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Vacuum Tube Theory

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Vacuum Tube Theory David Andresen May 8, 2009 Applied Science Research Dr. Dann Vacuum Tube Theory I. The Big Idea For my second semester project, I have chosen to research and build a vacuum tube amplifier. I will learn about the components of an amplifier circuit, including vacuum tubes, capacitors, resistors, and transformers, while also trying to understand how each of the parts perform a specific function that allows the circuit to run as a whole. After I have gained a sufficient amount of knowledge on the inner-workings of vacuum tube amplifier circuits, I will build my own based off of an online schematic. From the experience gained from building a pre-designed tube amplifier, I will hopefully know enough to design my own circuit one day. II. Introduction Although I have used both solid state and vacuum tube guitar amplifiers many times, I have never actually learned how either of them work. I specifically chose to focus on vacuum tube amplifiers because they are generally regarded as having a better and more natural tone than solid state amplifiers [4]. Expert musicians and audiophiles claim that the sound of a tube amplifier is superior to that of all other amplifiers, and the majority of high-end guitar amps are built with tubes. However, many people consider tubes as obsolete because the solid state devices that have begun to replace them are more efficient and less expensive. Apart from a diehard collection of vacuum tube lovers, many people think that tubes are a thing of the past, just like how digital TV is rapidly replacing analog. Part of my project will be to try and understand why tubes have such a distinct sound that is loved by many who know their way around amplifiers, and why the same sound cannot be reproduced with a solid state device. The ideal solution would be to build an amp that didn’t have the complexity of a tube amp but had the same dynamic tone that music lovers are looking for. III. Tube Theory Triode Vacuum Tube Fig. 1: Diagram of Single Triode Vacuum Tube [1] There are three types of basic vacuum tubes- the triode, the tetrode, and the pentode[ 1]. The simplest of the three, the triode, is pictured above and consists of three major elements. For now, we’ll consider the cathode and filament as one component. A triode has a cathode, grid, and plate. The cathode is heated by the filament, to the point at which loose electrons are kicked off of it through the vacuum and towards the positively charged plate. The grid is a mesh of fine wire, to which a voltage is applied with respect to the cathode. A negative grid voltage will choke off the flow of electrons. As a voltage is increased between the cathode and the grid, more and more electrons will flow until the tube reaches its saturation point [2]. Normally when AC current flows through the filament and heats the cathode enough to release electrons off towards the plate, a current is produced through the plate and circuit connected outside the tube. The plate current and voltage is much larger than the initial voltage between the grid and cathode. The voltage between grid and cathode is the input signal- a small voltage you would get from the source of the audio, such as an electric guitar, or electric piano. The signal is then amplified through the properties of the vacuum tube, and in the end you have a much larger voltage and (with and impedance matching transformer) current that can drive speakers. The other types of tubes have different components, including a screen between the grid and cathode, and a suppressor between the grid and plate. Fig. 2: A simple Triode amplifier circuit with tube and power supply [2] IV. Properties of Vacuum Tubes The general idea behind the operation of amplification in a vacuum tube may be easy to understand, but it is also important to determine what the significant properties of the tube are and why they lead to amplification. The process of amplification starts at the cathode and the filament. As in all metals, electrons are moving freely along the cathode, and some move towards the surface of the metal. Like water molecules boiling off and becoming water vapor, they escape the surface and fly off into the vacuum. It takes a considerable amount of energy for the electrons to escape the attraction of the positively charged metal atoms. The energy required for an electron to break free is called the “work function” and different for different kinds of metals [6]. Tungsten, the same kind of metal used in filaments and vacuum tubes has a work function of about 4.5 electron Volts. When thorium is added, the work function is lowered to 2.6eV[6]. The function describing the emission vs. temperature is an exponential one, meaning a slight change in the temperature has a huge affect on the emission. It is this emission that serves as the basis of amplification in a vacuum tube. The flow of electrons to the plate is controlled by the input voltage, and the plate current is the amplified signal. V. AX84 P1 For the amplifier I plan to build, I have chosen one with two stages of pre- amplification and one output stage. The amplifier is called the AX84 P1, and the website which designed the circuit also provides the schematic, as well as a building reference guide and a parts list. The AX84 uses two 12AX7 pre-amp tubes and one EL84 power tube [2]. The 12AX7 is a dual triode, meaning it has double of everything a triode would- two filaments, cathodes, grids, and plates. The EL84 (also knows as the 6BQ5) is a tetrode with a 9-pin miniature base, and it is used in the output portion of the circuit [3]. The AX84 P1 has three tone controls for treble, bass, and middle frequencies. It is run off of two Hammond transformers and produces about 5 watts of power. The schematic and layout of the amp is included below VI. Sample Circuit Preamp Stage 1 Fig. 3: The first preamp stage of the AX84 P1 [2] This is the first stage of the AX84 P1 amplifier. It consists of a 12AX7 triode, several resistors and some capacitors. The 12AX7’s grid is grounded through R9 and R12, so it has no voltage with respect to the cathode when there is no input signal. From looking at the 12AX7 spec sheet, it can be found that when the plate voltage is about 150V, and the grid voltage is approximately -2.2V, 0mA of current will flow through the plate [2]. As I discussed before, when the grid voltage is made more negative, electrons will stop flowing to the plate at a certain point. If the voltage is increased, electrons will flow until they reach the saturation point of the tube and no further increase of voltage after that can increase the current. There is a voltage in-between the cut off and saturation values that provides the most stability and least distortion of the signal that is called the bias point [4]. Again, from the spec sheet we can find that the cut-off point is -2.2V and the saturation point is 0V [2]. The bias point for a 12AX7 preamp tube is -1.1V. Instead of making the grid have a negative voltage, it is possible to raise the cathode’s voltage to 1.1V. If you consider R4, which has across it of 74V, we can find the electron flow into the plate. Plate Current = (74V)/(100K Ω) = .74mA By using Ohms law, we can see that .74mA is flowing through the 12AX7’s plate, and that most of the current exits through the cathode and through R13. By adjusting R13’s resistance to the correct value, we can make sure that the tube will be at the correct bias point [2]. So, if .74mA is flowing through the cathode and R13, Cathode Voltage = (.74mA) * (1.5K Ω) = 1.1V Setting the Cathode’s voltage to 1.1V is essentially the same thing as having a grid voltage of -1.1V, so the tube is at its bias point and will operate at the most optimal voltage. The input signal is amplified a little bit in preamp stage 1, a little bit more in preamp stage 2, and a lot by the EL84 power tube in the output stage of the circuit. By the end, if each of the vacuum tube’s bias point is set correctly, the input voltage will be greatly amplified without changing or distorting the signal VII. Simple Tube Circuit As a proof of concept before attempting to built my main tube amplifier, I built a simple amplifier circuit with a single 12AX7A dual triode vacuum tube. There were two major parts to the tube circuit, the actual amplifier circuit and the power supply. I built the amplifying section of the circuit with the two plates of the two triodes wired in parallel on a breadboard because though it was a very high voltage circuit (the plate voltage coming from the power supply should have been running at about ≈ 170 volts [5], but when measured it was slightly lower at about 159 volts), it drew less than an amp of current. To connect the nine pins of the 12AX7A tube to the sockets in the breadboard, I soldered leads onto each of the pins. Later, I tried placing the tube into a socket specifically designed for the 12AX7A and connecting leads to the breadboard directly without soldering them.
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