Back in the days that marked the inception of the consumer audio industry, valves (or tubes as they are frequently called) were the only electronic devices capable of amplifying signals. A valve in its most basic form is a glass 'envelope' with an internal vacuum, containing a number of small metal components. The simplest valves have a heating element and two metal parts. Basically put, due to the heat, electrons are released from the heated element and received by a second part. Between these, a 'grid' is placed and varying the voltage applied to the grid regulates the flow of electrons. In this way, the grid is powered by the incoming audio signal and controls the larger voltage, producing an amplified signal.
During the course of the twentieth century, valve design became more complex, with specific tubes being manufactured for specific purposes. From the simple 'direct heated' valve, through valves combining two sections in one housing, right up to radio and microwave transmitting valves, the list became almost endless. Small signal valves usually operate with a power voltage around 90 volts and milliamps of current, but some of the most powerful radio valves operate at thousands of volts and need intense cooling just to work in the first place.
For most of the twentieth century, all amplifiers were valve amplifiers and there was a sizeable industry producing the tubes themselves. Things probably reached a first zenith during the 1950s and 1960s, with a number of critically acclaimed amplifiers available to the public, broadcasters and recording studios. Some of the most famous names in audio were born during this period, with people like Leak, General Electric and Radford becoming feted by the audio industry.
Although relatively simple in many ways, designing and manufacturing higher quality valve amplifiers is quite a task. Inherently heavy, inefficient and generating a fair amount of heat, designs require extremely high quality components to work well. This is especially the case for the transformers involved, which were (and still are) often constructed by the manufacturers themselves - a lengthy and costly process.
The arrival of the transistor completely changed the market. Although originally patented as early as the 1920s, viable production of transistors and other 'solid state' components only really began commercially in the 1950s. However, by the 1960s, the devices were commonplace. The transistor had massive appeal to amplifier designers and manufacturers. Small and lightweight, they needed comparatively low voltages and power, and were inherently efficient. The floodgates opened and manufacturers around the world started to produce relatively low cost amplifiers and radios that took the home audio market by storm. The introduction of more complex integrated circuits allowed further developments and miniaturisation to the delight of audio enthusiasts.
Apart from a limited number of high power and specialist applications, the valve looked doomed to an early grave. Most valve manufacturers turned their attention to the production of cathode ray tubes for the home television market, only to be doomed to eventual failure when flat-screen and plasma TVs became commonplace. Many of the larger valve makers, such as the famed Telefunken of Germany, simply turned their attention to other things and abandoned production of vacuum devices entirely.
Yet those who assumed the days of valves were over, turned out to be wrong. Musicians in particular still remained loyal to valves, with many guitarists in particular aware of the fact that valves produce pleasant distortion when 'driven' too hard, as opposed to solid state designs, which can sound unpleasant and harsh. A small hard-core of audio enthusiasts also stuck firmly with valves, maintaining that the sound produced by valve amplifiers was more pleasing than that from their solid-state counterparts.
In fact, this hi-fi use turned into a minor landslide, with a number of new manufacturers entering the market, some producing relatively 'budget' equipment, but others manufacturing valve amplifiers of immense quality and commensurately high cost. Against all odds, companies started to produce vacuum tubes again. Mostly in the Far East and Eastern Europe, new firms sprang up to supply the small, but stable and growing market. The overall irony is that many of today's users find that older valves are preferable to those currently being produced and a significant market has opened up, with some older, rarer valves becoming increasingly and massively valuable.
Whatever the future brings, the one thing that now seems certain is that no matter how small the market, valve amplifiers are here to stay in the long term!