The Thermionic Tube
Mr.John Ambrose Fleming invented the thermionic tube in 1904. But when the transistor was invented in 1947, it marked the beginning of a new era. Due to the implicit advantages of this new "device"; smaller outlines, more rough construction, lower supply voltages, less power consumption and no heater (filament). Around 1970 the transistor had found its way into almost all areas of electronics – consumer electronics as well as the professional audio world. One area, though, still remains unconquered territory for the transistor: Power amplifiers with extremely high outputs (above 100 kW) as used in radio- and television transmitters. This territory is still firmly held in the realm of the tubes!
Why continue to use tubes in audio equipment?
Tubes did receive a rather poor reputation during the 60's and 70's. Many people claimed that tubes were noisy, unreliable, and that they furthermore produced high distortion and were very fragile. In the interest of historical honesty, most of these accusations were the direct result of poorly designed (read: low-cost) equipment, poorly manufactured tubes and low performance in- & output transformers.
It is true that a tube won't last as long as a typical transistor (due to the fact that it's a thermionic device) meaning that the cathode material will eventually wear out, so that emission of electrons can no longer take place. But this wear-out process does take quite a few years…
Use & abuse
In a well-designed amplifier (excluding power amplifiers) the tubes can run for many years. It is not uncommon to see tubes performing perfectly after 40.000-60.000 hours or more. If the amplifiers are well designed, the components are of good quality; you will have a well sounding and reliably device, which will serve you with aural dedication for years without ever breaking down. Another advantage of tubes is, that they will survive even a rather substantial short-term overload (both current and voltages), whereas the transistor will more or less "vaporise into thin air", when submitted to even small overloads for a very short time. Most of the newer types of tubes are quite sturdy and can stand even very rough handling. Its true that the older types of tubes were pretty fragile (before approx. 1935), but think about this for a while: ALL equipment made for military purposes until the 1960's were made using tubes – and only tubes. Tubes can take quite a beating!
Tubes can be designed with a very high frequency bandwidth. Distortion figures can easily be below 0,1 % and are always of lower order harmonics, which are far less disturbing on the ear – even when the design includes in- and output transformers. And the noise won't be higher than in a digital 16 or even 20 bit system: A 20 bit digital system has a theoretical noise floor of 120dB below maximum output, but in practice it is way less. If you take a TUBE-TECH CL 1B, the output noise of < -90 dBU (at 0 dB gain) and the maximum output of > +27 dBU, this gives a signal to noise ratio of 117dB!
Another advantage of an analogue system is that it doesn't have a limit for the reproduction of low-level signals (a 1 kHz tone can be heard even if it is 30dB below the noise floor).
Focussing on a specific purpose
But of course it is not always practically feasible solution (although it is possible) to make tube based equipment with the same high amount of features that you find in digital equipment.
But even this so-called "shortcoming" has a positive side effect: All analogue equipment is consequently equipped with dedicated controls (no multi-purpose switches and knobs) and only very seldom contain menus, sub-menus and sub-submenus etc. QED: You have a more straightforward layout of functions and a better overview of what is actually going on!
Today, almost a century after Mr. I Flemings invention, tubes are still prominently represented in the upper end of the world electronics scene. And that is where they deserve to stay for at least another century...