Post
by Eben » Thu Feb 12, 2009 12:41 pm
I warned y'all...
Yeah, D, we can cover acoustic amps too – They are predominantly solid state, (There are exceptions, the Rivera Sedona comes to mind), so there’s less to tweak for us common folks, but they’re still good fodder for discussion!
OK, let’s start with way basics: What does an amp do? Fundamentally, it takes a very weak, low power signal and jacks it up to everything from nice and clean to Zack Wylde and everything in between. Your electric guitar takes vibrational energy from the strings and translates that to an electric signal by way of your pickups. The current within your guitar is very small for obvious reasons, (A guy fries quick, struck by lightning), while the current within your amp can be and is quite high indeed, in fact lethally so; safely taking that electrical value from A to B is really the amps biggest chore.
The next question that usually comes along is, “OK, so why are there different amps for acoustics versus electrics, keyboards, etc? Well, in a nutshell, yes amps make the signal louder, but they also have to account for what range and combination of frequencies the instrument needs to have reproduced. Whereas an electric guitar amp needs to account for further sound shaping, (Effects, distortion, etc), an acoustic guitar amp is designed with keeping the signal as faithful to the original as possible.
So, how do they do it? There are three basic components to an amp; the pre amp, the power amp, and the speaker. Nowadays, a lot of amps have built in effects, which just means another sound shaping layer is added to the signal chain. Other than these, you have controls that allow you to tweak the various aspects of your amp, and a cabinet to house everything – really, it’s that simple. In very simple terms, the signal from your axe is gonna enter the amp through a cable, then hit the pre amp, then the power amp, and then the speaker, and if you have onboard effects, like reverb for example, the signal’ll hit that right after it passes through the pre amp phase.
So, on to the components, and again, we’re keeping this simple and brief. The pre amp is there fundamentally to juice the raw signal out of your axe; again, what comes down the cord from your Strat is a tiny, weak signal, so much so that you just can’t pass it to a power amp and expect stuff to happen: This section is very important in the tone shaping world too, though, because it is here that things like specific kinds, brands or derivations of tubes will have tremendous impact on your sound. Remember I mentioned my amp uses 12AX7s in the pre amp phase? Those are pretty legendary; loved by some, disdained by others. Seriously, when you get into tube jargon, it really does start to sound like you’re at a wine tasting; “Warm syrupy mids with sparkling highs,†and stuff like that… The amount of air in the tube, the type and size of plates used, all of this will impart different tone characteristics that you may or may not want.
Onward! Once that signal is boosted by the pre amp phase, it’ll go through any onboard effects, (And let’s face it, even rudimentary amps usually have at least a reverb section, and that is in fact an effect!) Once it’s through there, it’s on to the power amp which is where all your good clean and dirty fun comes from.
Now, amps are generally split into solid state and tube categories, but truth be told, there are quite a few out there that really are hybrids of the two. In any case, you hear guys like Dennis and Vince and me who are tube only guys; so why and what are the fundamental differences? Well, at the most basic level, a tube amp uses vacuum tubes, (AKA dinosaur technology), to amplify the signal we’ve been discussing, while solid state uses diodes, transistors and other little naught bits to achieve the same end. One thing that occurs to a lot of folks at this point is the thought, “Well, if they do the same thing, then why are folks so fired up about one or the other?†and therein lies the rub; while things should be the same, they ain’t and that’s why folks are drawn to one camp or another. I saw a show the other night about the largest record pressing plant in L.A. They had gone from big highs in the 60s to serious lows in the 80s and 90s to…. Now they are pressing as much or more than they did in their heyday; why? ‘cause some folks dig that sound, and CDs just don’t cut it. It’s why Jackson Browne’s studio has legendary tube effects and not computer based stuff – They sound different, and for some of us, that difference is what we’re after. It is a general truism that solid state can and are made lighter than tube amps. The same can be said of power output versus use, in other words, a 15 watt tube amp probably sounds as loud as a 30 watt solid state without going overboard; just another one of the subtle but important differences.
The hybrid amps are simply a derivation of these two; a lot of them will employ tube pre amp sections and solid state power amps. Most do that because the predominant tone shaping is done in the pre amp phase, and because solid state power amps are cheaper, tougher, and again, often lighter than their tube brethren. There really is not a ton of difference between a solid state and a tube power amp section, but I am here to tell you that one can hear the difference. I can’t tell you exactly why I sold my Tech 21 Trademark 60, (A very nice semi-boutique solid state amp), but I’d say that while the clean was clean and the dirty was dirty, it sounded like it was the diet version of what I was after; pretty much there, but not leaving the right taste in my mouth… Nowadays, tube emulation circuitry is all the rage, but once again, you don’t need to be a complete purist or tone snob to be able to tell the difference; when real versus emulation is pointed out to you, it’s obvious as all get out!
Within tube amps, you’ll hear terms thrown around like Class A, Class B and Class AB. Without drawing diagrams with little squiggly things on them, in general, things work this way.
Class A amps mean that generally, they are always putting out a fair bit of juice; the hip term is that they have large quiescent current, meaning the current level in the thing when it is producing no output. You’ll hear folks refer to Class A amps as running “Wide open, pedal to the metal, etc,†and that’s why. Class A amps vary that big ol’ quiescent current to create a varying current and load, and as such, they ain’t exactly what you’d call paragons of efficiency.
Class B differs fundamentally by changing that quiescent current (AKA the bias level), so that they are not effectively running flat out all the time. While this arrangement brings advantages in terms of efficiency, it all so brings more challenges! In a Class B amp, the tube inputs are linked by pairs of diodes. The problem is that we’ve brought a new pair of things into the mix, and differences therein can cause unwanted reactions. ‘Bias’ as we use it here refers to the nature of the current flow around and through your amp; it therefore makes sense that if the bias is maladjusted or not to your liking, the sound you get won’t be what you want. “Adjusting the bias†basically means setting the idling current in the power tubes, and then doing so for the surrounding voltage conditions when one can do that. At issue is the fact that all tubes are different, and especially brand to brand and over time. Some amps have circuits to do this automatically, and they work OK, but nothing’s perfect, of course. Some have bias controls, (Certain Fender Twins come to mind), and that might be giving more leeway than intelligence dictates, ‘cause after all we’re talkin’ about guitar players here, right? What all this means is that Class B is not necessarily a better mouse trap; if all we were after was a more efficient amp, then yes, all is groovy, but practically speaking, to really drive these bad boys, voltage drops across them diodes needs to be exactly the same, and if it ain’t, welcome to “Crossover distortion: Y'all know as well as I do that this ain’t ever gonna happen. Last but not least, Class B requires a significantly greater input signal and the output current is only gonna be about half the waveform, so we got another exercise in compromise!
Class AB lies somewhere between these two examples, and again, while fixing some things adds others – There just ain’t no free lunch.
Well, as Clint Eastwood says when he directs, that’s about enough of that; for now!
E
‘I love America - I just dont know how to get there anymore.’
John Prine
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