Your rock 'n ' roll heroes may go on-stage with half a factory in amplification, but if your auditorium is a suburban sitting-room, a practice amp is more likely to fit the bill.

INVESTING IN AN AMPLIFIER for your first electric guitar is not in uplifting affair. The hallowed tones of Johnny Famous do not emanate from a £50 combo with a four-inch speaker - a painful truth soon realised at the music shop.

This assumes you feel it necessary to indulge in buying an amp in the first place. My local dealer recently recalled (with some glee) the aspiring axe hero who felt an amp to be surplus to requirements. After buying a brand-new Strat, our Mensa candidate indulged in a spot of DIY electronics. Removing the jack plug, he wired one end of the guitar lead to a standard three-pin plug. Plugging his Strat into the mains resulted in a pretty "hot" guitar and a hefty repair bill. The moral? Plugging your guitar into an amp instead of the mains increases your chance of survival by a considerable margin.

Having decided an amp could come in handy, if you're just starting out, money will no doubt narrow your choices. At your local dealer, Mr Commission will probably advise shelling out a couple of hundred quid for a "proper amp". However, even if the Megablast Deluxe 400 is the amp to have, most of us have to settle for something a little more modest.

As a bedsit strummer, I've got through a number of small amps, each with its own endearing and infuriating features. My first acquisition was a flaccid eight-watter inflicted on an unsuspecting public by Laney. Although it had all the musical sophistication of a kazoo, it was capable of the most incredible distortion and feedback when turned up to its full volume level of eight. (You could tell it wasn't a real amp - it didn't even get up to 10!) With this beastly rasp now part of my sonic arsenal, I and a number of equally inept friends would merrily thrash away for hours. Our version of Bowie's 'Jean Genie' sounded like the Jesus & Mary Chain, and it was still only 1982!

Feedback is a likely occurrence with a small amp, as you're more likely to be playing at full whack. The resulting banshee wail is either an exciting addition to your sound or an annoying intrusion, depending on your taste. To master the art of feedback - or to strangle it at birth - you first need to understand it. Feedback is caused when the pickups on your guitar receive the sound from the speakers and feed it back through the amplifier repeatedly. If you experiment with your setup, you'll find that feedback is more likely the nearer you are to the amp and if your guitar and amp are directly face to face. (Why do Heavy Metal guitarists use feedback? Could it be that they're so busy pulling serious rock 'n' roll poses, they keep standing in the wrong position?) Anyway, with a bit of practice it's not too hard to coax some feedback out of a small amp.

All well and good if that's what you're after. But if feedback ain't quite your bag, there are a few ways you can go about trying to reduce its likelihood. Use a long lead, and stand to the side of the amp, angling your guitar away from parallel to the speaker. The natural resonant qualities of hollow-bodied guitars make them more susceptible to feedback than solid-bodied types, but there are a number of options apart from filling your beloved 355 with cement. Some "experts" recommend removing metal pickup covers as a way of reducing feedback from hollow bodies. Since the covers are usually magnetic steel, they are capable of generating a current when vibrated by the top.

By far the most effective way to reduce the feedback tendencies of a hollow body is to cover the f-holes with a tape or some plastic, or stuff the body with rags, foam rubber or something similar. Beware, though, that these "customised" alterations will interfere with normal resonance and change the tone of the instrument.

Whichever preventative method you choose, be careful not to get glue on the finish of the guitar, and if you're inserting sound-deadening material into a hollow body, avoid contact with the internal wiring. You don't, after all, want to imitate our friend with the "hot" Strat.

So shriek freak or not, feedback is a phenomenon you've got to consider when dealing with a small amp. Yet assuming you can overcome/learn to love the effects, amps of relatively low output can hold up well even alongside larger models; those with valves, for example, only give their best when driven hard. And with the right small amp, a high-quality sound can be achieved at quite low volumes. Small Fender amps such as the Champ and the Princeton are classics in this respect. My late lamented Laney, I fear, was not.

Despite their low power output, some small valve-amps are frighteningly loud. I once owned a neanderthal 20-watt combo made by Park, which when turned only half up was guaranteed to cause a serious breach of the peace. The problem was that, even at half volume, the background hum was appalling. It had more snap, crackle and pop than... well, it wasn't very good anyway.

Herein lies another bugbear for users of small amps. Noise (the unwanted kind, that is) abounds in cheap equipment. If your guitar has pretty basic pickups and produces a weak signal, then when the amp boosts that signal it will also boost a lot of noise. One possible remedy is to buy an external pre-amp, which boosts the signal before it reaches the amp. This was only half a solution for me: a pre-amp reduced the noise and allowed me to overdrive the amp for some nice distortion, but it also made the amp louder than ever. Anyhow, part of the noise was due to the general buzziness associated with valve amps. Technology has moved on slightly since then, and some modern valve amps have a hum-balance control to minimise the effect. Using a noise gate can be a smart move, too.

The dodgy old Park is the only valve amp I've owned, and although transistor ("tranny" in rockspeak) amps do not generally suffer from hum, the more basic models lack the tonal quality of valve amps. There are plenty of amps (of both tube and tranny varieties) at 20 watts or so that can produce a rich jazz sound. Jazz musicians will often choose a tranny amp for exactly the same reason a rock or blues player would avoid it - distortion-free clarity. The problem is that most practice amps lack the tonal sophistication in the midrange (ie. the bit that's neither bass nor treble) to accommodate players with jazz leanings.

My Marshall Reverb 12 is a good case in point. Nice little combo - compact, fair overdrive, reverb. But the midrange punch is Joe Bugner when it should be Mike Tyson. In fact, it sounds so thin I don't think I've ever turned the treble more than half up, and I always use the neck pickup on my Les Paul copy. While Marshall's engineers have probably slaved for months in pursuit of the perfect valve-imitation overdrive, scant attention seems to have been paid to the basic tone of the amp.

But enough axe-grinding for now. Practice amps vary wildly, and it's wise to try as many as possible before buying. They're clearly designed for a limited purpose (you can't even sit on half of them) but remember, nice things come in small packages. On the other hand...