Why vacuum tube amplifiers?

An increasing number of high-end audio lovers around the world recognizes that high quality tube amplifiers can sound superior to the best transistor (or IC or mosfet) based designs. In general there is no agreement on the technical reasons that tube amplifiers can sound better, although the percieved differences are often significant.

The main audible differences with respect to 'silicon' based amplifiers are:

• An improved repreduction in the high part of the spectrum, with more fine detail and accuracy.
• Better reproduction of the sound stage, the ability to focus sources in the stereo and depth image.
• Reduced output power: the single-ended triode based designs often don't exceed something like 12 watts.
• Reduced damping factor, which can lead to a weak control of the bass speaker.

The latter two aspects cause that the largest problems with these tube amplifiers are in finding good high efficiency loudspeakers, with a smooth impedance characteristic.

Many people say the nicer sound of tube amplifiers is due to significant harmonic distortion in the tube amplifiers. Although this might be true for 'guitar amplifiers', this is not true for high-end audio equipment, where such distortion is well noticable and negatively impacts the sound. High quality tube amplifiers surely have low distortion figures.
Then what is the technical explanation for their superior sound? My personal guesses are currently that:

• Tubes can result in a better timing of transient signals: Tubes don't store as much charge as semiconductors, and the involved (parasitic) capacitances are nicely constant and free of long-term discharging effects.
• Most good tube amplifiers are designed with little or no feedback, which makes the distortion spectrum to go down much faster, thus generating fewer high-frequency intermodulation products.

Note that the global transition from vacuum tube to semiconductor based audio amplifiers was driven by cost and compactness but NOT by sonic quality. Also in the tube era, there has been a transition from early 'single ended' tube designs to 'balanced, push-pull' designs (roughly happening around the '40s), because the latter allowed more compact and less power-hungry designs. However, the best sounding tube amplifiers are still of the oldest single-ended triode-based style: Like the first amplifiers which were built for cinemas when Holywood started to produce the black-and-white Charly Chaplin movies with sound....

And -last but not least- it is really fun to redevelop and build such things again at home, and caring more for quality than production cost.

Jos van Eijndhoven

Some nice links are:
the tubestore
triode electronics
Steve's Vacuum Tubes
Angela Instruments
Cuno's triode site

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