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Amp FactsArticle from Making Music, August 1987 |
Stewart Ward is a musician who got into electronics in the 1960s when he realised he couldn't afford to buy a Vox AC30. So he built his own, and gradually became more involved in designing and building amps. In 1980 he set up Axess Electronics and showed his first commercial product to the British trade. Now his famous Session series of combos is established and successful, as a glance at the Buyer's Bible will usually show. Stewart here outlines an informal and often controversial A to (more or less) Z of Amplification, as told to Tony Bacon.
"We get amps sent back for blown fuses. About 90% of the time the wrong type of fuse has been fitted, people don't understand the difference between F fuses and T fuses. F means fast-acting; T means time-lag. For mains, you must fit a time-lag fuse, otherwise when you switch it on the surge of the transformer will take the fuse out. So people send back amps with blown fuses and blame the amplifier."
"Capacitors don't have a sound. Resistors don't have a sound. It brings to mind something that happened in the early 1970s: the vintage guitar thing was just taking off, and everyone was replacing capacitors in the tone controls of their Fender Telecaster or Strat, and it had to be the Fender capacitor. Any old capacitor out of their radio and TV shop just wasn't good enough, it wouldn't give that sound. Which of course was rubbish. The capacitor doesn't know it's going in a vintage Strat."
"It's impossible to make two amplifiers sound exactly the same. The electronics can — that's easy to do. It's the electro-mechanical things that make them sound different, and particularly speakers. Who was operating the cone press that day? How much moisture was there in the air that day? There are all kinds of variables."
"A continually problematical amp becomes known as a rogue amp, a Friday afternoon job. They don't exist, actually, but I can tell you how that idea comes about. The guy takes it back to the dealer, and the dealer's got a little man who tries to repair it, and really hasn't got a clue. Keeps it for a week, guy comes in next Saturday, yeah yeah, all done for you. He takes it away, and it hasn't been repaired properly. Comes back in Monday morning — "the same thing's gone wrong". Shop says OK, we'll sort it out for you, gives it back to the same 'repairman', he scratches his head, takes the back off, has a poke around, puts the test meter across it — ah! dry joint. They always put that down to justify taking the back off and charging the £10 minimum service charge: you wouldn't believe the number of amplifiers that have dry joints. So, after six months the customer is absolutely irate, demanding his money back, and the maker gets to hear of it last. We eventually get back this amplifier, all the wiring is burnt through, the whole thing's an abortion. In a case like this, we replace the amplifier, write to the customer and the dealer, and send a copy of each to the other. That usually solves it. And all because of the dealer and his back-room boy who hasn't got a clue."
"A guitar can be handmade — it's something that's carved out of a block — but the components that go into an amp cannot be handmade. There are some people who, rather than doing a proper, serious job of manufacturing reliable equipment, will make one amplifier a week and charge £2000 for it. Which has got to be a rip-off."
"Most amp makers never quote what the mains voltage was when they did their spec-sheet measurements — was the mains voltage up or down that day? Was it 250 volts, 240 volts, or what? Mains varies. Plug your gear in at three in the morning when very few people are using the mains, and you might have considerably more than 240 volts. Some clever electronics engineer could measure the power at that time and get considerably more power coming out the speaker at that particular time of the day."
"On the Rockette 30 we say typical output power is 30 watts RMS into a pure resistive load. A lot of makers do a sum based on the voltage measured across the speaker — at what frequency? A speaker is a frequency-selective thing. But by quoting a resistive load, it doesn't know what the frequency is, it just sees a power being dissipated across it. It's the only meaningful way of conveniently measuring power. Other people just say it's '30 watts', or, worse, '30 watts music power' or 'peak power'. What you really need is an 'RMS' figure, which gives you an idea of what the thing is capable of doing on a continuous basis. So when you see a 'watts' figure quoted for power you should ask is it RMS, and is it across a resistive load? The other consideration is the speaker. The RMS power may be quoted honestly, and be the same for two amplifiers — but what speakers do they each have fitted that's turning that power into sound itself? Once you've got the RMS figure, it's only half the question answered — you still need to know what the average sensitivity of the speaker is, which is measured in SPL. For a 12 inch guitar speaker you should be looking for no less than 98, 99dB; the SPL of the average Celestion rock 'n' roll speaker is around 98, 99, 100 or 101dB. On the other hand, car radio speakers, for example, are all of very low efficiency, around 95, 96dB."
