| Vacuum Tube |
Article Index for Vacuum |
Website Links For Vacuum Tube |
Information AboutVacuum Tube |
| CATEGORIES ABOUT VACUUM TUBE | |
| vacuum tubes | |
|
The vacuum tube is a voltage-controlled device, which means that the relationship between the input and output circuits is determined by a Transconductance function. The solid-state device most closely analogous to the vacuum tube is the JFET . However, the vacuum tube typically operates at far higher voltage (and power) levels than the JFET. Diode Triode OPERATION ''Vacuum tubes'', or ''thermionic valves'', are arrangements of Electrodes in a Vacuum within an insulating, temperature-resistant envelope. Although the envelope was classically glass, power tubes often use ceramic and metal. The electrodes are attached to leads which pass through the envelope via an air tight seal. On most tubes, the leads are designed to plug into a Tube Socket for easy replacement. The simplest vacuum tubes resemble '''. The resulting negatively-charged cloud of electrons is called a Space Charge . These electrons will be drawn to a metal "''plate''" inside the envelope if the plate (also called the Anode ) is positively charged relative to the filament (or Cathode ). The result is a current of electrons flowing from filament to plate. This cannot work in the reverse direction because the plate is not heated and cannot emit electrons. This very simple example described can thus be seen to operate as a Diode : a device that conducts current only in one direction. DEVELOPMENT The 19th century saw increasing research with evacuated tubes, such as the Geissler and Crookes Tube s. Scientists who experimented with such tubes included Eugen Goldstein , Nikola Tesla , Johann Wilhelm Hittorf , Thomas Edison , and many others. These tubes were mostly for specialized scientific applications, or were novelties, with the exception of the Light Bulb . The groundwork laid by these scientists and inventors, however, was critical to the development of vacuum tube technology. Though the Thermionic Emission effect was observed as early as 1873, it is Thomas Edison's 1883 investigation of the " Edison Effect " that is the best known. He promptly patented it (), but as the particle nature of the Electron was not known until 1897, he did not understand the process. Diodes and triodes John Ambrose Fleming had worked for Edison; in 1904, as scientific adviser to the Marconi company, he developed the "oscillation valve" or kenotron. Later known as the Diode , it allowed Electric Current to flow in only one direction, enabling the Rectification of alternating current. Its operation is described in greater detail in the previous section. In 1906 Lee De Forest placed a bent wire serving as a screen between the filament and Plate Electrode , later known as the "grid" Electrode . As the voltage applied to the grid was varied from negative to positive, the amount of electrons flowing from the filament to the plate would vary accordingly. Thus the grid was said to electrostatically "control" the plate current. The resulting three-electrode device was therefore an excellent and very sensitive Amplifier of voltages. DeForest called his invention the " Audion ". In 1907, DeForest filed for a three-electrode version of the Audion for use in radio communications. The device is now known as the Triode . The non-linear operating characteristic of the triode caused early tube audio amplifiers to exhibit distortion at low volumes. This is not to be confused with the distortion that tube amplifiers exhibit at high volume levels (known as the Tube Sound ). To remedy the low volume distortion problem, engineers plotted curves of the applied grid voltage and resulting plate currents, and discovered that there was a range of relatively linear operation. In order to use this range, a negative voltage had to be applied to the grid to place the tube in the "middle" of the linear area with no signal applied. This was called the idle condition, and the plate current at this point the "idle current". Today this current would be called the Quiescent or Standing Current . The controlling voltage was superimposed onto this fixed voltage, resulting in linear swings of plate current for both positive and negative swings of the input voltage. This concept was called '' Grid Bias ''. Batteries were designed to provide the various voltages required. "A" Batteries provided the filament voltage. "B" Batteries provided the plate voltage. To this day, plate voltage is referred to as B+. "C" Batteries were used to provide grid bias, although many circuits used Grid Leak Resistor s or Voltage Divider s to provide proper bias. In Britain "A" Batteries were known as "wet" batteries, and "B" batteries were known as "dry" batteries. The "wet" batteries were rechargable - usually of the Lead-acid type ranging from 2 to 12 volts with 6v being most common. "Dry" batteries were typically 45, 60, 90 or 120 volts. Many further innovations followed. It became common to use the filament to heat a separate electrode called the cathode, and to use the cathode as the source of electron flow in the tube rather than the filament itself. This minimized the introduction of hum when the filament was energized with Alternating Current . In such tubes, the filament is called a heater to distinguish it as an inactive element. Tetrodes and pentodes |
|
|