VACUUM TUBES

A Research Paper

Presented to the Faculty of
School of Engineering
Asia Pacific College
Magallanes, Makati

by

Pamela Joyce M. Espigol

In Partial Fulfillment
Of the Requirements in
Electronic Devices and Circuits Lec

Date Submitted
October 30, 2015

II. INTRODUCTION a. RESEARCH PAPER SUMMARY NO. | ABSTRACT | METHODOLOGY | RESULTS | CONCLUSION | RECOMMENDATION | 1 | The objective of this work is to simulate a tube guitar amplifier, the Giannini True Reverber designed by Carlos Alberto Lopes in the nineteen sixties. The nonlinear “overdrive” characteristics of these devices make them attractive for guitarists since odd harmonics are added into the guitar sound as well as sound compression. The shortcomings of these amplifiers led to the development of DSP simulation techniques over the last few years. Many past DSP simulations of tube amplifiers were implemented using Static Digital Wave shappers for the task of replicating the tube transfer characteristics. Since the physical behavior of such systems is quite complex, physically informed models are necessary for more precision in the simulation, requiring more computer power. A Wave Digital Filter (WDF) simulation of the Giannini True Reverber double 12AX7 preamp is accomplished in this work using Koren’s triode equations and Block Compiler, where each parameter was acquired in the original electronic schematic or by measurement of the real amplifier. The real preamp is compared to the WDF model using the following test signals: single tone, logsweep and transient signal analysis. The results suggest that the current triode models do not cover all circuit topologies, so that further research is required. | A. Giannini True Reverber * In recent years Brazilian made “vintage” tube amplifiers have been rediscovered by musicians. A characteristic of these amplifiers are their high output power, ranging from 18 W to 300 W. * A double triode 12AX7 vacuum tube is the amplification device. B. Tube Guitar Amplifier Simulation * The models must be categorized as either linear or nonlinear since analog amplifiers have corresponding linear circuits (RC or LCR filters) and nonlinear circuit elements (transistors, operational amplifiers, diodes and electron tubes) * B.1 Nonlinear Digital Filters * The most straightforward way to generate nonlinear distortion in digital audio signals is by applying a nonlinear function into each sample of the signal. * An example of these functions was implemented by [Gallo, 2011] proposed in a patent that comprises a complex waveshaper to emulate the effects of vacuum-tube amplifiers:f(x) =(k1+x)/(k2-x) if x<aX if a<x<b(x-k3)/(x+k4) if x>bwhere k1 = a2, k2 = 1 + 2a, k3 = b2 and k4 = 1 − 2b. The values of a and b can be freely chosen between −1.0 and +1.0 in order to control the characteristics of the nonlinear function. C. Physically Informed Amplifier Models * It is necessary to convert a circuit schematic into a system of equations that correspond to the amplifier circuitry. Circuit elements such as capacitors, resistors and inductors are modelled directly using classical circuit equations whereas nonlinear circuit elements such as diodes, transistors and vacuum tubes are modelled with more complex equations to approximate their electric transfer. * C.1 Koren’s Vacuum Tube Equations * These models were successfully used in most of the physically informed vacuum tube guitar amplifiers digital emulators developed over the last few years, and in SPICE (Simulation Program with Integrated Circuit Emphasis) simulations. D. Physically Informed Amplifier Models * It is necessary to convert a circuit schematic into a system of equations that correspond to the amplifier circuitry. Circuit elements such as capacitors, resistors and inductors are modelled directly using classical circuit equations whereas nonlinear circuit elements such as diodes, transistors and vacuum tubes are modelled with more complex equations to approximate their electric transfer. E. Wave Digital Filters * The main advantages of this model methodology are: high modularization potential, energy preservation by the use of Kirchoff laws and good numerical properties in its implementations, leading to efficient real time digital models of virtual analog circuits for audio effects. F. Nonlinear Audio Metrics * One special testing methodology was created by [Pakarinen, 2011]. This methodology uses single tone, intermodular, logsweep and transient signal analysis of a system´s output signal and also the input signal. These techniques were used in a case study by [Oliveira et al., 2012] to characterize nonlinear distortion of an all tube Giannini True Reverber amplifier. G. Giannini True Reverber WDF Model * The digital WDF simulation of the True Reverber pre-amplifier was accomplished by the use of two WDF triode models. * These models use Koren’s triode equationsinordertoproducethenonlineartransferandhighgainofthecommon-cathode triode amplifiers. | * The virtual and real True Reverber pre-amplifier have distinct plot patterns for the test results. This suggests that WDF’s and Koren’s triode models led a simulation that does not match the real amplifier. * Many models were biased since SPICE also uses