Vacuum Tubes, Physics tutorial

Introduction:

Let us return into history; to be exact; in the year 1883. That is when Thomas Edison discovered that electric current flowed from the hot filament to a metal plate at the bottom of a light bulb. This invention was termed as the 'Edison effect' at that time and it exhibited that electrical current didn't require a physical conductor, and that certainly, it was possible to make current flow via a vacuum.

As you might anticipate, this discovery which current can travel via a vacuum at that time appeared illogical to the number of people and was not put into practical utilization till 1904 when a British scientist named John A. Fleming made a vacuum tube termed as the diode. This acted as a valve as it forced current in the tube to travel in one direction. This unidirectional flow is much significant in turning alternating current into the direct current.

The vacuum tube undergoes a main transformation when Lee De Forest discovered a vacuum device that not only forced current to move in a single direction, however as well raised the current as it passed via the vacuum tube. Lee De Forest put a metal grid in the middle of the vacuum tube and by employing a small input current to change the voltage on the grid; Lee De Forest could control the flow of a more powerful current, via the tube. The strength of two currents was not essentially associated as a weak current may be applied to the tube's grid, however a much amplified current outcome at the main electrodes of the tube.  

Turning the weak currents into strong currents was vital for a number of new methodologies at that time and Bell Laboratories made instrumental in the growth of this technology which found applications in all from hearing aids to radios to televisions.

Vacuum tubes are made up of a glass tube surrounding a vacuum by embedded electrical contacts at the ends; Vacuum tubes are as well termed to as electron tubes and thermionic valve. They are devices employed to amplify, to switch or else modify, or make an electrical signal controlling the movement of the electrons a low-pressure space.

Description of Vacuum Tubes:

Vacuum tubes have been important to the growth of electronic technology that has driven the expansion and commercialization of the radio broadcasting, radar, television, sound reproduction, huge telephone networks, analog and digital computing and industrial process control.

A few of such applications pre-dated electronics, however the vacuum tube made them extensive and practical. As for most of the purposes, the vacuum tube has been substituted by solid state devices like transistors and solid-state diodes that last much longer are smaller, more proficient, more consistent, and cheaper than equivalent vacuum tube devices. Vacuum tubes still find out broad applications in specialized regions such as in high-power radio frequency transmissions; different sound signature, and as cathode ray tubes in oscilloscopes. A specialized form of the electron tube, the magnetron, is the primary source of microwave energy in microwave ovens and radar systems. The klystron that is a powerful however narrow-band radio-frequency amplifier is generally positioned by broadcasters as high-power UHF television transmitters.

Vacuum tubes comprise of electrodes in a vacuum all within an insulating heat-resistant envelope. As this envelope is frequently in the shape of a tube, early references to this device termed to it as vacuum tube.

Most of the vacuum tubes encompass glass envelopes, via some types like power tubes might be constructed by ceramic or metal envelopes. The electrodes are joined to leads that pass through the envelope through airtight seal. Most of the tubes, almost devoid of exception feature socket pins via which the leads are designed to plug into tube sockets for simple placement and substitution.

The simplest vacuum tubes encompass a filament termed as the cathode that is housed an evacuated glass envelope. If hot, the filament via thermionic emission discharges electrons into the vacuum. These electrons outcome is a negatively charged electron cloud termed as space charge. The space charge electrons are drawn to a metal plate anode within the envelope if the anode is positively charged relative to the filament. This transforms to a total flow of electrons from the cathode to the anode. This means that conventional current flows from the anode to the cathode.

Vacuum tubes require a considerable temperature differential among the hot cathode and the cold anode. Due to this, vacuum tubes are relatively power-inefficient as the heating of the filament uses energy. When the vacuum tube is covered in a heat-retaining envelope of insulation, then the whole tube would reach the similar temperature. This outcome in the emission of electron from the anode which would counter the normal one-way current. Because the tube requires a vacuum to operate, convection cooling of the anode is not usually possible except the anode forms a portion of the vacuum envelope.

