Inside Track: Why Valve Amplifiers Are Hot Stuff
Back in the nineteen-eighties, telling your fellow audiophiles that you were a fan of valve amplification would result in either sympathy or derision – or even both. Cheap, reliable semiconductor-based amplifiers had taken entirely over the hi-fi world – thanks in part to the widespread use of MOSFETs – so it was hard to see any role for vacuum tubes outside of a museum…
By the mid-nineties, however, many people seemed to be tiring of the semiconductor sound. It may have been clean and punchy – as well as affordable and reliable – but many thought it lacked a certain musicality or even charm. The valve revival began in earnest, and since then this vintage technology has been slowly rehabilitated back into the respectable hi-fi world. People don't laugh or sneer anymore. Indeed valve amplifiers are widely regarded as interesting or even magical. How times change.
For many years, valve technology was widely regarded as being seriously flawed. People said it sounded dull and lifeless, and was lacking in vitality. That was true, but only up to a point. Indeed, the iconic Leak Stereo 20 valve amplifier – although having a fantastically fluid midrange – did suffer from a rolled-off bass and treble response. Nowadays though, we know that such audio classics aren't necessarily representative of the breed. Vintage designs may have a vintage sound, but modern valve amps need not – indeed many are seriously fast, punchy and powerful. As ever, it's not what you do, but the way that you do it.
The valve was the first-ever 'active' electronic component. It was invented by the American Dr Lee De Forest, who in 1906 produced his 'Audion tube' or 'De Forest valve' – which became known as the triode from 1919. In the USA it is commonly called a tube – because it's a 'vacuum tube' – but in the UK it's usually called a valve – because that's what, in effect, it is. A valve in its simplest form has just two electrodes contained within a vacuum inside a glass envelope. One is the anode (or 'plate' in the US), and the other is the cathode. The cathode is heated up and electrons are emitted from it by a process called thermionic emission. If a positive voltage is applied to the anode with respect to the cathode, the negatively-charged electrons emitted from the cathode will be attracted to it, and a current will flow. If the anode is made negative, no current will flow – a process known as 'rectification'.
De Forest added a wire grid between the two electrodes – a third electrode. With a positive anode and therefore the valve conducting, when a voltage is applied to the grid, the current flowing between the anode and the cathode can be modified. Only a small change in the applied grid voltage produces a large change in the current flowing between the anode and the cathode, resulting in amplification of an applied signal. Without the invention of this triode, no further development in recording techniques would have been possible, as the limits of purely mechanical recording and playback had been reached – so it was a momentous occasion. Over the years, additional electrodes have been added to improve valve performance, such as with tetrodes (four electrodes) and pentodes (five electrodes). By the twenties, innovation in electrical recording and amplification systems, and also the advent of magnetic recording, had become the driving forces behind the recording industry.
THE BIRTH OF HI-FI
Rolling forward to the nineteen fifties and sixties, and high fidelity sound reproduction began to capture the public's imagination. Of course, at this time it was still all valve-based. The first production portable transistor radio – the Regency TR-1 – had only been released in October 1954, and the semiconductors of the day could only handle small amounts of power – so didn't pose a threat to valves. Then in April 1958, Leak introduced its landmark Stereo 20 valve amplifier to herald the introduction of the (then) new stereo LP. This iconic product had a quintessentially 'valve sound' with a sweet, open and vibrant midband, but was a distinctly soft in the bass and treble.
The main reason for this was its output transformers. Valves are very high impedance devices and operate at relatively high voltages and low currents. Transistors, on the other hand, are high current and low voltage devices. To drive a loudspeaker, which is a low-impedance device, from a valve output stage, a transformer is required to reduce the voltage of the output audio signal and increase the current. The Leak's transformers are small by modern standards. Even when the company upgraded it by fitting different valves to increase the output power, the transformers stayed the same, and the overall result was actually a deterioration in sound quality! Nowadays, valve power amplifiers are characterised by meaty transformers that weigh a ton, but offer vastly superior audio quality as a result.
There have also been improvements to power supply design, and as a result, the mains hum that bedevilled vintage equipment is now largely gone. Modern toroidal transformers benefit from improvements in both the design of the core and the materials used in their construction (the coils of wire within a transformer are wound onto a core), resulting in a wider and flatter frequency response. For example, some output transformers are wound with solid silver wire instead of copper.
Turn your attention to the other passive components in modern valve equipment and compare them to their counterparts of fifty years ago, and you soon see how things have changed. Gone are the paper-wax capacitors and cheap carbon resistors, which by modern standards were relatively unstable and whose values would drift over time – especially given the heat that they would be subjected to as part of a valve circuit. One particular issue with capacitors is that – in addition to their capacitance – they also have an ESR (Effective Series Resistance) and a leakage resistance, both of which cause detrimental effects to the circuit and ultimately the sound. Also, the leakage resistance tends to decrease over time, resulting in higher leakage currents, and the ESR tends to increase. Here again, modern components have much-improved stability and, in the case of capacitors, lower ESR and higher leakage resistance.
However, simply improving the design and using better components doesn't explain the renewed interest in valve technology over the years. There's clearly something about the modern valve sound that's very desirable. Now that the deficiencies and limitations of older designs have largely been overcome, what is it about the valve sound that so many people love? It tends to be fuller, more rounded and less clinical than most solid-state amplifiers. The reasons for this are harder to explain, but I believe it's to do with the fundamental differences in the way that valves and transistors amplify a signal.