Koren’s triode equations for triode models and it is also a digital simulator * The power supply of the these virtual circuits utilize lower voltages, such as 250 V. Koren’s Equations were designed to work in that range, as opposed to most real amplifiers that operate in the 350 V - 450 V range. * The simulation of the Giannini True Reverber amplifier was accomplished using parameters from the real schematic and measurements from the real amplifier (435 V power supply voltage), suggesting that in cases of real physical parameters the simulations may be unable to match the transfer of real tube amplifiers using Koren’s triode Equations. | * The results of Wave Digital Filters simulation using Koren’s triode equations were unable to reproduce the transfer of a real amplifier, the Giannini True Reverber, suggesting that current Vacuum tube amplifier simulation techniques were unable to precisely match the sound characteristics of this amplifier. * For real time simulations, there is always a trade-off between accuracy and computation time. * The quantitative data of this study describes that in 90 % of the blind tests, a trained listner can distinguish if a distorted “Crunch” guitar sound implemented in was either genterated by a simulation program or by the actual physical analogue equipment. * The same investigation for the Fender style “Clean” by [Macak, 2012b] sound alson described that in 83 % of the blind teste the listeners were able to correctly identify if the sound was generated by a simulation program or the actual amplifier. * This leads to the conclusion that vacuum tube amplifiers and related audio equipment will not be replaced by simulation programs in the near future. However this does not discart the emulation softwares as accessible tools for the simulation of these devices, as the simulations can be consider as satisfactory for the less trained listeners and musicians. * The flexibility of these emulation tools is also an advantage over expensive, large, heavy and hard to handle vacuum tube amplifiers. It is also worth noting that these simulation softwares can be utilized as a sound reference tool for musicians to be acquainted to the classic electric guitar vacuum tube sound. | * More research should be conducted in order to derive a set of equations that will satisfactorily describe the physical behavior of vacuum tube amplifiers, as proposed by [Cohen and Helie, 2012], but these new triode equations have yet to prove their potential, since they were developed very recently. | 2 | The OPTHER (Optically Driven THz amplifier) project supported by the European Commission within the Seventh Framework Program (FP7) represents the first joint European attempt to realize vacuum electron devices in THz range. The target of the project was to design and realize the first 1 THz vacuum tube amplifier. The challenges of the presented task and the innovative solutions adopted established a new level of knowledge in the field. The main aspects of the OPTHER project are described, focusing on challenges and adopted innovative solutions. | A. Cathode and Electron gun * Two novel approaches were considered: the optically modulated electron beam and the carbon nanotube cold cathode * The optical control is able to convert an incident optical beam with wavelength within a suitable span into a localized surface wave such that the electric field is oriented perpendicular to the surface and is enhanced. That would ensure maximal interaction with carbon nanotubes, thereby enabling fast modulation of the emission of the CNT cold cathode by an optical control * A relevant investigation was performed, and is still in progress, to realize a carbon nanotube (CNT) cold cathode. Different patterns of CNT emitting structures were investigated and realized by the CVD technique. * The cold cathode approach requires further investigation to be used in THz tube. To test the tube, a thermionic electron gun was designed and realized. B. Slow Wave Structure * The available high aspect ratio fabrication processes due to their properties, limit the degrees of freedom of the structure shape to two dimensions. * Since the sheet beam technology is not so mature, it was decided to adopt a cylindrical beam. This posed a great challenge that was overcome by the double corrugated waveguide C. Input and Output Coupling * The feeding of the RF signal at 1 THz was a critical issue due to the high level of losses and to the lack of reliable measured values. * A vertical tapering of the double corrugated waveguide (Fig.5a), provided the best input/output coupling performance, but the number of steps required for its realization made this unfeasible. * The solution was to taper the lateral position of the corrugations (Fig.5b). This approach also guaranteed a good coupling and compatibility with the technological process D. Photolithographic Fabrication * Two fabrication techniques were used: SU-8 UV-LIGA and X-ray LIGA. The results obtained by SU-8 UV-LIGA were promising, but the X-ray LIGA was more reliable and accurate. E. Assembly * The assembling of the tube represented a challenging phase due to the dimensions of the parts and alignment issue. The assembling of the tube