The effects loops on guitar amps were created to let you insert certain types of effects into your signal chain after the amplifier pre-amp section, so any overdrive or tone shaping generated in the pre amp is incorporated in these effects. Depending on the effects used, the resultant sound can be drastically different depending on if it was put in through the effects loop or through the input of the amplifier. When people talk about effects (FX) loops, they would either be referring to a parallel FX loop or a series FX loop. The difference hinges upon how the amp routes the guitar tones.
The serial loop would run the effects in line: the guitar tones enter the input, flow through the pre-amp section, then through the effects loop, going out the effects loop send and returning on the effects loop return. Then the signal from the return of the effects loop goes through the power amp section, and finally comes out through the blaring loudspeakers. This basic loop is viable for most applications.
The parallel loop is located at the same place in the signal chain as the serial loop, but it splits the signal after the preamp section and puts one part of the signal through the effects loop send and the other part, which is unaffected, gets summed back together with the return signal of the effects loop. Often times there will be a mix control on a parallel loop allowing you to adjust the level of the returning effect loop signal mixing with the amp’s unaffected original signal output from the preamp. Parallel loops are less common, but can be an effective tool with heavy effects or noisy or lower quality effects. Because the parallel loop is a mix of the effect and your original signal, your tone is not compromised or altered as much, and you can adjust the amount of effect you want.
The majority of effects fit in these categories: filter (envelope followers, wahs, auto-wahs), gain (overdrive, distortion, fuzz, clean boosts, compression), or time-based (delay, chorus, reverb, phase, flange). While each of these help you get cranked up tones at the stomp of a pedal, each type works its magic in a different way, and even effects from the same category can have varied functions. Here’s a breakdown of each effect to demystify its workings and the results of putting them either up front or in the effects loop:
Besides the faulty pre-amp channels and old cramped amps, a fuzz pedal is the grandfather of the dirt boxes. It was initially intended to help the guitar player mimic the reedy, raspy intonations if a saxophone. It creates a vintage fuzz tone; a slightly rounded, wooly, and warm yet sparkly distortion in the guitar signal, to render more girth, meat, and sustain in the sound. Like a sixties barber, everything that goes in to it comes out with a neat flat top! More imposing pedals control the signal and alter it based on the specific synthetic requirements, while usually pedals which are more playable let you keep some aspects of your feel, dynamics, touch, as well as the core tonality.
When working with a dynamic fuzz pedal, you can crank it to the max to create heavy distortion, or increase the tube amp gain to glean a gentle overdrive. Fuzz pedals which use a hollowed out germanium transistor work to unleash ungodly sonic chaos on your tone, but it’s mayhem with a warm, smooth, and furry heart. On the other hand, silicon transistor based fuzz pedals are renowned for their crisply defined and slightly harder tones.
Up front or in the loop? Fuzz is almost always in the front of the amp. It is not as useful to run your fuzz in the effects loop, because the output of the effects loop is usually too much for the fuzz pedal’s input. It can be put in the effects loop, but the result is not as useful. Typically you want to adjust your tone controls on your amp to maximize the output of the fuzz, but in the effects loop it would be after the tone controls and they would have little effect on the fuzz. The amp’s tone controls would be boosting the level of the input into the fuzz in the effects loop, making more fuzz tone, and not modifying the output tone of the fuzz.
2) Wah Pedals
Effect pedals might be the masters of audio wizardry, but nothing rivals the fabulous expression permitted by a wah pedal. While it was originally intended to mimic the sound of a muted trumpet, to produce an expressive crying tone, eminent guitarists, such as Frank Zappa and Jimi Hendrix, soon turned that sweepingly sweet “wah-wah” sound in to an imperative element of the rock and roll design. The voice-like quality and the human element of the manipulation of the pedal has carved a niche in our hearts (and on the pedal board). The other function of the wah pedal is to enhance certain frequencies by keeping the pedal in a single position, accentuating the "sweet spot" in the tonal spectrum of an instrument.
Up front or in the loop? The wah pedal is also almost always used in the front of the amp. This is because the wah affect is a form of EQ adjustment and works best on your clean guitar tone. When a distorted signal, which has huge harmonic spectrum, is put through the input of the wah effect it can become very hissy and narrow sounding and does not present as much wah guitar tone affect. Although, if this narrow, hissy sound is what you want then put it in the loop. Also, on two channel amps you may want the wah in the clean channel in the loop to adjust your amp’s tones to work with the input of the wah pedal. For the most part the wah pedal is an up front effect.
The intention of installing an overdrive pedal is twofold: to alter and smooth the slightly distorted sound of an overdriven tube amp or to provide a gain boost to “overdrive” a tube amp into distortion. However, most overdrive pedals do a little of both. Overdrive, derived from the sound of a saturated vacuum-tube guitar amplifier, are probably the most famous single breeds of pedals and the genre continues in the form of Boss, MXR, DOD, and other issued formative overdrives. Remember that a good overdrive is the foundation of any comprehensive pedal board, and the most useful tone twister at hand. One of the best amps to use with an overdrive pedal is the V3, as it offers pure tones that can go from smooth to shimmering to accommodate infinite sustain and crunch.
Up front or in the loop: Because of the overdrive’s twofold use it is great in both the front of the amp and in the effects loop. In the front of the amp, it can drive your amp’s preamp for more sustain. In the effects loop it works as a boost pedal with the bonus of a little more sustain for solos. This is why you may need two of these pedals.
4) Delay and Reverb Effects
The massive capabilities of a delay pedal, including clean signal reproductions, protracted delays, and the endless fun of up to 16 seconds of looping delay, have contributed to the fame of these. The delayed sound can be played back multiple times or between recordings to replicate the effect of a reiterating, dying echo.
As for a touch of reverb on your guitar’s signal, you don’t have to be surf rock legend Dick Dale to appreciate the friendlier, warmer, and twangier sounds produced by a smattering of the reverb, which infuses a touch of ambience into your tones. It simulates the reverberating sound of a guitar, played in a reflective, empty room. The majestic spring reverb pedals are what you should be after. The tones will almost make you drool.
Up front or in the loop: Delay and other modulation effects are useful both up front and in the loop. This is similar to the wah pedal. In the up front position, these modulation effects are affecting your clean original guitar tone. For chorus and flange modulation effects this concentrates the effect on the simple guitar tones. When in the loop it also affects the saturated distortion overtones that may sound cool, but also may dominate the overall tone. Also, modulation effects like chorus manipulate the phase of the signal, and using a parallel loop to mix it with the original signal can increase the effect. For reverb, it is almost always in the effects loop. This is because the reverb effect is simulating the sound of a room and if your guitar had reverb, but not the distortion, your guitar would sound like it is in a bigger room than your distortion was. Usually amplifiers with built in reverb have it located after the effects loop.
Effects loops are another tool for creating guitar tones and sounds. There is no right or wrong way to connect your effects, so experiment with them both up font and in the effects loop.