Ben Duncan looks deep into the back of his 'Voicemaster Megoamp' and the funny glass things glowing softly. Valves. Not trannies. He speaks.

For the first half of the 20th Century, there were no transistors. In fact, up until the late 1960s the world of rock'n'roll sound amplification revolved around valves (vacuum tubes in the US).

Today, 99.9% of the world's wonderful electronics technology (cough, splutter) owes its existence to the transistor, invented in 1947. The transistor does everything a valve can do, only more. The silicon chip is but one example, where hundreds of thousands of transistors are bundled into a tiny space to provide powerful processing of complex information. Take music: in one form or another, transistors have expanded the whole process, from computer composition to FM synthesis.

In everyday life, transistors have no competition if we make the comparison on strictly physical and economic grounds. Unlike tubes, they don't have to glow red, so they consume less power and run cooler. They're also smaller, and don't require bulky and heavy components to back up their action. This in turn makes big and complex arrays of electronics feasible: a computer or synth built with valves would occupy a whole room!

But the first transistors were delicate, finnickity devices and it wasn't until the early 1960s that they began to challenge the role of valves in music amplification.

Twenty five years later, valve electronics have all but disappeared, with three exceptions. Big radio transmitters, Russian jet fighters, and instrument amps all share the religious observance of yesteryear's technology.

Because valves control the flow of electricity in a vacuum, whereas transistors are made of solid materials, Americans soon coined the words 'solid state' for the new, superior technology. More recently, in the wake of a backlash, 'tube' equipment has badges to proclaim 'vacuum state technology'. The big piss-take. Or is it?

How the valve works

Valves and transistors are both active components, the crucial parts in any electronic circuit, without which they'd be no amplification. But the elementary behaviour of the transistor is a conceptual nightmare compared with the valve which is simplicity in itself.

The most basic amplifying valve has three electrodes (ie bits of metal), mounted in a vacuum. Because live electrons are whizzing around inside, and tubes get hot, we encase the vacuum in a heatproof insulator — hence the familiar glass envelope. The electrode in the middle is coated with a special chemical which 'boils off' electrons when heated. For this to happen, there's a couple of extra wires, connecting to a miniature electric element. Called the beater, this is the bit which glows red. The electrode it's heating is called the cathode. The second electrode is wrapped around the cathode; it's called the anode.

It's a fact of the cosmos that electrons are charged negatively, and also the opposite poles attract. So the anode is made sexually alluring by connecting it to the positive (+) side of a high voltage power source. Having done this, the electrons get shot off into the vacuum, and make an immediate beeline to the anode, which ensnares them. Except for the intervention of the third electrode, a mesh of fine wires, interposed between the anode and cathode. Called the grid (think of a cattle grid), this can exert control on the flow of electrons out of all proportion. How? Well, like poles repel, so if the grid has only a small negative voltage applied to it, the electrons are repulsed before they reach the anode; instead, they remain in a cloud around the cathode. And by varying the voltage on the grid, we can control the flow of electrons from cathode to anode. This arrangement constitutes amplification, because just three or four volts on the grid can control hundreds of volts on the anode. In effect, a varying voltage (like music) modulates (wiggles!) a steady (DC) voltage in its own image, creating a magnified replica. Exit a louder sound.

The valve we've just described is called a triode, since it has three electrodes. It's the original and simplest species, first devised in 1904. Later, in the 1930s, extra electrodes were added to soup up performance. A valve with two grids is a tetrode, while a valve with three grids has five electrodes, hence pentode.

From the 1950s onwards, most amplifiers have settled on pentodes (eg EL84) or tetrodes (eg 6L6) for power amplification, but triode valves (like the famous model ECC82) still do good work in preamp stages.

Valve Sound

Even if it's positively old-fashioned, big and heavy, amplification by valve remains de rigeur for serious guitarists and a whole bunch of dedicated hi-fi listeners, because transistor equipment sounds different (a fact), and quite possibly worse (it all depends on what turns you on).

But we needn't jump to the conclusion that these two disparate groups are listening in the same way, to the same thing. For in creating music, just where do we draw the line? Valve instrument amplifiers have become part of the sound of the guitar; if they add their own character to the sound, that's great. Compare this with hi-fi reproduction, where the amplifier had better not add to or subtract anything from the sound. Why? Because the music is then being developed beyond the state where the artist has handed it over. Sacrilege!

Since the mid 1970s, when hi-fi journalists woke up to the fact that most guitarists were still using valve instrument amps, many reasons have been advanced to account for our apparently: 'irrational preference'. As every guitarist knows, a good valve instrument amp has unsurpassed warmth, depth, and soul. The sound can be fat, creamy, brassy, punchy or raunchy. These adjectives suggest that a good instrument amp is doing much more than producing an exactly magnified replica. But what's the source of these magic qualities?

First, all active devices exhibit distortion, meaning that the amount of amplification isn't constant, across the board. But valve distortion is very different to that produced by transistor amps. There's more to it, and the overtones produced are much more likely to be euphonic (agreeable in sound). For example, when overdriven, a transistor amplifier produces lots of odd harmonics, like the third, fifth and seventh. These sound thin, rather hard, metallic and dissonant to our western ears. But a good instrument amplifier has been tuned (perhaps unwittingly) to balance out the overtones, so there's a roughly equal amount of the even harmonics (second, fourth, sixth etc). These overtones sound fat, warmer and 'rounded', leading to a teasing combination of opposite qualities, a sonic game of yin'n'yang.

Secondly, valve circuits are frequently microphonic. Not just the valves, but the components too. Just try tapping your valves lightly with the volume turned up. Ping! Pyonng! So in a combo, the vibration coming from the speaker is 'picked up' by the parts inside, meaning that a portion of the sound goes round in circles, a subtle feedback loop. The effect isn't easily put into words, but is unmistakable; it's an ethereal kind of reverberation or faint chorus which contributes to a rich sound.

Thirdly, valve amplifiers of a given power rating (let's say 100 watts) are commonly found to be louder than a transistor amplifier of the same nominal rating. As a rule of thumb, a guitar played through a 50 watt valve instrument amplifier can do battle with a 500 watt (transistor) PA. This paradox arises because amplifier ratings are a measure of continuous power, and take no account of an amplifier's ability to produce much higher (but short lived) bursts, useful for rhythmic music.

This happens in valve amplifiers because the high voltages from which they run allow the power source to store an unparalleled reserve of energy measured in Joules. Why? Well Joules = C x V²/2, where C is the size of Capacitor storing the power supply's 'oomph', and V is the Voltage.

In a typical 100 watt transistor power amplifier, V is 100 volts, while C = 4700μF (micro-farad). In a typical 100 watt valve instrument amp, C is somewhat smaller, say 1000μF, but V is higher (say 450 volts), so V² is a lot higher.

And the score? The trannie amp manages 24 joules, against 100 joules for the valve amplifier. In other words, our valve amp can put out around four times as much instantaneous welly.

More valve technicalities next month.