In our unending quest to forge a guitar tone that sounds like somebody opening the Ark of the Covenant inside a tyrannosaurus, many of us will eventually find ourselves considering the possibility of using more than one amplifier.

As the more mathematically astute amongst you will doubtless be well aware, employing just one additional amplifier will give you precisely twice as much potential to irritate people as you’d get with a single amp rig. That being said, it is an enterprise that is not without its challenges. How should one split the signal from the guitar? Which combination of amplifiers is best? What if the electrons get into a fight about who goes to which amp, causing widespread unrest and undermining your precious tone? Well, without getting too bogged-down in “science” and “facts” and “electricity doesn’t work like that, Nicholas”, let’s go ahead and examine some of the basic considerations for any guitarist looking to run more than one amplifier.

Splitting the signal

When it comes to splitting your guitar signal to feed the inputs of two different amplifiers, the temptation is to just take a $5 Radio Shack “Y” cable and MacGyver that bad boy in there. You will get some slight attenuation of the signal, similar to backing off the guitar volume a touch, but this method is probably the easiest way to dip your proverbial toe in the proverbial water. Dipping of proper toes in actual water is not encouraged whilst experimenting with amplifiers.

A rather more sturdy method would be to buffer the signal with some intervening device. A simple option is to add an effects unit with stereo (or dual mono) outputs at the end of your chain, with one output going to each amplifier. Stomp boxes that commonly feature stereo outputs include chorus, delay and reverb.

However, depending upon your requirements, you should ideally deploy a gizmo that caters for negating the load splitting of running into two inputs, allowing you to change the phase of one of the outputs, and which offers an isolated output to prevent ground loop issues.

That’s quite bag of technical cats to suddenly put amongst your experimental pigeons, so let’s take a closer look at some of the issues that such a device would help avoid.

Common problems

There are always a heap of things that can go wrong with any guitar rig, but when it comes to using more than one amplifier, I’m going to focus on two of the most common problems:

Ground loops

The dictionary defines a ground loop as: “an unwanted electric current path in a circuit resulting in stray signals or interference, occurring for example when two earthed points in the same circuit have different potentials.” If, like me, you failed high school physics, now would be an appropriate time to say “Oh. Interesting…” in a thoughtful voice, before deftly changing the subject. In any case, for our purposes we can define a ground loop thus: “an obnoxious hum in your rig, probably caused by a demon or something.” The best way to get rid of this hum – short of perhaps an exorcism – is to isolate one of the amps from the ground (route to earth) of the other.

UNDER NO CIRCUMSTANCES should you try to be clever by removing or “lifting” the line ground from one of your amps. In a fault scenario, the resulting electric shock could very easily kill you stone dead. Thwart the nefarious electrickery of the Reaper – use a buffering device or isolation transformer instead.

Phase cancellation

This is where things get a bit hectic and I have to draw a graph with some wavy lines on it, from which you can use a jolly expensive calculator to “extrapolate” the “amplitude” and determine the “sinus wave” or something. In any case, the upshot is that if the invisible sound bacon of your two amplifiers collide at a disagreeable point in their oscillations, you will experience what is known, in technical circles, as a “meh.” This usually presents itself in the form of sub-optimal roar, but can quickly be thwarted by reversing the phase of, or adding a minuscule delay to, one of the channels. Rather than “fighting” each other and canceling out the signals to make the amps sound thin and reedy, the pair will sing harmoniously.

Typical multi-amp configurations

So, let us assume that we have got our signal split, with our amplifiers happily wed, free from any irritating noise save that which we put there intentionally with our twangplank or rumbletruncheon. What then, are the basic options for actually belching forth great slabs of music from this multi-mouthed monster?

A/B or A/B/Y

A/B is the most basic configuration. You only run one amplifier at a time, switching between the two with an A/B box. A typical ploy would be to set one amp up for your clean or rhythm sounds and the other for your dirty or lead tones. George Lynch has been known to favor this method (he even has his own signature pedal for such capers), and many other players switch between different amps for different sounds.

A/B/Y is the same as A/B but you also get to run both amps at once. This gives you the variety of textures possible with differing gain, EQ and voicing as discussed above. An A/B/Y box will give you the choice of running each amp individually or both together.

If you have a posse of enormous roadies, you could even go all Mark Tremonti and have a couple of amps for your clean tones (often Fender combos, in his case) and then an A/B/Y setup for your heavy rhythm and leads. Tremonti seems to favor a Mesa Triple Rectifier mixed with one or more from a gnarly lineup that has included Bogner, Dumble and Cornford.

Stereo

Whether or not stereo rigs are practical for most of us is an entirely different discussion, but it is, without doubt, an enormously fun way to use multiple amps. Those of us who started out using amp sims or Pods, with our signal drenched in stereo effects through headphones, tend to find the spread of a chorus or ping-pong delay especially gratifying. Stereo delay and reverb will give you an absolutely enormous sound when you’re sat in the sweet spot, although anyone standing slightly to either side of your speakers may not understand quite what all the fuss is about.