represented a challenging phase due to the dimensions of the parts and alignment issue. | * Transverse electron velocity effect is greater without compensation than with compensation * The X-ray LIGA was more reliable and accurate. X-ray LIGA is based on illuminating the mask by the synchrotron light. * The feeding of the RF signal at 1 THz was a critical issue due to the high level of losses and to the lack of reliable measured values. A vertical tapering of the double corrugated waveguide, provided the best input/output coupling performance | * The first ever designed and realized 1 THz cascade backward wave amplifier is presented. * Due to the lack of experience and knowledge for amplification at THz frequencies, a huge theoretical and experimental effort due to the lack of experience and knowledge for amplification at THz frequencies was required from the Consortium. | N/A | 3 | Vacuum devices, such as TWTs and BWOs, for THz regime, represent a new challenge in vacuum electronics. Structures with dimensions in the range of microns are required to optimize the energy transfer from an electron beam to the RF field. The availability of codes to accurately analyze micro slow-wave structures at THz frequency is fundamental for a reliable and fast design. A procedure to design a THz backward-wave oscillator based on an analytical code to compute the cold parameters in backward-wave mode and 3-D electromagnetic codes for device performance is presented. An output power of 190 Mw is demonstrated at 1027 GHz. | A. Analytical computation of the cold parameters * The field expansion in is used to compute the dispersion equation and the backward wave interaction impedance of a corrugated waveguide. * The schematic of the corrugated waveguide: * The dispersion equation is:where: * The interaction impedance of the n-th space harmonic is defined: B. 3. 1-THz Backward wave oscillator * A backward wave oscillator based on a corrugated rectangular waveguide is designed and simulated. The first step is to design and optimize the corrugated waveguide SWS (Fig. 1), by the presented analytical model, to get the required central operating frequency at about 1 THz. * The dimensions of the corrugated waveguide SWS are reported in Table I. The dimensions are in the range of high-aspect ratio micro-fabrication processes. A beam voltage of 12Kv is chosen to cross the interaction impedance curve in its central region. This corresponds to an operating frequency of 1.027 THz (Fig.2) and assures a wide tuning range. * The width (Table I) of the sheet electron beam (Fig.4) is chosen by observing in Fig. 3 that, in the region included between 80 µm and 160 µm, Kp has a maximum 20% reduction with respect to its maximum value. * The beam current is fixed at 8Ma * A uniform focusing magnetic field of 0.9 T is applied to assure a proper beam confinement. To reduce the computational effort in the simulation setup, the cathode was synthesized with an equivalent emitting surface. | A. Analytical computation of the cold parameters * After some calculations results: B. 3. 1-THz Backward wave oscillator * The dimensions of the corrugated waveguide SWS are reported in Table I. The dimensions are in the range of high-aspect ratio micro-fabrication processes. A beam voltage of 12Kv is chosen to cross the interaction impedance curve in its central region. This corresponds to an operating frequency of 1.027 THz (Fig.2) and assures a wide tuning range. * The width (Table I) of the sheet electron beam (Fig.4) is chosen by observing in Fig. 3 that, in the region included between 80 µm and 160 µm, Kp has a maximum 20% reduction with respect to its maximum value. | * An analytical method to compute the dispersion and interaction impedance in backward wave mode for the corrugated waveguide SWS has been used in conjunction with 3-D codes for the design of THz backward wave oscillators. * The high output power obtained demonstrates the effectiveness of the method and the feasibility of vacuum devices in THz frequency range. | N/A | 4 | Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (Mw-to-GW) and two orders of magnitude in frequency (from to THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques. | A. BACKWARDS–WAVE OSCILLATORS (BWOS) * We have a closed feedback loop and the BWO is capable of oscillation providing that either the beam current or the SWS length exceed the threshold necessary for start oscillation. * The fact that the beam propagates in the opposite direction to the growth of the wave means that the beam current bunching is greatest at the downstream end of the circuit while the wave amplitude is greatest at the upstream end of the circuit. B. EXTENDED INTERACTION KLYSTRONS (EIKS) * The high gain per unit length of the extended interaction circuit reduces the total length of the interaction circuit, simplifying the design of the permanent magnet focusing system typically used for the EIK. * Current Capabilities of the EIK: The EIK concept has proved highly scalable at lower frequencies 220GHz , both up and down infrequency. This has the advantage of short design cycle times, as well as preserving a well proven rugged design. * Physical Limitations of the EIK Approach for THz