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Diode
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 were attached to leads which passed through the envelope, in a way that was sealed air tight. On most tubes, the leads were designed to plug into tube sockets for easy replacement.
The simplest vacuum tubes resemble incandescent light bulbs, in that they have a filament sealed in a glass envelope, which has been evacuated of all air. When hot, the filament releases electrons into the vacuum, a process called thermionic emission. 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. This results in a current of electrons flowing from filament to plate. Obviously this does not work the other way round, because the plate is not heated, so we have a diode, a device that conducts current only in one direction.
The non-linear operating characteristic of the triode gave early valve audio amplifiers a distortion. This distortion was an undesirable distortion at low volume, not to be confused with the desirable distortion that tube amplifiers exhibit at high volume levels (the latter known as the valve sound). To remedy this 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". 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 resistors or voltage dividers to provide proper bias.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.
When triodes were first used in radio transmitters and receivers, it was found that they were often unstable and had a tendency to oscillate due to parasitic anode to grid capacitance. Many complex circuits were developed to reduce this problem (e.g. the Neutrodyne amplifier), but proved unsatisfactory over wide ranges of frequencies. It was discovered that the addition of a second grid, located between the control grid and the plate and called a screen grid could solve these problems. A positive voltage slightly lower than the plate voltage was applied, and the screen grid was bypassed (for high frequencies) to ground with a capacitor. This arrangement decoupled the anode and the first grid, completely eliminating the oscillation problem. This two-grid tube is called a tetrode, meaning four active electrodes.
However the tetrode had a problem, especially in higher-current applications. At high instantaneous plate currents, the plate would become negative with respect to the screen grid. The positive voltage on the second grid accelerated the electrons, causing them to strike the anode hard enough to knock out secondary electrons which tended to be captured by the second grid, reducing the plate current and the amplification of the circuit. This effect was sometimes called "tetrode kink". Again the solution was to add another grid, called a suppressor grid. This third grid was biased at either ground or cathode voltage and its negative voltage (relative to the anode) electrostatically suppressed the secondary electrons by repelling them back toward the anode. This three-grid tube is called a pentode, meaning five electrodes.
Tubes with 4, 5, 6, or 7 grids, called hexodes, heptodes, octodes, and nonodes, were generally used for frequency conversion in superheterodyne receivers. The additional grids were all " control grids", with different signals applied to each one. A special grid acted as a second plate to provide a built-in oscillator, which mixed with the incoming radio signal. These signals create a single, combined effect on the plate current (and thus the signal output) of the tube circuit. The heptode, or pentagrid converter, was the most common of these. 6BE6 is an example of a heptode.
It was common practice in some tube types (e.g. the Compactron) to include more than one group of elements in one bulb. For instance, an early type of multi-section tube, the 6SN7, is a "dual triode" which, for most purposes, can perform the functions of two triode tubes, while taking up half as much space and costing less. The 6AG11 Compactron tube contained two triodes and two diodes. Currently the world's most popular vacuum tube is the 12AX7, with estimated annual worldwide sales of greater than 2 million units. The 12AX7 is a dual high-gain triode widely used in guitar amplifiers.
The beam power tube is usually a tetrode with the addition of "beam forming electrodes", which take the place of the suppressor grid. These angled plates focus the electron stream onto certain spots on the anode which can withstand the heat generated by the impact of massive numbers of electrons, while also providing pentode behavior. The positioning of the elements in a beam power tube uses a design called "critical-distance geometry", which minimizes the "tetrode kink", plate-grid capacitance, screen-grid current, and secondary emission effects from the anode, thus increasing power-conversion efficiency. The control grid and screen grid are also wound with the same pitch, or number of wires per inch. Aligning the grid wires also helps to reduce screen current, which represents wasted energy. This design helps to overcome some of the practical barriers to designing high-power, high-efficiency power tubes. 6L6 was the first popular beam power tube, introduced in RCA in 1936. Variations of the 6L6 design are still widely used in guitar amplifiers, making it one of the longest-lived electronic device families in history. Similar design strategies are used in the construction of large ceramic power tetrodes used in radio transmitters.