The cooling of anode takes place in most of the tubes via black body radiation and conduction of heat to the outer glass envelope through the anode mounting frame. In cold cathode tubes several form of gas discharge underlies the operation as they don't rely on thermionic emission at the cathode. They are generally applicable in lighting like neon bulbs, and in voltage regulation.

When a control grid, is added between the cathode and the anode the vacuum tube is termed as a triode as it now consists of three electrodes. A triode is voltage-controlled in that a voltage applied as an input to the grid can be employed to control the flow of electrons between cathode and anode. The relationship between this input voltage and the output current is found out by transconductance. Control grid current is practically negligible in most of the circuits.

History and development of Vacuum Tubes:

Let us go back into history again. In the nineteenth century, scientists experimented by such tubes mostly for specialized scientific applications or innovations with the exception of the light bulb, and the foundation laid through nineteenth century scientists and inventors was vital to the growth of vacuum tube technology.

The thermionic emission is more frequently credited to Thomas Edison as he patented his Edison effect even although he didn't comprehend the underlying physics, or the potential value of the discovery. It was not till the early 20th century that this result was put to employ by John Ambrose Fleming and Lee De Forest as the diode and the triode correspondingly.

The growth of the thermionic diode and the triode led to great enhancements in telecommunications technology, specifically the birth of broadcast radio.

Diodes and triodes:

John Ambrose Fleming, a physicist who worked as an engineering consultant for such communications firms as Edison Telephone and the Marconi Company in the year 1904 invented the Fleming valve which was employed as a rectifier for alternating current and as a radio wave detector. All this was as an effect of experiments he conducted on Edison effect bulbs imported from the USA. 

In two years Robert Von Lieben filed for a patent on a three electrode vacuum tube able of amplification however it was Lee De Forest who in the year 1907 positioned a bent wire to serve as a screen between the filament and anode that brought the triode to the fore-front in radio communications application.

Lee De Forest concerned against operation that might cause the vacuum to become too hard however in the year 1915, Langmuir was one of the first scientists to realize that a harder vacuum would enhance the amplifying features of the triode.

The non-linear characteristic of the triode caused harmonic distortions at low volumes in early vacuum tube audio amplifiers. This non-linearity is prepared by applying a grid bias negative voltage.

Tetrodes and pentodes:

Early application of the triodes in radio transmission and reception was plagued through uncontrollable oscillations that resulted from the parasitic anode-to-grid capacitance. Some of the efforts to decrease such parasitic capacitances were unsatisfactory over a broad spectrum of frequencies till it was invented that the addition of a second grid, positioned between the control grid and the plate resolved the problem. This grid is termed as the screen grid and a positive voltage slightly lower than the plate voltage applied to it fully removing the oscillation problem. An effect of the screen grid is that the Miller capacitance is decreased with enhancement in gain at high frequency. This two-grid tube is termed as a Tetrode as it consists of four active electrodes.

In all the vacuum tubes, electrons hit the anode hard adequate to knock out secondary electrons and in triodes these electrons being less energetic can't reach the grid or cathode - they are re-captured by the anode.

Though in a Tetrode, they can be captured through the second grid, decreasing the plate current and the amplification of the circuit. As secondary electrons can outnumber the primary electrons, if anode voltage falls beneath the screen voltage, the valve can illustrate negative-resistance termed as the Tetrode kink. This can as well overload the screen grid and can cause it to overheat and melt, devastating the tube.  

This problem is resolved by introducing the suppressor grid biased at either ground or cathode voltage. Its negative voltage relative to the anode voltage electro-statically suppresses the secondary electrons through repelling them back toward the anode. This is the five electrode vacuum tube that is as well termed as the Pentode. The pentode was discovered in the year 1928 via Bernard Tellegen.

Improvements in Vacuum Tubes:

Early vacuum tubes look like incandescent light bulbs and were made via lamp manufacturers, who had equipment to manufacture glass envelopes and powerful vacuum pumps needed to vacate the enclosures. Later, specialized manufacturers employing more economical construction processes were set up to fill growing demand for broadcast receivers and bare tungsten filaments operated at a temperature of around 2200oC.

