdrive pedal ever designed is the Ibanez Tube Screamer. A great deal of modern overdrive pedals are nothing more than Tube Screamer clones, although a great deal of creedance appears to be attached to the early TS808 models. In fact many pedal modifiers such as AnalogMan and Keeley offer upgrades from Ibanez's subsequent releases (and indeed Boss' SD-1 Overdrive) to TS808 standards. The characteristic Tubescreamer tone is a low-medium gain drive with a noticeable mid-frequency boost or 'hump' and a rolloff of the bass frequencies. This sonic coloration makes the tubescreamer equally popular and loathed among the pedal using guitar community - some adore the way a tubescreamer can be pulled out of a live mix easily whereas others dislike the "honky" tone noticeable when playing distorted chords.
Distortion pedals drive the guitar signal into a fiercely overdriven saturated state, where the guitar signal waveform is vastly distorted and clipped from the original input. Distortion has a more metallic, abrasive edge to it's tonality than a warmer overdrive sound, partly due to the use of solid-state diodes and bipolar transistors in most common distortion circuits as opposed to the "tube-esque" compression of JFETs often used in overdrive pedals.
Distortion lends itself perfectly to more aggressive styles of music such as Punk, Metal, Grunge and Industrial. The thick, chunky guitar tone produced by distortion effects via pedals or high-gain amplifiers such as those manufactured by Soldano and Mesa-Boogie was the de-facto guitar sound of the late 80s and early 90s. With the return of more retro-styling and more vintage sounding genres distortion may have been overtaken by Overdrive in popularity, but will always retain a place in most guitarists' sound palettes.
Some popular distortion pedals include the following:
Equalization effects modify the frequency content of the guitar signal. Several types of equalizer exist, from simple two or three knob versions controlling Treble, Bass or Treble, Middle and Bass content to Graphic Equalizers with a series of sliders for separate frequency bands or Parametric Equalizers with a frequency select and boost/cut control structure. Equalizers are designed to provide the guitarist with a great degree of control over the overall tone of the guitar signal being amplified for a variety of applications, some of which are listed below:
1) adding a bass boost for more thickness if a small amplifier/speaker is used
2) providing a mid-range 'scoop' for a modern metal distorted tone
3) boosting a guitar sound for lead playing by applying a mid-frequency boost
4) compensating for a difference in sound between two guitars or pickup arrangements
A Wah or Wah-Wah pedal is a rocker-pedal controlled dynamic filter which can be manipulated by the guitarist's foot in a rocking motion to provide a significant vocal effect to the guitar signal. In fact the name 'Wah' and the brand name for Jim Dunlop's best-selling pedal series; the 'Crybaby' are derived from the vocal sound the effect produces when engaged. The Wah is simply a parametric notch filter controlled by the rocker pedal. When the toe of the pedal is fully depressed, the treble frequencies are dramatically amplified, while the bass frequencies are attenuated. When the heel of the pedal is kicked back - the opposite occurs; the bass frequencies are boosted and the treble cut, producing a wooly loose tone. When the pedal is in it's middle position, a strong mid boost (useful for soloing) is applied. The beauty of the effect however comes not from placing the rocker in a single position, but in transitioning between heel and toe in a rhythmic motion with the phrasing of the guitarwork played at the same time. Jimi Hendrix, according to many "the only person to play a Wah pedal effectively" illustrates this best on the song 'Voodoo Chile (slight return)'
Modulation - Chorus/Flanger/Phaser/Vibrato/Tremolo
Modulation Effects generally process the input guitar signal through summing the original signal with a number of delayed and/or pitch shifted versions to create anything from a subtle swooping wash effect to a watery shimmer. This section will aim to describe the differences between the various modulation effects and how they are produced, as well as detailing some popular pedal examples of each effect.
