Up until the invention of the transistor amplifier in the late fifties, valves or vacuum tubes were the only form of amplifying device available to power audio equipment. On paper, valves are hopelessly deficient by transistor standards, they produce significantly less power, high levels of harmonic distortion and require an output transformer in order to be able to drive a loudspeaker. Their continued popularity is purely due to their sound quality which is generally considered to be more natural, dynamic and transparent than the majority of transistor designs. Their distortion is largely even order and thus not ugly or uncomfortable but rather is almost euphonic, it adds a natural sound that emphasises the timbre of instruments and the reverb of an acoustic. This is one reason why valves have never gone out of fashion in the world of guitar amps, an area where distortion of the right quality is held in very high regard.
The fact that valves are usually run purely in class A also adds to their appeal, as this brings a purity and transparency that even the best solid state alternatives struggle to equal - in fact few ever do. Against this it's important to remember that valves rarely compete with transistors when it comes to bass reproduction, an area where absolute power is critical to controlling a loudspeaker. Hybrid amps combine valves and transistors in an attempt to try to get the best of both worlds and brands like Copland and Croft have produced some very good examples. For the true glory of tubes however there's no beating the purist approach.
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| Icon Audio Stereo 20
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Preamps
Valves come in high and low power varieties, the latter can be found in preamplifiers and the output stages of a few CD players, while output tubes, as the high power variety are known, are used as the output device in a power amp. Valve amps tend to be split into preamps and power amps, you get a few power amps with a passive volume control for use with a single input but results are usually improved by the use of a preamp. Integrated tube amps are made but they tend to be variations on the passive pre plus power amp theme.
If you want to play vinyl with a moving coil cartridge through a tube preamp, assuming it has a phono stage onboard, you will require a step-up transformer. These tend to add at least £300 to the overall bill, but can give results that exceed those offered by solid state phono stages.
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| Kondo KSL-Kegon
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Power amps
Tube power amps are made in two basic varieties, single-ended (SE) and push-pull (PP) or ultralinear. Of these, SE is the least powerful but has a purist appeal that causes enthusiasts to spend fantastic amounts of man hours developing speakers that are efficient enough for power outputs that don't make double figures.
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| Kondo KSL-M77
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Hi-fi as a hobby can be obsessive and rarely more so than with valves. SE amps are usually built using triode valves, these are the simplest form of vacuum tube and hark back to the 1920s. Most of today's SE triode (SET) amps use valves designed in the forties such as the 300B created by Western Electric.
PP amps on the other hand produce greater power and less distortion with 25 watts being a common output for a pair of EL34 tubes. They don't however have quite the same degree of transparency.
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| Quad QC-24/II-40
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Drawbacks
The obvious drawback with the low power of valves is that you need unusually sensitive speakers to be able to produce decent volume. Where most speakers offer around 87dB per watt sensitivity, something suitable for an SET has to quadruple that (an extra 6dB) to work well. Large speakers are therefore very popular among tube aficionados.
Some valve amps worth trying:
Budget
Icon Audio Stereo 20
£650
www.iconaudio.co.uk
This Chinese integrated design uses EL84 valves in PP operation to produce 15 watts per channel. It has four inputs and a sweet and open sound. Named after a classic British amplifier of the 1950s (the Leak Stereo 20), it provides a great introduction to the charms and capabilities of valve amplification without hefty cost penalty.
Midrange
Quad QC-24/II-40
£4,007
www.quad-hifi.co.uk
This preamp and pair of monoblock power amps is a revival of a classic British valve amplifier. Based on the most famous and revered of the Quad designs this KT-88 tube-powered amp evokes the Quad 22/II pairing of the sixties with its retro styling and distinctive finish. It has a big muscular sound but is easy on the ears in true valve style.
Dream on...
Kondo KSL-M77/KSL-Kegon
£60,000
www.audionote.co.jp
Kondo or Audio Note Japan makes what many consider the world's finest valve amplifiers, using hand-wound silver output transformers and a lifetime of experience. The KSL-M77 preamp is available with a phono stage and offers three line inputs, while the KSL-Kegon is a parallel SE power amp using 300B output tubes. As a combination they transcend normal hi-fi description and take you to a place where music is paramount. It's quite simply the best there is.