The growth of oxide-coated filaments reduced filament temperatures to around 700 oC that in turn decreased the thermal distortion of the tube structure and allowed closer spacing of tube elements. This enhanced tube gain, as the gain of a triode is inversely proportional to the spacing between grid and cathode. Growth of the indirectly-heated cathode, by the filament within a cylinder of oxide-coated nickel, further decreased distortion of the tube elements and as well allowed the cathode heaters to be run from an AC supply devoid of the super imposition of the mains signal on the output.

Heat Transfer and Appearance of Tubes:

As considerable amount of heat is generated if vacuum tubes operate, they are usually around 30 to 60% efficient that means that 40 to 70 % of input power to an amplifier stage is lost as heat. The needs for heat removal significantly changed the appearance of the high-power vacuum tubes.

Most of the tubes include two sources of heat when operating. The first one of such is the filament or heater. As some vacuum tubes have directly heated cathode others employ the indirectly heated cathode. This generally comprises of a nickel tube, coated on the outside with the similar strontium, calcium, barium oxide mix employed in directly heated filaments and fitted by a tungsten filament within the tube to heat it. This tungsten filament is generally uncoiled and coated in the layer of alumina to insulate it from the nickel tube of the actual cathode. This form of construction lets for a much greater electron emitting area and, as the heater is insulated from the cathode; the cathode can be positioned in a circuit at up to 150 volts more positive than the heater or 50 volts more negative than the heater for most general types. This as well lets all the heaters to be simply wired in series or parallel instead of some requiring special isolated power supplies like specially insulated windings on power transformers or the individual batteries.

The second source of heat is produced at the anode, whenever electrons, accelerated through the voltage applied to the anode, hit the anode and impart a considerable fraction of their energy to it, increasing its temperature. Tubes employed in power amplifier or transmitting circuits, this source of heat will surpass the power dissipated in the cathode heater.

This heat generally escapes the device through black body radiation from the anode or plate as infra red light. A few is conducted via the connecting wires going to the base however none is convected in most kinds of tube as the vacuum and the absence of any gas within the bulb to convect. It is the manner tubes get rid of heat that most influences their overall appearance, next to the kind of unit (that is, triode, pentode and so on) they contain, or whether they have more than one of these fundamental units.

For devices needed to radiate more than 500 mW or so, generally indirectly heated cathode types, the anode or plate is frequently treated to make its surface less shiny, and to make it darker, either black or gray. This assists it radiate the produced heat and maintain the anode or plate at a temperature significantly lower than the cathode, a need for proper operation.

Other variations:

Pentagrid converters were normally utilized for frequency conversion in super heterodyne receivers in favor of a combination of a triode and hexode vacuum tube combination. Other process of frequency conversion was based on octode tubes that featured eight electrodes and in which the additional grids were either control grids having various signals applied to each one, or screen grids. In most of the designs a special grid acted as a second anode and given a built-in oscillator that coupled this oscillator signal having the incoming radio signal to make a single, joint effect on the anode current. This was equivalent to the similar effect as an analogue multiplier while the helpful component of the output was the difference frequency between that of the incoming signal and that of the oscillator.

Multiple Vacuum Tube Designs:

In order to decrease the cost and intricacy of radio equipment, it was general practice to join more than one tube function, or more than one set of elements in a single tube. An illustration is the RCA Type 55 was a double diode employed as a detector, automatic gain control; rectifier and audio preamplifier in early AC powered radios.

Special-Purpose Tubes:

Different inert gasses like Argon, Neon or Helium will ionize at predictable voltages and this served up as the basis for the construction of these special-purpose devices as voltage regulator tubes.