The Chorus effect is designed to emulate a series of guitars playing the same thing at the same time. Even if exactly the same phrase or chord sequence is played by a group of guitars, a subtle thickening effect will be produced because of slight delays between each guitar or the difference spatially between the amplifier speakers. This effect is produced by a chorus pedal in a simple but ingenius manner. The input signal of the guitar is delayed and summed with the original signal, but the time delay applied is variable, and governed by an LFO (Low Frequency Oscillator) which can be controlled by the parameters on the chorus pedal. This variable delayed signal produces from a subtle thickening of the original signal to a watery-swirl through to a pronounced 'wobble' depending on the settings of the LFO. A chorus pedal typically has two adjustable controls; Depth (or the amplitude of the LFO - affecting how pronouced the pitch shifting of the delay pedal is) and the Rate or speed at which the LFO cycles from it's maximum to it's minimum value and back again to the maximum peak. The delay applied to the original signal before summing is normally adjustable between 20ms (20 milliseconds, or thousandths of a second) and 30ms in a chorus pedal.
Some popular Chorus pedals include the following:
The Flanger is very closely related to Chorus in that it too achieves it's effect, which is a more pronouced background 'swoop' across the signal by summing the original signal with a variably delayed version of itself, however to produce a flanger effect the delay applied to the signal varies between much smaller limits. Whereas a Chorus pedal operated by using an LFO that can be varied between 20ms and 30ms, a Flanger works within bounds closer to 1ms-10ms. At this level of delay the sound cannot be perceived by the human ear as an 'echo' and instead manifests itself as a pitch shift in the background. As with Chorus pedals, Flangers normally feature at least a Depth and Rate control to tweak the amplitude and speed of the LFO respectively. However flangers may also feature the ability to feed a proportion of the output signal back into the input of the Flanger (i.e. before the LFO controlled delay circuitry). This creates a more pronounced and aggressive "metallic" sounding Flange effect, and if the feedback control (often known as Manual or Resonance) is set to a large amount of feedback, self-oscillation can be acheived.
Some popular Flanger pedals are as follows:
A Phaser produces it's distinctive sound by summing the output of a filter with the original guitar signal, much like a Chorus pedal sums the output of a variable delay circuit with the original circuit. However the Phaser works off a much more subtle relationship between the input and filter output waveforms. The filter does not actually remove any frequencies as a filter would traditionally, and is actually termed an All-pass filter. Instead it operates purely on the phase of the waveform, which can be seen to be a delay of sorts, but in terms of the period of the waveform's oscillation rather than a definitive millisecond time period. The filter also has a distinctive frequency-phase response meaning that different frequencies (i.e. treble or bass content of the guitar signal) will be phase shifted by differing amounts. When a waveform is summed with another, constructive or destructive interference can occur depending on the phase relationship between the two signals. When two signals are out-of-phase by 180 degrees (or half the period of the oscillation) then they interact in a totally destructive way - i.e. that frequency is not present in the result of the summation. This creates a notch filter which moves through the frequency range dependent on the phase relationship between the two waveforms. A series of these phase-filters are applied to the signal to cause the Phaser effect, and the number of filters used is referred to as the number of stages the Phaser has. The MXR Phase 90 for example is a 4-stage Phaser, while Phasers from 6-stages up to 12-stages are available (generally at increasing cost). Phasers can have several adjustable parameters but the most common are Depth which controls the amount of filter output summed to the original sound (i.e. the amount of phasing occuring) and Speed/Rate which controls the speed at which the notches sweep up and down over the frequency range.