Applications: The dimensions of the RF cavities can indeed be scaled, but other factors such as conductivity, electron velocity spread and thermal dissipation become more significant than at lower frequencies. * Solutions for THz Applications: The trade-off is to maintain a rugged mechanical structure for acceptable thermal performance while maintaining sufficient coupling impedance with the electron beam and reducing the sensitivity to manufacturing tolerances. There are several factors to consider, including: 1) Formation of a suitable electron beam including reduction of velocity spread; 2) reduction of ohmic losses (possibly achieved by overmoded operation); 3) improvement of thermal stability.C. TRAVELING-WAVE TUBES (TWTS) * The TWT amplifies by converting kinetic energy from an electron beam to an RF electromagnetic wave. * Several researchers are developing 220 GHz TWTs where critical challenges include the fabrication of the interaction circuit, a high current density cathode structure that can be accurately integrated with the circuit and achieving stable high power electron beam transport. * Based on their success at 656 GHz, Northrup Grumman is usingcoupledfoldedwaveguideinteractioncircuitsat220GHz and an electron beam array consisting of five separate beamlets.D. GYROTRONS * The dependence of an electron’s cyclotron frequency on its energy is a relativistic effect. The result is a coherent, macroscopic, transverse, cyclotron frequency current that generates transverse EM waves. * The resonance condition for the excitation of the cyclotron resonance maser instability is met when the electron beam satisfies the cyclotron beam mode dispersion relation: * High power THz Gyrotrons using CW magnets, high power THz Gyrotrons using pulsed magnets, CW THz Gyrotrons, CW THz Gyrotrons for DNP/NMR D. FREE ELECTRON LASERS (FELS) * This slippage has significant consequences for the optical pulse lengths and bandwidth. * Practical Implementation: The capability of an FEL is to a great extent governed by the performance characteristics of the accelerator that drives it. Since only relatively low electron beam energies are required for THz generation, another sort of accelerator is feasible, the field emission diode driven by a Cockroft-Walton or similar pulsed high voltage generator. Voltages of up to a couple of MeV are applied to a field emission cathode and significant currents can be produced. E.RELATIVISTIC BEAMLINE SOURCES * If the electron bunch is compressed to subpicosecond duration, the emitted radiation pulse can be less than a picosecond in duration and therefore contain coherent spectral content up to a few THz. This is readily illustrated by modeling the radiation pulse’s electric field as a differentiated Gaussian function * Nonlinear cross-phase modulation (XPM) in an electro-optic crystal was demonstrated by the strong electric field associated with intense THz radiation * The direct excitation of coherent lattice modes in a semiconductor was investigated using intense, ultrashort terahertz pulses where there laxation dynamics can be studied with time-delayed optical/IR pulsesF. MICROFABRICATION AND ADVANCED CATHODES * SU-8 is notorious for being extremely resistant to attack by chemicals and is therefore very difficult to remove, although certain molten salts have been shown to work very well. An alternative photoresist for UV-LIGA that has been used for MMW VEDs is KMPR, a photoresist similar to SU-8 that is much easier to remove. * Processes for UV-LIGA and DRIE illustrating pathways to form both all-metal and metalized silicon illustrate the UV-LIGA method: first, a silicon wafer is coated with a thin layer of SiO , followed by a thin coating of photoresist. After UV exposure, the photoresist is stripped leaving a micropattern of exposed SiO. The exposed SiO is then etched away and then the photoresist is removed, leaving behind a micropattern of SiO on silicon. | * Table II shows the projected capability of EIK based devices in the high MMW and low THz region. Development work is currently underway to explore the performance of this device at frequencies up to 1.03 THz * Fig. 15. Current achievements in power versus frequency for THz and near-THz VED sources. The sources include three cateogories: (1) compact sourceswith high mobility; (2) compact gyrotrons with moderate mobility; and (3) stationary accelerator-based sources. Specific VED types include: BWOs, clinotrons, orotrons, TWTs, EIKs, gyrotrons (gyro), FELs, and BL sources using either CTR or CSR. | * The THz gap is far from an empty wasteland of technology choices. THz VEDs cover over 12 orders of magnitude in power and over two orders of magnitude in frequency. In terms of power frequency characteristics, the device choices include roughly three classes. :1) Compact sources with high mobility. These include BWOs and their relatives, EIKs and TWTs. They currently fill a performance window of 0.1–1.0 THz and 10 –10 W (CW and pulsed). 