What is a vacuum tube or Thermionic Valve? A Thermionic Valve (also referred to as a vacuum tube or electron tube especially in North America) is an electronic device consisting of electrodes in an evacuated or low pressure envelope, frequently of tubular glass form - hence vacuum tube. Most valves do indeed have a vacuum inside the tube envelope, but there are many specialised forms of valves that use inert gasses in the envelope. Developed initially in the late 19th and early 20th centuries, they are still in use today despite the rise of the transistor and integrated circuits. Why? Well, for some uses because they can handle very high power and in the case of Hi-Fi, because there are still plenty of audiophiles that rate the valve / tube sound above all others. How does a Valve / Vacuum Tube Work? The simplest valve has two electrodes - a filament and a plate (called the Anode) - and resembles nothing more than the humble light bulb in operation. When a current is applied to the filament, it heats up (as does the filament in a light bulb) causing it to emit electrons creating a negatively charged cloud of electrons in the vacuum envelope in a process known as thermionic emission. In the case of the light bulb, that's as far as it goes. In a valve however, the second electrode (a plate called the anode) if positively charged with respect to the filament will attract these electrons. So, there will be a flow of electrons from the filament to the Anode called the electron current. As the anode plate is not heated in any way, it cannot emit electrons itself, so the current can only flow one way. A device that shows this property (only allowing current to flow one way through it) is called a diode - di (two) electrode. In later valves, another electrode was introduced called the cathode which was heated by the filament. The cathode became the source of electron emission and the filament was then referred to as the 'heater'. This scheme reduced the effects of 'hum' if an alternating current was used to heat the filament.As they were originally designed to be operated from batteries (e.g. in the earliest military radio sets) and the associated terminology still applies - in the US the power supply for the heater is called the A battery; that for the plate the B battery or today B+. These are more commonly referred to as the LT (low tension) and HT (high tension) supplies. You may sometimes see reference to the C battery or grid bias supply.Valves are not the most energy efficient devices due to the heat required to start the thermionic emission process. In addition, they must not become too hot or thermionic emission will start from the anode also - they rely on the temperature difference between the cathode an anode to work. Basic Construction As mentioned previously, the valve or tube consists of an arrangement of electrodes inside an envelope containing a vacuum or various low pressure gases, the latter being described as 'soft' valves. This envelope is frequently tubular in shape and is classically constructed of glass. Other materials are also used, including metal and ceramics. The electrodes are usually connected to pins on the base of the envelope which then plug into a valve socket, this arrangement having been designed to facilitate easy replacement of the tube if required. A number of different electrode arrangements exist, with extra electrodes being added to improve performance.The vacuum in the tube must be a very 'hard' vacuum - as near a perfect vacuum as is possible. This is because if any molecules of gas remain in the valve's envelope they may ionise and interfere with the tube's operation - possibly causing catastrophic failure and certainly poor performance. A special process is used in manufacture to alleviate the problem of any remaining gasses in the tube. A "getter" is used to absorb the remaining gas. In modern tubes this is usually a trough filled with a metal that oxidises rapidly (usually barium). After the tube has been sealed the getter is heated by RF induction until it vaporises and reacts with any gas still in the tube. This leaves a silvery deposit on the inside of the tube. The getter will continue to react with and remove any gas that leaks into the tube in its life, turning white as it does so. As an aside, if looking at a tube - if there are white deposits inside it, it is likely to be failing or has done so. Also, contrary to popular belief, black spots on the inside of the tube are not bad news and don't indicate failed getter and a valve on the way out. Reliability Tubes are perceived as unreliable which was seen as being one of the benefits of solid state electronics. Valve sockets were developed to facilitate easy changing of tubes. However, a modern power tube such as an EL34 should last 1000 - 1500 hours of use, pre-amp tubes longer still. One of the possible causes of failure is air leakage into the tube, although getters can help as described above. The heater filament may also break like that in a light bulb, though as the heaters are run at much lower temperatures than a light bulb this is less of an issue - the heaters tend to be most stressed by the switch on process. Interestingly, the Colossus electronic code-breaking computer used to crack German cyphers during the second world war was valve based and was left permanently switched on as the designer felt the tubes were more reliable that way (despite it consuming 1.5kw!). The cathode efficiency is also slowly reduced due to contamination by atoms from the other elements in the tube, restricting its ability to release electrons.Despite this, there should be little to worry the owner of a modern valve amplifier, tube preamp or tube guitar amp. Valves and Tubes Today Valves today are mostly used in specialised applications, but have been replaced by solid state electronics in everyday use. Today they are commonly used in audiophile high-end Hi-Fi for music reproduction, guitar amplifiers (where the valve sound is still sought after) and certain high-power radio frequency uses - where ceramic power tetrodes are still used in transmission for military and commercial purposes. Why? Because of their high power handling capabilities - a modern water cooled power tetrode is capable of handling 1.5 megawatts (1.5 million watts) of power - a 2 megawatt power amplifier could be constructed from two of these. In contrast, the highest power that can be dissipated by a transistor is about 1 kilowatt (1000 watts) - so a high powered transmitter requires and array of many transistors and bulky associated cooling equipment to do the job a single vacuum tube can!The humble valve has come a long way since the early 20th century - mostly replaced in modern electronics it still has its place to play in specialist applications not least high-end audiophile amplifiers - it is no coincidence that the most expensive amplifier in the word is a valve design. They are also still very widely used in the music industry - some of the characteristic guitar sounds of the last 60 years are due to the overdriven valve sound. There is even a PC motherboard that uses a valve in its audio output and a new docking speaker set for the iPod uses tube output too. Widely manufactured still, especially in China (Shuguang), Russia (Svetlana and others), Slovakia (JJ-Electronic) and even the USA (Westrex Inc.) - they will be around for many years to come.