Some popular Phasers are as follows:
-- add Vibrato pedal details here
Some popular Vibrato pedals include:
Boss VB-2 Vibrato - now discontinued, and commanding a large resale price on ebay
Danelectro Chicken Salad Vibrato - now discontinued
Perhaps the simplest of the modulation effects, Tremolo pedals operate purely on altering the volume, or amplitude of a guitar signal relative to time. The effect is characterised by a "chopping" or "throbbing" tonality to the guitar sound, caused by the ramping up and down of the guitar volume by the LFO (Low Frequency Oscillator). Most tremolo pedals contain at least two adjustable controls; Depth, or how much the volume is altered by the LFO (a higher depth setting would yield a more pronounced effect) and Rate, or the speed at which the volume is affected. Vintage tube guitar amplifiers such as the Vox AC30 often featured the effect built-into the amplifier and it is this tube-driver tremolo (warm and attractive in tone) that is often emulated by tremolo effects pedals. Also equally useful and emulated is a more aggressive Square-Wave Tremolo which drops and restores the volume of the guitar signal at a much quicker rate - producing a hard-edged effect that is very noticeable when combined with a distorted or overdriven guitar signal.
Some popular tremolo effects pedals are the following:
Delay pedals sample and repeat the input guitar signal. Most pedals contain controls to adjust the delay time applied before the repeats (Delay Time), the number of repeats themselves (Feedback) and the level the repeated signal appears in the mix (Mix). Delay pedals can be set accordingly to produce close "slap-back" single repeats or long washes of repeated decaying notes.
Delay pedals typically are produced in two flavours; Digital and Analogue. Digital Delays sample the signal after quantization through an A/D chip and are characterised by a pristine, noiseless quality to the repeated sounds. Analogue delays use all analogue components and are typically more "warm" and "dark" in the repeated tone, due to degradation of the treble content of the original signal in the circuitry. For some guitarists, true analogue delay is the holy grail of delay tone and old Boss pedals such as the DM-2 and DM-3 Analogue Delay models command a high resale price on ebay.
Some popular delay pedals include the following:
Reverb pedals aim to replicate the natural reverberations caused by playing a guitar in a particularly sized room or hall. Most Reverb pedals will have a setting for Room and Hall reverb, if not several settings of various sizes and shapes. Two major parameters that are normally adjustable include the decay time (i.e. the amount of time for the original note reverb content to decay to inaudible level) and the mix level or how loud the reverb signal is relative to the original guitar signal. While reverb is an extremely useful tool when recording (to add some depth to a dry recorded track), the use of the effect in a live scenario is somewhat more of a niche. While some guitarists enjoy a slight amount of reverb on their guitar sound, many appreciate that this is provided by the very room the performance is being held in naturally. However some styles of music, such as surf lend themselves particularly to the effect, when crystal clear clean lead guitar passages can be liberally treated with a heady wash of reverb to good effect.
Some popular reverb pedals are the following:
Loop pedals simply record and repeat a user-determined section of guitar playing. Normally the Guitar player selects the record start point, and then presses the pedal again to stop recording and the end of the loop. The loop then starts playing and loops continuously until it is terminated by the player. Depending on the looper, it may be possible to overdub more parts over the original loop. Some loop pedals allow loops to be saved to memory and recalled.
Looping is particulary useful for practise and composition at home and on the road, writing complementary guitar parts, or for use in a live environment, either as part of a band setup without a lead and rhythm guitarist to provide a backing rhythm part for soloing or for buskers to build up layers of sound to form an entire song of several loop tracks in parallel.
Some commonly used loop pedals are:
The simple answer to the question "Which order shall I place my effects?" is normally "The way they sound best to you", and while this approach should not be overlooked and there are several different sounds that can be produced through non-standard effects ordering, there is a basically accepted order which is useful as (if nothing else) a starting point for effects ordering:
Compression ----> Wah ----> Overdrive/Distortion ----> Equalization ----> Modulation ----> Delay/Reverb
This is the standard suggested ordering of effect pedals/modules. It is generally accepted that if one is being used that a compressor is best placed at the beginning of the signal path, so that it can operate most clearly on the guitar output. Wah pedals also appear to function best when placed near to or at the beginning of the chain, although some people find they prefer the sound after a distortion pedal. Overdrive/Distortion pedals also operate at their optimum as close to the guitar as possible, to preserve playing dynamics which can be lost in a long signal chain. Equalization can be placed before "dirt" pedals such as distortion or overdrive devices to boost or 'push' the circuits into earlier saturation or to shape the tone of the distortion/overdrive, although post-dirt shaping is the more popular option for most guitarists incorporating an Equalizer into their setup. At this point the pedal gives you a great deal of control over the tone produced by the dirt section of the pedalboard, be it mid-scooped or boosted, or filtered in a more complex manner. Modulation is normally placed after dirt and equalization to modulate the resultant sound. Modulation pedals placed before drive tend to overwhelm the sound when engaged and produce strange harmonics in the distortion/overdrive units. Finally time variant pedals such as Delay and Reverb units are placed last, in order to work on the whole sound before it is to be amplified, although some people prefer to apply some modulation such as chorus after delay to thicken out the sound (if this is not produced within the delay pedal itself).
