During the 1960s and ’70s, many manufacturers marketed their amps on the basis of wattage. One amp could look like another, but if it didn’t have twice the wattage, how could it possibly be better? And the stats included in manuals and ad materials made meaningful comparisons difficult, with wattage expressed in various ways, including peak, RMS, and continuous power. Eeek! In retrospect, I can understand why salespeople gave an annoyed or a blank look when asked, "What kind of watts?"
As if it mattered. What really matters is how your speaker and amplifier circuitry work as a system to deliver the tone and volume you want, and provide a certain degree of reliability.
A powerful amp can indeed be a wondrous thing. However, what you hear isn’t the amp, but the speakers converting electrical energy from the amp into sound. More specifically, you hear the speakers, their enclosure, and the environment in which they’re operating.
Before we go any farther, let’s see a show of hands: How loud is a watt? Before you answer, ponder that the typical household lightbulb is rated at 60 or 75 watts. How loud is that? Yeah, it’s a trick question. So, what are watts, anyway? A watt is a measure of power. Applied to amplifiers, it means a given amount of power when delivered to a load of a specific impedance. In terms of speaker ratings, it means how much power can be poured through the speaker without it going ka-boom.
It isn’t just watts you’re interested in, if you want loudness. It’s SPL, short for sound-pressure level. An SPL rating doesn’t give a rat’s tail whether an amp has tubes or MOSFETs, is stuffed with one 12" speaker or four 10s, or has cool grille cloth–just how many decibels are produced at a given frequency when a specific signal level is fed into the speaker (the sound’s decibel level is measured a specific distance from the speaker enclosure–typically 1 meter). Usually, this kind of measurement is employed for high-fidelity gear such as P.A. speakers, studio monitors, etc.–not guitar amps–because another factor is included: total harmonic distortion. Whereas a lack of harmonic distortion in sound-reinforcement or recording gear is desirable, in a guitar amp it can make the tone seem dry or cold. Bottom line: Trying to compare power ratings and clarity among amps and recording gear is a tricky and unrewarding game.
Speakers can be designed to handle just about any number of watts. They also come in different frequency-handling flavors. That is, some are better suited for highs, lows, or what-have-you. Some are intended to operate over the full human-hearing range of 20 Hz to 20,000 Hz. Still others have resonant peaks and dips that make them sound better for, say, crunchy rock riffing than mellow jazz chording. However, a speaker fresh out of the box is sort of like a heart or brain without a body. It won’t work by itself (in fact, if you run an amp into a speaker that isn’t mounted in an enclosure, you can cause it to quickly fail). The raw speaker’s performance is radically affected by the type of enclosure it’s placed in. Take a 12" speaker and stick it in an open-back enclosure and then in a closed-back enclosure with the same dimensions, and it’s going to sound way different when driven by the same amplifier with identical settings. Part of this is because every speaker has a free-air resonant frequency, the frequency at which it vibrates most easily when it isn’t mounted in an enclosure, and putting the speaker in an enclosure changes that frequency. Simultaneously, the air in a cabinet acts as a cushion against the speaker cone’s movement, creating "damping." The volume of the cabinet and whether (and how much) it’s open combine to give the speaker its body and tone.
A basic guideline is that the bigger the enclosure, the lower the resonant frequency (and the greater the bass response). Of course, then it’s a matter of balancing the "tightness" of the tone; a smaller closed box provides a tighter sound, whereas a big box doesn’t give as much "snap" to percussive attacks. These are, of course, only vague parameters. Clever designers turn to an amazing number of tricks to coax a blend of efficiency, durability, tone, and output from a speaker/enclosure combination.
Which brings us to the subject of replacement speakers. Mercifully, speakers are expensive enough that guitarists don’t rip out perfectly good ones from their amps and replace them with the latest, hottest, loudest models endorsed by today’s favorite shredder. Otherwise, pickers would be replacing speakers left and right and finding yet another reason to be dissatisfied by their gear.
Speaker manufacturers sometimes design in conjunction with amp makers, or an amp maker may design an enclosure around a particular type of speaker. Sometimes it’s just a happy coincidence that a certain speaker and enclosure sound good together. More often, though, it’s a matter of painstaking "blindfold testing" in the enclosure’s development stage to get the most tonal bang for the buck. Note that efficiency isn’t emphasized quite as much as tone. And that’s good for you, because if your speakers can handle the 10,000 watts that your Kill-O-Matic amp delivers, but it sounds like a sack of potatoes being dumped on a kitchen table, then what? Okay, it’ll be loud, but it’ll suck, too.
Part of the charm of vintage amps is their old, funky speakers, some of which were designed for radios and hi-fi systems, and not specifically to make guitars sound good. A lot of the speakers are unimaginably inefficient and tonally challenged, by today’s design standards. But the amps sound great. Beefy or midrangey, bright or dark, each has its own sonic "fingerprint." So, what do you do if you should inadvertently blow up one of those old speakers? Good question. You can have it rebuilt, or you can replace it. That’s pretty much it.
Baby your speakers! If you have an open-backed cabinet, don’t use it as a storage locker for your cords, wah-wah pedal, string winder, and other small objects. One bad bounce in transit, and these objects can rip through the speaker cone, costing you tone and money. If your speaker is held in the cabinet with capture nuts, check that they’re tight. If you can’t turn them with your fingers, they’re probably okay tight enough; it’s the ones that have loosened enough to let the speaker rattle that should be tightened. If any need tightening, don’t over-tighten them! Just make them snug.
Keep this in mind: You only have to be loud enough to be heard. If you like the tone your amp and speakers give you, then don’t muck with them. If you’re not loud enough, whether to hear yourself or to bathe the audience in your tone, don’t look to speaker replacement as a way to increase volume. Get a second amp or an extension cabinet (if your amp can safely drive one), or put the amp up on a chair so that you can hear it better and it has more projection. If you’re playing in big halls, the sound people are going to plop a microphone in front of your speaker and put your sound through the P.A. system, so don’t sweat it.