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Home , Amplifier, EL84, Triode, Vacuum tube

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 tube . I will learn about the components of an amplifier circuit, including vacuum tubes, , , and , 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 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 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 claim that the 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 Vacuum Tube [1]

There are three types of basic vacuum tubes- the triode, the , and the [ 1]. The simplest of the three, the triode, is pictured above and consists of three major elements. For now, we’ll consider the 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 are kicked off of it through the vacuum and towards the positively charged plate. The grid is a mesh of fine , to which a is applied with respect to the cathode. A negative grid voltage will 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 - a small voltage you would get from the source of the audio, such as an , 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 ) 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 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 to break free is called the “” and different for different kinds of metals [6]. , the same kind of metal used in filaments and vacuum tubes has a work function of about 4.5 electron Volts. When 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, , 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 . 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 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 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 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 . I built the amplifying section of the circuit with the two plates of the two wired in parallel on a breadboard because though it was a very 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. The power supply circuit needed to be able to handle more current, so I built my own turret board where all the components could be easily and securely placed. For the plate power, a 24-volt variable AC power supply was used. 12 volts were fed into a 12:120 volt AC transformer to simulate power coming from a wall socket and then into the power supply section of the circuit. An additional 12-volt DC power supply was used to power the tube’s filament. The advantage of having two separate power supplies for the plate power and filament was that the plate power could be turned off to allow work to safely be done on the circuit while the filament could remain on and would not be damaged by powering it off and on multiple times. A generator was used for the audio input, and the output of the amplifier circuit was connected to a standard 8Ω .

Fig.4: Amplifying section of experimental circuit [5]

Fig. 5: Power supply section of experimental circuit [5]

Fig 6: Layout Diagram [5] VIII. Prototype Results

From the data taken and also just by listening to the audio output of the amplifier circuit, it was clear that something unintended was happening to the . Though I was able to get some output from the amplifier, the audio output was somewhat distorted and de-amplified. The first power supply with which the circuit was tested added excess to the circuit, causing the output signal to be very distorted and barely audible. Another power supply was tested as well, one known to be working in perfect order through testing on an , and it provided a much cleaner output signal. However, when the frequency generator was connected directly to the 8Ω speaker, the audio output there was much louder and clearer than when it was fed through the amplifier circuit. With the frequency generator set to 100Hz, I plotted the sound pressure vs. time. There are many deformities in the graph of the signal fed through the amplifier. As a control, I connected the frequency generator directly up to the speaker and measured the pressure again. The result was a nearly perfectly formed sine wave with only barely noticeable deformities at the peaks. The of the frequency generator was ≈ 3.16, much higher than the amplifier amplitude of ≈ 2.77. This could be due to many different reasons. When pairing devices of differing impedances, it is an important practice to match the two impedances [5]. In the case of this experiment the two devices are the amplifier and the 8Ω speaker. The amplifier is a high voltage, low current device, and the speaker is a low voltage, high current device [5]. The speaker could be connected directly to the amplifier without an impedance matching output transformer, but the power output would be minimal. The vacuum tube circuit that was built has a Thevenin resistance of about ten thousand ohms, and the speaker of course has 8 ohms of resistance. Therefore, to effectively match the two impedances a transformer with the ratio of 10,000:1 one is need to ensure maximum power output [5]. This also means that because the impedance of a transformer is the square of its turns ratio, it would needs a turns ration of 100:1. In the experimental plans, an automotive ignition coil is used because it has about the right ratio of turns and can be used at very high . Because these are somewhat expensive, and I could not find one at nearby stores, I opted for an output transformer. However, I mistakenly purchased one with an impedance ratio of 100:1 and a turns ratio of 10:1. This is off by a factor of ten, and could have been why the amplifier seemed to be de-amplifying the signal. I tried connecting the amplifier signal directly up to the speaker without the output transformer, and the audio output was even quieter, suggesting that the output transformer was at least helping a little bit. I checked the wiring many times, and could find no flaws, but there is always the chance of a wiring error or that the capacitors and resistors were not to spec. Finally, I tried using a brand new 12AX7A tube plugged into a in case it was the vacuum tube that was causing problems, but I received similar results to before.

IX. Results

After a lengthy construction process, I have completed the build portion of the AX84 P1, but have not yet been able to troubleshoot it. Overall I am pleased with the construction of the amplifier considering how much work went into it. Though I did expect some aspects of building the amplifier to be challenging, I was surprised how often I had to sit down and think through roadblocks on each step of the construction. Drilling, preparing and sealing the chassis was a great way to learn how to effectively and efficiently use the drill press when you have many precise holes to drill. Installing the components onto the eyelet board taught me the importance of patience and double-checking my work. Installing all of the components including the transformers, tube sockets, jacks, knobs, , , and pilot light showed me that I could depend on the previous work I did with the drill press. And finally, installing and soldering the internal wiring gave me a valuable lesson on assembling electronics and how complicated they can get. Upon completion of the wiring inside the amp, I took it home to test it on a speaker cabinet. I carefully (while wearing gloves and glasses) switched the standby to the on position, and let the tubes warm up. When I switched on the main power the pilot light lit up and there were no significant problems. I switched it off, checked the fuse, and plugged a guitar into the input and a cable going from the output to the speaker cabinet. I powered the amp back on, only to get no signal coming from the output. I tried the 4Ω, 8Ω, and 16Ω output jacks, but I did not get any sound going to the speaker cabinet. I am still pleased that the amp turned on without any problems considering there is usually a long process of debugging that is needed before an amp is working properly. As of now, while the amp does power on and off just like it should, there is a mistake somewhere in the wiring that needs to be fixed in order for the amp to work properly

X. The Next Step As a novice amp builder, I am sure that the problem is linked to some trivial mistake, and the first place I will look at is the input and output wiring. After that, I will do a complete check of the internal wiring because I have not yet had a chance to double-check my work. In reading the AX84 forums, there are countless posts of people worried that their amp has no output at all. However, most of these people’s problems are solved after a little trouble-shooting and help from the amp building community. I am confident that with a little time I will be able to figure out what went wrong and fix it.

Resources

[1] All About Circuits : Volume 1: Chapters 1, 2, 3, 5, 7, 8, 9, 13 Volume 3: Chapters 1, 13

[2] AX84.com: Schematic, building plans, tube theory, circuit design

[3] diyaudio.com: various threads on AX84 P1

[4] Basic : The essentials of electron tubes and their circuits, J. Barton Hoag

[5] All About Circuits : Vacuum Tube Audio Amplifier

[6] Tubes 201 , http://www.john-a-harper.com/tubes201/.