Whether you're a relative beginner seeking to expand beyond your 'starter pack'1, or a more advanced player who's only just really starting to think beyond technique and getting into the realms of gear, one thing is certain: if you play electric guitar with any level of seriousness at some point you'll need to go amplifier shopping for the first time. It is probably more important to upgrade your amplifier before upgrading your guitar. A great amplifier can make a bad guitar sound good, but a great guitar played through a bad amplifier will always sound bad.
Where To Start?
Buying an amp can be a bit daunting at first, even if you're only seeking a small, simple amp with a built in speaker. There are a lot of decisions to make: valve or solid state? How much power is appropriate? Head and cab or combo? Analogue or digital? New or second-hand? There is a wealth of information out there, but the thing to remember is that 95% of it is opinion. The only person who can really judge the correct amplifier for you is, well, you.
You should be prepared to be flexible and 'think outside the box' a bit. While many articles (this one included) will tell you certain types of amps are better for certain applications, sometimes you might get a really good sound out of something that conventional wisdom tells you is all wrong for the job. For example, Duane Eddy uses a 150W amp with a single 15" (380mm) speaker... something a lot of people would tell you is entirely wrong for twangy rock 'n' roll!
It's also very important to pick a budget and stick to it, especially if you intend to buy from a guitar shop. As with any business, they need to make money and will often try to sell you something far more expensive than you actually need!
Valve or Solid State?
At one time all electronics were based on circuits containing thermionic valves2, over time these were replaced by transistors, which now in turn are being increasingly replaced by integrated circuits3. But there is one area where both valves and transistors are still in common use: guitar amplifiers.
Broadly speaking both valves and transistors do the same job, they take a signal and amplify it. The more valves or transistors in the circuit, the more amplification there is. A simple amplifier will probably only have two of whichever component. One in the pre-amplification stage, which brings the guitar's actually quite weak signal up to a usable level. Another one will be found in the power amplification stage where the signal is further boosted to a level that will 'drive' the speakers4.
But what anyone really wants to know is, what differences are there in the sound?
There is an old cliche which states that every good guitar sound you've heard came from a solid state amp, whilst every great sound came from a valve amp. This is broadly true, but often has more to do with the guitarist than the amplifier; Jimmy Page would probably be able to sound exactly like Jimmy Page on all but the cheapest solid state amp! Also, certain sounds suit one amp type better than the other; thrash metal5 generally works better through a solid state amp, whilst anyone wanting to play blues or classic rock should ideally be looking at valves.
It is said that valves sound 'warmer' than transistors; the sound is less 'artificial' than solid state, they respond better to variances in instrument volume and 'attack'6 and have that lovely sounding distortion you can only get by overloading the valves. However, they are also more expensive to buy (especially new), and more expensive to maintain as these days only a very few factories worldwide still make valves; you will need to replace the valves in your amp at least every couple of years, unless something goes drastically wrong you should never need to replace a transistor. Valve amps can also weigh an awful lot - mostly due to the large input and output transformers used in them - which can be problematic if you carry all your own gear.
Another option is hybrid amps, which utilise valves in the pre-amp stage and transistors in the power amp stage. Depending on who you ask, this is either a pointless compromise or the best of both worlds.
After deciding whether you want to go for valve or solid state, the next important decision is choosing the power of the amp. Generally speaking the more watts it has, the louder it will be, but this will also be limited by how many speakers you are outputting through and the old roadie's7 rule of thumb is that one valve watt is the same as five solid state watts. Obviously the actual maths and science are more complex than that, but it is true that if you were to run a 50W valve amp next to a 50W solid state amp using a set up that was otherwise identical the valve amp would easily drown out the solid state. However, you are also able to get more overall power out of solid state electronics, hence why powerful bass amplifiers (which need more output to be heard over the guitar) are always solid state or hybrids.
The biggest thing to consider is where and how you'll be using your amplifier. If you're only ever intending on playing blues licks at home for your own entertainment you won't ever need a 200W all-valve monstrosity, wheras if you want to play raucous hard rock in pubs and clubs a 10W solid state practice amp will just get you laughed off stage!
Head and Cab or Combo?