The Thyratron vacuum tube is the special-purpose tube filled by low-pressure gas or mercury, a few of which vaporizes. It includes a hot cathode and an anode, however as well comprises a control electrode that behaves similar to the grid of a triode. If the control electrode begins conduction, the gas ionizes, and the control electrode no longer can stop the current; the tube latches into conduction. Eliminating the anode voltage lets the gas to de-ionize, restoring it to its non-conductive state. This function is identical to that of the modern Silicon Controlled Rectifier (SCR). Thyratrons are recognized to carry large currents in comparison to their physical size by the hydrogen filled versions being broadly applied for radar transmitters as their extremely reliable time delay between turn-on pulse and full conduction.

Tubes generally encompass glass envelopes, however metal, fused quartz silica and ceramic are possible choices. The initial versions of a few vacuum tubes employed a metal envelope sealed by glass beads. Metal and ceramic are employed almost completely for power tubes above 2 kW dissipation. In a few power tubes, the metal envelope is as well the anode and air is blown via an array of fins joined to this anode, therefore cooling it. Power tubes employing this cooling scheme are available up to 150 kW dissipation. Higher than that level, water or water-vapor cooling are employed. The highest-power tube presently available is a forced water-cooled power Tetrode capable of dissipating 2.5 megawatts. Whereas the other very high power tube is a 1.25 megawatt Tetrode employed in military and commercial radio-frequency installations.

Vacuum Tube Power Requirements:

Batteries:

The voltages needed by vacuum tubes were given by batteries in early radio sets and as many as three different voltages were needed that signify three different batteries were employed. The low voltage battery provided the filament voltage and vacuum tube heaters were designed for single, double or triple-cell lead acid batteries, giving nominal heater voltages of 2 V, 4 V and 6 V correspondingly. Portable radios at times employed dry cell batteries 1.5 or 1 Volt heaters. Decreasing filament consumption enhanced the life span of batteries and through 1955, radio receiver tubes requiring between 50 mA and 10 mA for the heaters had been developed, this didn't last long though as the advent of transistors rendered numerous vacuum tube applications obsolete.

The Anode voltage was given by high tension supply or batteries that were usually dry cells having many small 1.5 volt cells in series. They generally came in ratings of 22.5, 45, 67.5, 90 or 135 volts. A few sets employed a grid bias battery and however most of the circuits employed grid leak resistors, voltage dividers or cathode bias to give proper tube bias, they had extremely low battery drain.

AC power:

Battery replacement stands for a main cost of operation for early radio receiver users and the growth of battery eliminators decreased operating costs and contributed to the growing popularity of radio. A power supply employing a transformer having several windings, one or more rectifiers and big filter capacitors given the required direct current voltages from the alternating current source.

As a cost reduction measure, particularly in high-volume consumer receivers, all the tube heaters could be joined in series across the AC supply, and the plate voltage derived from the half-wave rectifier directly joined to the AC input, removing the need for a heavy power transformer. As an extra feature, such radios could be operated on AC or DC mains. As this arrangement limited the plate voltage and indirectly, the output power that could be acquired, the resultant supply was sufficient for numerous purposes. Filaments tap on the rectifier tube given the 6 volt, low current supply required for a dial light.

Direct and indirect heating:

It became very common to utilize the filament to heat an individual electrode called the cathode, and to utilize this cathode as the source of electron flow in the tube instead of the filament itself. This minimized the introduction of hum if the filament was energized by alternating current. In such tubes, the filament is termed as a heater to differentiate it as an inactive element. Development of vacuum tubes which could employ alternating current for the heater supply let removal of one rectifier element.

Tutorsglobe: A way to secure high grade in your curriculum (Online Tutoring)

Expand your confidence, grow study skills and improve your grades.

Since 2009, Tutorsglobe has proactively helped millions of students to get better grades in school, college or university and score well in competitive tests with live, one-on-one online tutoring.

Using an advanced developed tutoring system providing little or no wait time, the students are connected on-demand with a tutor at www.tutorsglobe.com. Students work one-on-one, in real-time with a tutor, communicating and studying using a virtual whiteboard technology.  Scientific and mathematical notation, symbols, geometric figures, graphing and freehand drawing can be rendered quickly and easily in the advanced whiteboard.

Free to know our price and packages for online physics tutoring. Chat with us or submit request at info@tutorsglobe.com