Pedalboards, cabling, buffering
Buffers are kind of like a relay pump station for your water service. From the main source, it is pushed by pump miles away to get to your house. This distance causes it to lose pressure. So, they add relay pump stations along the way, to keep the pressue up. Buffers act the same way with your electrical signal.
When driving long (20 foot) cables from your guitar can cause signal loss due to the capacitance of the cable. Longer the length the more the loss. This is especially true when every pedal on your board is "true bypass". In these cases, when you have no pedal engaged, the true length of the "cable" from guitar to amp is the sum of ALL of the cables in your chain until it gets to tha amp. Putting a buffer in your chain as the first or even last effect will reduce the signal loss and keep your sound/volume consistant while switching your effects on and off. Famous effects-maker-for-the-stars Pete Cornish has the following to say on this subject:
Take for instance a 15 ft guitar cable linked to ten pedals, each linked by a 2 ft cable, and then onto the amp by a 30 ft cable. If all pedals have "true bypass", and are off, then the total cable length hanging on the guitar output will be 63 ft. This will cause a huge loss of tone and signal level particularly if the guitar is a vintage type with low output and high impedance. The amp volume is then turned up and the treble control increased to compensate for the losses. The inherent background noise now increases by the amount of the gain and treble increase and is usually, in my experience, too bad for serious work. If one of the pedals is now switched on, then it's (hopefully) high input impedance (and usually low output impedance) will buffer all the output cables from the guitar and the signal level will rise due to the removal of some of the load on the pickups (i.e.: 17 ft instead of 63 ft of cable). The treble will rise and the tone and volume will not be as before. If that pedal was say a chorus or delay, devices which are usually unity gain, then your overall signal level and tone will vary each time an effect is added...not a very good idea. ... So the answer to your question re "true bypass" is no, I do not use this system in my Effects Boards/Racks as it can seriously degrade your sound. All my effects pedals which are derived from our large systems have, as the main input stage, a fiendishly clever pre amp that has the same characteristics as the input of a tube amp (1 Megohm/20pF), a highly efficient filter to eliminate the possibility of radio breakthrough and a low output impedance so that any following pedals/ cables etc. will not impose a load on the guitar signal. This pre amp is fitted to all our large stage systems and has always met with huge approval; not only from the guitarist but also the PA operator who is so happy to have constant level and tone presented to his mixing board. source
There are cases where a buffer is not desired. Certain pedals, such as wahs and fuzz faces, are extremely responsive to the direct signal and impedance from your pickups. They like to be driven directly. This is a case where you want to NOT have a buffer before them. Also of note, is that too many buffered pedals in a chain can also cause undesired effects on the guitar sound. Some people even prefer the "less-bright" sound that comes from such signal loss.
Harmony-Central Effects Explained - extremely useful and in-depth explanations of the main effect types.
Build Your Own Clone - one of the best ways to learn about effects pedals and electronics is to build your own. BYOC offer kits to build your own effects, from a simple booster to a complex Boss VB-2 Vibrato Clone
Aron Nelson's DIYStompbox forum - currently one of the THE top forums for DIY pedal building. Even some big name builders post there. You can learn nearly everything you want to know about effects pedal building from the FAQ, Schematics, Links, and forum members contained here.