You will often see amplfiers described as either a head or a combo. A head is the amplifier chassis mounted in a self-contained unit which must have separate speaker cabinets connected to it via speaker cables8, wheras a combo is the amplifier chassis and one or two speakers all contained in one convenient box.
Whether you want a head and cab or a combo is really a question of space and practicality. A combo with a single 12" (305mm) speaker will obviously take up much less room than an amplifier head sat atop a speaker cabinet containing four 12" speakers!
In real terms there is little difference between a 50w combo amp with two integral speakers and the same amp as a head attached to a cabinet with two speakers. The latter does have the advantage that it is easier to add more speakers by using more or bigger cabinets (most amp heads will support two speaker cabinets). With a combo though you never have to worry about impedance matching9 or power handling10 as the speakers it comes with should always be the right ones for the job!
Any good speaker enclosure, whether part of a combo or separate cabinet, should be sturdily build from good, thick plywood. MDF is sometimes used, but is not generally desirable in a guitar cabinet as it is much heavier than plywood and does not resonate as well. The sound made by any speaker cabinet, whether a separate unit or built into a combo, is also affected by whether the cab is fully enclosed or open-backed; enclosed cabinets tend to produce more 'bottom end' (bass frequencies). If you do get an enclosed cabinet make sure the back is made of plywood as this will be less inclined to move than less sturdy materials.
Analogue or Digital?
Most people will say analogue, every time. This is not snobbery as much as practicality. Analogue amps are simpler and thus there is less that can go wrong. While digital amps offer all kinds of inbuilt effects and 'modelling' (usually simulations of famous models of valve amps), once you get to the price bracket where the models actually sound the way they should, you might as well be buying a classic valve amp anyway! That being said, a good digital modelling amp can be extremely useful in a cramped studio where it might be impractical to fit a full stack or two.
New or Second-Hand?
Really this depends on what you want out of the amp in terms of both sound and bragging rights. New amps offer advantages in that you know they haven't been 'repaired' by less-than-skilled hands, and you'll be able to get support directly from the manufacturer. Second-hand amps can, depending on make and model, be either wonderfully cheap or ridiculously expensive but due to different construction methods will often be easier and cheaper to maintain yourself. In either case it's best to try before you buy, to make sure the amp will do what you want it to and to check whether the controls are noisy when adjusted or if there are any unwanted hums, pops and crackles (this being especially important with second hand amps). If money is no object you'd be hard pressed to find anyone who won't advise you to go for an amp made in the 60s or 70s by Orange, Marshall or Sunn. Whereas if your budget is pretty tight it is well worth looking for a Peavey amplifier made in the 80s.
Trying It Out
If you can, always try before you buy! Take your own guitar with you, as you'll always get a better impression of the sound quality, if using your own instrument. When first trying an amplifier connect your guitar directly to it, with no effects or anything else that may colour the signal. Turn the volume and tone up to maximum on the guitar and if the amp has two or more channels make sure you are using the cleanest one. Pick your open strings one at a time, this will immediately tell you how the amp, and the speakers, respond to various frequencies and will help you work out how to initially adjust the amp's equaliser (EQ) controls to get your desired sound. The EQ is usually just three knobs marked High, Middle and Low. More complex equalisers are usually found as separate units. Next play a few chords and a scale or two. Listen to the sound, is it particularly harsh on the high notes or too muddy on the low notes? Again, adjust the equaliser to taste.
If you can, stand well back, especially if the speakers are at floor level; you don't have ears in your shins! Next try the 'lead' (also called 'dirty' on some more rock-oriented amps) channel if there is one, this is where any inbuilt distortion kicks in. Fiddle with the gain and master volume and listen to the sound, is there a point where the sound becomes too harsh to be really usable for anything other than obscure noise rock? Does it feed back too easily11? Sometimes an amplifier's inbuilt distortion can be very unsatisfactory while the clean sound is a thing of beauty... that's the point where you may want to start considering the use of a separate distortion/overdrive effect if you don't already have one, but that's a subject for another Entry!
Go Forth And Amplify!
And so with this knowledge you can confidently go out and buy a new (or second-hand) amplifier. Have fun, explore all possibilities and try not to annoy the neighbours too much!