2) Compact gyrotrons with moderate mobility. These currently fill a performance window of 0.1–1.0 THz 3) Stationary accelerator-based sources. These include FELs and beamline sources. They currently fill a performance window of 0.2–10 THz (and beyond) and 10–10 W (average and pulsed). * Further advances of gyrotrons will derive from methods to operate at high harmonics and the development of cryogen-free, superconducting magnets. * Advances in accelerator-based sources such as FELs and beamlines will follow from better fundamental understanding and control of mode hopping and stability, broadband radiation output coupling and control and pulse stacking for higher single pulse energies. * New methods for THz metrology and electromagnetic characterization of material properties and components’ electromagnetic performance will benefit all classes of high power THz VED sources | * Future advances in THz gyrotrons could come from many directions. Operation at very high harmonics would be very attractive; it may be possible with axis-encircling beams. The advent of cryogen-free, superconducting magnets will also simplify the implementation of gyrotrons in the THz frequency band. * Further progress to develop microfabricated VED circuits for high power THz radiation sources must certainly continue this path and abandon the standard fabrication techniques that have been around for decades. Since component and assembly tolerances will be a key factor, there is a strong incentive to move away from heated cathodes, if any other technology can prove itself. * The interaction with wave guides and transitions to free space modes is sufficiently complicated that there is still much to learn about optimization of such designs. Issues include mode hopping and dead bands which appear in THz FEL systems when users would really like continuous tunability. Also the practical issues of making a broadband cavity outcoupler and pulse stacking for higher single pulse energies are still open to identification of better solutions. * Further progress is already underway to increase power and bandwidth, improve stability and continuous tunability, decrease size and weight, increase efficiency and reduce costs for THz-regime BWOs and clinotrons, EIKs, TWTs, gyrotrons, FELs, and BL sources. * Further development of materials and microfabrication techniques, new, high-current-density, long-lived, miniature cathodes, novel beam configurations and more precise alignment and assembly techniques are especially important to enable further advances of the most compact VED sources. * Further advances of gyrotrons will derive from methods to operate at high harmonics and the development of cryogen-free, superconducting magnets. * Advances in accelerator-based sources such as FELs and beamlines will follow from better fundamental understanding and control of mode hopping and stability, broadband radiation output coupling and control and pulse stacking for higher single pulse energies. * New methods for THz metrology and electromagnetic characterization of material properties and components’ electromagnetic performance will benefit all classes of high power THz VED sources. | 5 | The operation of high-power, high-frequency vacuum tubes requires an appropriate protection method to avoid significant damages during arcing. Fast closing switches like spark gaps , thyratrons, ignitrons and semiconductors acting as charge-diverting bypass switches are the most commonly used protection method. These “crowbar” switches cause hard transient conditions for all subcomponents involved and usually result in a significant post-fault recovery period. The availability of fast high-voltage semiconductor devices, with flexible on/off control function, makes opening switch topologies possible and attractive to improve this situation. This paper describes a circuit topology to protect an Inductive Output Tube which is expected to operate within RF subsystems for accelerator applications. The topology is characterized by using a commercial available high voltage MOSFET switch with direct liquid cooling and completed with essential snubber extensions. The advantages of the opening switch approach are faster action, smaller fault energy, faster recovery, and more compact design. Initial test results of this topology are presented. | A. Main Characteristics * Operating Conditions * The ratings are 48 kV dc operating voltage, 3.8 A dc operating current and fault energy smaller 10 Joule during arcing. * Pulsed mode operation is characterized by 1 Hz repetition rate and duty factor ranging from 0.1 to 0.5 respectively. * Preceding Closing Switch Protection * Accomplished by the application of the classic closing switch approach by means of Light Triggered Thyristors. * For sufficient margin in case of arcing, additional current limiting resistors had to be applied. The protection efficiency of this previous test configuration will be compared with the new opening switch. * Main Components for Opening Switch Protection * Semiconductors and Auxiliaries: For fast switching operation a BEHLKE High-Voltage Transistor Switch type HTS 701-10-LC2 based on MOSFET technology in combination with external control electronic is chosen. Because of the heat load of dc mode operation, a direct liquid cooled version is deemed necessary. The MOSFET module has been modified compared to the standard version by means of additional series diodes to protect the intrinsic body