"If you were to ask a service engineer in this country which amplifier they'd had the most problems with, they'd say H/H, those bloody old 'valve-sound' heads. But H/H turned out thousands upon thousands upon thousands upon thousands of the things, more than anyone could ever dream of. If you took 0.1 per cent of those many thousands of amplifiers, it looks like a lot of faults. But it's not, relatively. Let's say that five per cent of Mesa Boogies fail on stage — and I believe it's more than that from unconfirmed rumours I hear. If you took that same supposed failure rate to something like Session, or H/H, or Carlsbro, or Peavey, the companies would be out of business by now because they couldn't finance such a high warranty comeback."
"Mostly, amplifiers don't go wrong as such. Things very often get sent back to manufacturers through misunderstandings, people not knowing how to use the product properly. People expect more performance than technology is capable of providing, and that's certainly true of amplifiers that overdrive. We've had amplifiers sent back because of hiss on the overdrive channel, but people don't realise that overdrive is a compressive thing — you cut the peaks off something to make it sound aggressive and distorted, but then you need a lot of gain to sustain the sound. So when the gain goes up, you amplify circuit noise as well. Nobody can actually do anything about that."
"A valve amplifier doesn't do something that a transistor amp doesn't do. It's the other way around."
"There are basically two types of speaker in terms of construction. There's the simple guitar speaker, fairly cheap to put together. It has built-in cone break-up — that stems from the idea that when you turned an old-fashioned amplifier up loud it started to get exciting because of the distortion that it made. You might wonder why some amp makers have stuck with one type of speaker for so long. It's because they daren't change, because that's what makes that particular maker's sound: not so much the amplifier or the electronics, but what's inside the speaker cabinet. The ribs that you see on a guitar speaker cone are there to create a certain type of distortion in the cone. That's fine on those old-fashioned type of amps. Unfortunately, if you take those same old amplifiers and replace all the speakers with, let's say, Electro-Voice PA speakers, which have very little cone breakup and colouration, and turn your amp up full bore, plug in your favourite SG and try to get that Clapton 'woman' sound, I'm afraid you're going to be very disappointed. Because what you're going to get is twice the sound pressure level of a boring sound.
"The second type of speaker is this JBL, Electro-Voice type of speaker. The geometry of the construction inside is a little bit different, it's got edgewound coils for maximum efficiency, and it means for the same amount of current flowing you get a lot more out of the speaker, sometimes twice the output — and that costs money to do. Interestingly, the cones of both speaker types all come from the same maker.
"There is this traditional romance of valves over transistors. A valve amplifier doesn't do something that a transistor amp doesn't do, it's the other way around. It's what the valve amplifier doesn't do that makes it sound the way it does. Valve amps tend to be made by specialists, people who understand the sound they're trying to get out of it — older people, usually. Then there's a company that calls everything 'transistorised', which is something that hasn't been heard of in consumer electronics since the 1950s — it's a term that we don't use in electronics any more. We don't in fact use transistors as such, we use integrated circuits (ICs). But valve amps are never slagged off... so nobody will be the first to publish the fact that valves are perhaps not all they're cracked up to be. The benefits of using transistors — reliability and performance — will mean that someone's got to kill off the valve eventually.
"Manufacturers have only got to put one valve in a transistor amp, and they can call it a valve amp. This has happened. That one valve might be the rectifier tube, but as long as it's in there it qualifies manufacturers to say it's a valve amplifier. And if it's a valve amplifier there are musicians who will go out and buy it because it's a valve amplifier. I get requests now, from purely commercial people, distributors and so on, saying for christ sake why don't you make an amplifier with a valve in it so that we can sell it as a valve amplifier. I think that is dishonest and I won't do it. Either it's a valve amplifier or it isn't."
"It's the mechanical things that go wrong: jack sockets, speakers, switches, fuses, that sort of thing. Unfortunately people tend to think now that if it's got Made In Japan written on it that it's going to be totally reliable and never go wrong. They forget that they have these parts that need servicing every now and again — particularly if you live in a big city with all that carbon monoxide about in the air, an alien film of crud that lands on things. People do actually leave their amp in the boots of their cars overnight, even in the winter, in the freezing cold, take it into a hot sweaty pub the next night, and the whole thing gets covered in condensation. Then when the thing breaks down six months later, they blame the manufacturer."
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Feature by Stewart Ward, Tony Bacon
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