One theory about why many prefer the valve sound concerns distortion. All amplifiers distort to some extent, but decent hi-fi designs only produce relatively small amounts. Valves tend to produce even-order harmonic distortion, while transistors produce odd-order – and many believe that even-order harmonics are easier on the ear. Simply put, the noise that valve amps introduce just happens to be subjectively more pleasant. For example, second harmonic distortion manifests itself as a smooth hazy sound, whereas first or third harmonic distortion seems hard and scratchy.
Many believe this to be true, but it's only a partial explanation. The circuit design of valve amplifiers plays a major part, too. For example, it is quite easy to design a valve amplifier with no negative feedback, while an equivalent amplifier based on semiconductor technology would be very unstable without some. Of course, this is a generalisation, but any negative feedback introduces colouration to the sound, however small. Generally, the less negative feedback, the purer the sound.1
There are other factors that influence our listening pleasure, but it's hard to measure them. Indeed, the more we understand how things work, the more we realise how little we know of the bigger picture – as the picture keeps getting bigger! I prefer to trust my ears when performing listening tests, rather than relying on measurements. The measurement approach assumes we can measure everything that affects audio quality, including the spatial cues and that difficult-to-articulate sense of realism. Although measurements have an important role to play in assessing audio equipment, they don't yet give us the full picture.
For example, if you think of the electrical current flowing in a circuit as a stream of electrons, there's a high density of electrons passing through the tiny elements of a semiconductor device. Contrast these electrons with the electrons streaming off a heated cathode in a valve and heading towards the relatively large anode through the grids inside the valve, attracted by the positive charge of the anode. Here, the density of the electrons is relatively low – a sort of 'free-range' (valves) vs. battery farming (semiconductors) approach! So the two technologies work very differently on a fundamental level, yet it's hard to express these differences using conventional measurements.
Venturing into the world of valve equipment twenty years ago wasn't easy, but now it is – due to a number of companies specialising in making new, affordable valve amplifiers in many different shapes and sizes. Icon Audio and Ming Da, for example, offer products to suit all pockets. Consider starting off with a 20W per channel push-pull design and you won't be disappointed. Somehow, power seems to go further with a valve design than for an equivalent semiconductor amplifier. That's because wattage is not the best indicator of loudness, and does not paint the full picture when it comes to an amplifier's volume potential. There are many characteristics of an amplifier's design, coupled with other factors, that determine how loud it can sound.
When you get hooked, you might want to move to a single-ended Class A valve amp, which is less efficient than a push-pull design but doesn't suffer from Class B's crossover distortion. Having the power amplifier separate from the preamplifier is another logical upgrade as it allows for the power supply that caters for the delicate signals of the preamp to be separate from the power supply that meets the hefty requirements of the power amp.
For anyone who is happy to wield a soldering iron, there are companies that offer build-your-own construction kits, such as World Designs. This is quite a feasible option as valve circuits are much simpler than their transistor counterparts and can be built on either purpose-made circuit boards or, alternatively, on tag strips for that more traditional look and ease of upgrading components at a later date. Apart from saving money by building your own equipment and having the satisfaction of doing so, you can easily employ upgraded or audiophile components in the construction, such as audio-grade resistors and capacitors.
Upgrading a valve amplifier is easy. More often than not, significant audible improvements can be achieved by replacing the supplied and low-cost valves with better quality makes of the same type. Changing a valve simply involves unplugging the old valve and replacing it with a new one. Bear in mind that valves don't last forever as they do wear out, although small-signal valves last considerably longer than power output valves. As a general rule of thumb, assume 50,000 hours for small-signal valves usually found in preamps (double triodes ECC82/12AU7, ECC83/12AX7, etc.) and 10,000 hours for a power output valve (300B triode, EL84 pentode, etc.). Of course, all the guesswork can be taken out if you have or know someone who has a valve-tester!
A good supplier, such as HiFiCollective will generally offer a whole range of valves at various price points. A small-signal valve like the ECC83 can cost from under £12 to about £45. The superb 300B directly-heated triode (where the cathode is the heater itself and not a separate electrode heated indirectly by a heater) is a truly wonderful output valve for audio applications. This valve is found in a wide range of power amplifiers – single-ended and parallel single-ended, as well as Class B push-pull – and costs from £90 to upwards of £480 if you can stretch to the Psvane replica of the highly sought-after legendary Western Electric version.
Not only does valve equipment sound great, but it can look great too – EAR Yoshino amplifiers are a perfect example of this. Power amps traditionally have their valves visibly on display – partly because their orange glow when switched on looks rather pretty, and partly because the amount of heat they generate dissipates away easily without harming the passive components underneath. Indeed some valves look particularly stunning, and the 101D valve is one such design. It was originally developed by Western Electric in the early part of the twentieth century and is another example of a directly-heated triode (like the 300B). As with the 300B, the 101D valve wasn't originally designed for audio use, but because of its high accuracy and fidelity, it's sometimes found in modern high-end valve preamps. The 101D also shows off its pip (where the glass envelope is sealed after evacuation) at the top of the valve, which is rather fetching!
If you've not heard a valve amplifier before, then you're in for a pleasant surprise. They have a way of taming the harsh edges of digital music without losing any clarity, openness or depth. The result is something that's altogether more musical than many may think possible. Ignore the naysayers because once you've heard one of these tubular belles, in my view it's seriously difficult to go back to solid-state.
A Chartered Scientist, Chartered Engineer, Chartered Physicist and a Fellow of the British Institution of Engineering and Technology, Neville has worked as a Director of the British National Health Service, for the Ministry of Defence and in private industry. He’s a lifelong audio enthusiast and regular contributor to British hi-fi magazines, with a passion for valves and vinyl.
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