diodes from reverse current. * Snubber Unit: The operation of the High Voltage Transistor Switch (HTS) for short circuit conditions (tube arcing) is only possible with an additional series connected snubber unit. The HTS has to be switched off during arcing by the control electronic fast enough before reaching a critical current level that might damage the output transistors. B. Test Circuit Topology * To evaluate the switching performance of the opening switch approach a test topology according Fig. 2 was applied. * Fault current sensing is accomplished with a Pearson monitor at low side and an appropriate level comparator to generate the /INHIBIT signal for a fast off command of the HTS. | A. Wire Test * A wire test has been prepared to prove that the energy transfer during arcing is well below the 10 Joule limit allowed. * The uncoated #36 AWG copper wire with a length of 12.8 inch has been specified by the manufacturer of the tube. Closing the short circuit a fast high voltage thyristor module (Behlke HTS 800-100-SCR) was used. This module is able to withstand 80 kV at a maximum turn-on peak current of 1 kA for 100µs, which is sufficient for the overvoltage and pulse current expected during testing. B. Snubber Unit Decoupling Feature * To prove the essential decoupling function of the snubber unit, the return current with respect to wire current was measured during short circuit operation. * The current rise of return current correlates the expected value of 100 ampere per microsecond. For proof of principle the parasitic oscillations observed are not considered within this context. C. Fault Current Charge Transfer * To demonstrate the significant improvement achievable with the new opening switch approach compared to the preceding closing switch crowbar the charge (Q) transferred through the test wire has been identified. | * The initial test results of this work prove the feasibility of the opening switch approach based on compact semiconductor modules for tube protection applications. The requested maximum fault energy smaller 10 joule can be accomplished easily. * In general there is significant potential of this circuit topology to achieve even smaller fault energy levels. This work is considered as a first step taken to prove the principle. | * The very fast switching speed of the MOSFET module under high voltage conditions causes significant interaction with stray capacitances and leakage inductances of the circuit topology. An improved construction of the circuit layout has to avoid possible resonant transients intrinsically. • For a very compact mechanical design alternate housing and cooling methods of the switch module in strong relation to the snubber unit needed have to be investigated.• High side current sensing for fault level detection and fast /INHIBIT command is the preferred solution. Solving the contradiction of sensitivity and robustness against electromagnetic interference is a challenging task. The availability of fast current monitors with dc capability is mandatory. • The overall reaction time of the system is 400 ns for now. Further improvement, if desired, is possible with a redesign of the external control unit to get faster response time for the /INHIBIT interface. |

III. RELATED LITERATURE
Here are other related articles to showcase that all vacuum tubes have the capability to amplify or modify any sort of signal which can be applied to many materials humans use on a day-to-day basis:
[1] Microwave vacuum tube device employing grid-modulated cold cathode source having nanotube emitters
Abstract:
An improved gridded microwave tube is provided, the tube containing a cold cathode, an anode, and a grid located between the anode and cathode. In one embodiment, the cold cathode has a refractory metal substrate and carbon nanotube emitters, the emitters having a diameter of 1 to 300 nm and a length of 0.05 to 100 μm. The grid-cathode spacing is 1 to 100 μm, the grid contains apertures having a maximum dimension of 0.5 to 100 μm, and the grid thickness is 0.5 to 100 μm. Emission from the cathode directly onto the grid material itself, which undesirably heats the grid, is reduced by either (a) the presence of a shadow mask between the grid and the emitters or (b) selective formation of the emitters in locations that correspond to the grid apertures. The microwave tube operates at a frequency of greater than 0.5 GHz, advantageously greater than 2 GHz.
[2] Thermal performance of a solar cooker integrated vacuum-tube collector with heat pipes containing different refrigerants
Abstract:
A solar cooking system using vacuum-tube collectors with heat pipes containing a refrigerant as working fluid has been fabricated, and its performance has been analysed experimentally. The experiments were conducted during clear days in July and August of 2002 in Elazığ, Turkey under similar meteorological conditions for three refrigerants and water. Detailed temperature distributions and their time dependences were measured. The maximum temperature obtained in a pot containing 7 l of edible oil was 175 °C. Also, the cooker was successfully used to cook several foods. The cooking processes were performed with the cooker in 27–70 min periods.