You will need a stereo effect unit, typically identifiable by it having twice as many output sockets as its mono brethren. Simply connect the left output of the effect to the input of one amp and the right output of the effect to the other amp. You will also need to make sure that the left and right channel separation is maintained via each stereo effect in your chain. If you run a lovely stereo chorus before an overdrive, for example, you’re only going to be able to plug one of the former’s outputs into the latter. This will cause your stereo signal to be smooshed (a technical term) into mono, tragically thwarting the juicy stereophony of your expensive wobblebox.

Wet/Dry

Wet/Dry rigs are often spoken of in hushed tones by those that feel like they don’t already have enough stuff to haul to their gigs. If some other bugger hauls your stuff to your gigs in an enormous truck, then you may even like to try Wet/Dry/Wet, or “none more wet” as nobody probably calls it. In simple terms, W/D and W/D/W rigs involve one amp being fed an uneffected guitar signal whilst the effects go to the other(s), typically with the dry signal removed entirely. The idea is that you still get your huge delays and reverb, but without the muddying of the main guitar signal.

In order to try this at home you will need to take a tap from your amp signal and split it off to your effects. This signal is then passed to a separate stereo power amp (or two mono amps) which can output the stereo effects into your “wet” speakers. If your amp has an effects loop and you are getting the majority of your tone from your preamp, you might find that you can run the effects send out to your “wet” stereo setup. Failing that, an amp or power-soak’s direct-out (DI) can sometimes work. Alternatively you can add a second mic to your “dry” cabinet, add stereo effects to that and run the resulting signal direct to the board, à la Eric Johnson.

What amplifiers should I use?

Most of us probably don’t start out with a vast array of amplifiers to choose from. In all likelihood, you already have a favorite amp that you have sworn you will never part with, perhaps funded by the knee-jerk eBay sacrifice of the previous amp that you swore you would never part with. So, how does one go about finding it a suitable mate?

Conventional wisdom suggests that the raunchiest of tones can oft be achieved by combining amplifiers that each feature different kinds of output tubes (or voicing, in the case of solid state). For example, any combination of 6L6 or 6V6GT; EL34 and EL84 tubes – typically characteristic of Fender, Marshall and Vox respectively – has the potential to produce a mighty alliance. Marshall and Fender often work well together, as does the high-end of a Vox with the bottom half of a Fender.

“If I may I interject?”, you interject, “What if I have hitherto maintained a brace of amplifiers with thermionic valves from the same sub-species? Is it imperative that I append new apparatus that exhibits an alternative tubular orientation?”

That’s quite the vocabulary you have there. In any case, you can definitely create splendid tones with similarly voiced or even completely identical amplifiers. For example, you could run both amps with wildly different gain and EQ settings, or you could perhaps apply a preposterously short (around 10-20m/s or so) delay before the input of the second amp to give you an agreeably plump “double tracked” sort of sound. This is actually what I often do, which is probably why I’m going to such pains to justify it as a valid option.

“But,” you persist, “what if I don’t want to run both amplifiers at the same time?”

Well that’s an entirely different kettle of fish. In an A/B situation you should simply choose whatever amplifiers make your desired noises by themselves. Employ the usual method for choosing an amp, but execute it as many times as necessary. Pro Tip: Ensure that the variety of sounds that you are after wouldn’t best be achieved using one amp with some pedals. Most pedals are significantly lighter and more portable than an amplifier.

“What about digital amps and modelers?” you further probe.

Modelers are typically the most practical (and affordable) way of experimenting with different “amplifiers.” Many systems, including the Line6 Pod HD series and Fractal’s Axe-Fx, will let you run virtual rigs with two amps, usually allowing you to pan each one to a different output. If you own such a unit, try creating a two amp rig with the amps panned hard to each side. Then plug each of the unit’s outputs into a different power amp, monitor or just into each side of your headphones.

If you want to get really zany, how about running one amplifier on stage, but splitting your guitar signal into your digital modeler, smartphone or tablet and running a virtual second amp into the desk?

In conclusion

I have vomited a grotesque puddle of words already, but there are still all sorts of factors that I haven’t touched upon: Multiple amps in a studio environment, daisy-chaining amps, multiple amps sharing one cabinet, plying the venue sound engineer with enough liquor that he’ll agree to mic more than one cabinet, the contempt of your bandmates as your preposterous paraphernalia occupies two thirds of the stage… tips on how to find a good chiropractor. There are just so many variables to consider.

In what ways have you used more than one amplifier? Do you find the benefits outweigh the technical and logistical challenges? Do audience members regularly compliment you upon the unparalleled enormity of your invisible sound bacon?