[3] The cool sound of tubes [vacuum tube musical applications]
Abstract:
Although solid-state technology overwhelmingly dominates today's world of electronics, vacuum tubes are holding out in small but vibrant areas. Here, the author describes how music applications are one of the last remaining domains dominated by vacuum tubes, and how the devices flourish and even innovate in this field

IV. ANALYSIS The first articled tackled about a project regarding using a vacuum tube called ‘Giannini True Reverber’ to amplify the sound of a guitar by using wave digital filters. Since the physical behavior of such systems is quite complex, a Wave Digital Filter (WDF) simulation of the Giannini True Reverber double 12AX7 preamp is accomplished in this work using Koren’s triode equations and Block Compiler. However, the results suggest that the current triode models do not cover all circuit topologies and further research is needed to accomplish such project.
The second article is about using vacuum tubes for THz amplification. The article mainly focused on the problems people who previously performed such project faced and what actions they have taken to resolve them. The vacuum tube that they used is about five times the highest frequency of any vacuum amplifier ever reported. They used two novel approaches: the optically modulated electron beam and the carbon nanotube cold cathode. According to the article, the optically modulated electron beam was an extremely advanced solution. The feeding of the RF signal at 1 THz was a critical issue due to the high level of losses and to the lack of reliable measured values. A vertical tapering of the double corrugated waveguide provided the best input/output coupling performance although there were intricate steps to be followed. The third article is pretty similar to the previous article. It also used various vacuum tubes, such as TWTs and BWOs, for THz application. A procedure to design a THz backward-wave oscillator based on an analytical code and used an output power of 190 mW is demonstrated at 1027 GHz. The high output power obtained demonstrates the effectiveness of the method and the feasibility of vacuum devices in THz frequency range. This states that it’s found a better method to amplify THz. The fourth article also revolves around vacuum tubes regarding Terahertz. It talks about the Vacuum Electronic High Power Terahertz Sources. Although in this article, it talks more about what our generation’s current capabilities are regarding this topic and how it can be further advancement and development with more research for new materials and fabrication methods and also improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control. Also, THz VEDs has three categories: Compact sources with high mobility, Compact gyrotrons with moderate mobility, and Stationary accelerator-based sources.
The fifth article talks about fast opening switch approach for high-voltage vacuum tube protection application. The operation of high-power, high-frequency vacuum tubes requires an appropriate protection method to avoid shortcomings and significant damages during arcing. Based on the project the author has undergone, the advantages of the opening switch approach are faster action, smaller fault energy, faster recovery, and more compact design. Although the author has said, there are still various methods to create another circuit topology with smaller fault energy levels. This project is just a stepping stone to farther prove the principle. A vacuum tube is a device that amplifies or modifies a signal based on the user’s intent of usage. A vacuum tube has a vast amount of classifications and testing the capacity can be done in also a vast amount of methods. Some, although, has high faulty effects while some has low. All it takes is more research and a wider set of scope to be able to reach the correct result of modified (or amplified) signals that the user desires.
V. CONCLUSION AND RECOMMENDATION Alternatively referred to as an electron tube or valve and first developed by John Ambrose Fleming in 1904. The vacuum tube is a glass tube that has its gas removed which creates a vacuum. Vacuum tubes contain electrodes for controlling electron flow in early computers that used them as a switch or an amplifier. In layman’s term, a vacuum tube is a device that modifies or amplifies a signal by controlling the movement of electrons in an evacuated space. Beyond that, a vacuum tube has various types and also to which has various methods of testing whether it is reliable on a said project or not. After performing such research paper, I recommend to those who desire to create similar work is to familiarize one’s self with the specifications of the device you have chosen to use. Also, practice and perform minor experiments with the device to be able to have more profound experience before delving into the major work. If done so, it would be much easier to give observations and find faults in a project since you are knowledgeable about the device you’re working on. VI. REFERENCES
[1] Computerhope.com, ‘What is vacuum tube?’, 2015. [Online]. Available: http://www.computerhope.com/jargon/v/vacuumtu.htm. [Accessed: 29-Oct-2015].
[2] E. Barbour, ‘How Vacuum Tubes Work’, 2003. [Online]. Available: http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm#top. [Accessed: 29-Oct-2015].
[3] M. Rouse, ‘vacuum tube (VT, electron tube or valve)’, 2011. [Online]. Available: http://whatis.techtarget.com/definition/vacuum-tube-VT-electron-tube-or-valve. [Accessed: 29-Oct-2015].