Moderators Kindness Posted February 17, 2009 Moderators Share Posted February 17, 2009 Although many players simply want to know how to turn on and up their amps and don Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 How a (Preamp) Tube Works: The amplification portion of a preamp tube is called a triode because it includes three parts; (1) a cathode, (2) an anode and (3) a grid. A preamp tube like the 12AX7 is called a twin-triode because it is a pair of triodes (shown as V1a and V1b) in a single tube. In addition to the two sets of triode elements, a tube also has heater elements. Here is a representation of the elements of a 12AX7 as taken from wikipedia: originally found at: http://upload.wikimedia.org/wikipedia/en/5/53/EIA-9A.png You will see that the base of the 12AX7 has 9 pins that correspond to the 9 elements within the tube; each is labeled in the figure. Elements 4 and 5 are the heaters (a.k.a., the filaments) and pin 9 is the connection for the center tap around which the AC heater voltage swings. Elements 1 and 6 are the plates (a.k.a., the anodes), which will attract the free electrons in the tube. Elements 2 and 7 are the grids, which are essentially loosely spaced mesh screens that will help control the flow of current in the tube. Elements 3 and 8 are the cathodes, which are metal plates which will be the source of free electrons in the tube. In operation, a voltage is applied to the heaters, which in turn heat the cathodes, which in essence boil off a cloud of electrons from the heated cathodes. Those free electrons float around inside the vacuum of the tube. If no other electrodes were present in the tube, the electrons would return to the surface of the cathode at the same rate they are boiled off. However, a positive voltage is applied to the plate via the power supply, which in turn attracts the free electrons, which causes a current flow from the cathode to the plate. As noted above the grid is a loosely spaced mesh screen. The grid is also negatively charged with respect to the cathode. Because the grid is loosely spaced mesh, it allows the vast majority of free electrons to pass through it to reach the plate or to return to the cathode. Because it is negatively charged, the grid partially shields the cathode from the positive plate voltage and limits the amount of current that flows in the tube. The more negative the grid, the fewer electrons makes it through the grid and to the plate. The less negative the grid, the more electrons make it through the grid and to the plate. The input signal is applied to the grid to swing it more or less negative, controlling the flow of electrons in the tube. Because of the structure of the tube (particularly the proximity of the grid to the cathode), and the voltages applied to the plates, a small change in grid voltage results in a large change in plate current. This is amplification. I Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 The Tone Stack The tone stack comes next in the signal chain. The tone stack is a circuit of capacitors, resistors and pots (variable resistors) which shape the frequency response of the signal. In other words, this is the equalization circuit. Although the circuit itself is fairly complex with a significant number of components affecting the audible frequencies, there is an exceptionally useful freeware program available from Duncan Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 The Power Amp The power amp in the JTM45 we are building consists of a pair of EL34 output tubes configured to receive the push-pull output from the phase inverter. The EL34 is an example of a beam power tube. (The original used KT66s.) Beam Power Tubes (Pentodes) The EL34 is a pentode. The pinout for the EL34 is shown below: originally found at: http://www.mclink.it/com/audiomatica/tubes/img/el34pin.gif Just as the triode described above, the EL34 has heater elements (pins 2 and 7), a cathode (pin 8), a grid (pin 5) and a plate (pin 3). In addition, the EL34 includes a screen grid (pin 4) and a suppressor grid (pin 1). The screen is similar to the grid in that it is also a wire mesh element located between the cathode and plate. The screen was initially implemented in radio amplifiers to reduce the capacitance of the tube (thereby increasing the high frequency gain of the tube). However, there are added benefits for use in a guitar amplifier. First, by placing a high voltage on the screen (just lower than the plate voltage) the tube is capable of a higher amplification factor for two reasons. First, because the screen is located closer to the cathode than the plate, its voltage has a greater effect on the plate current which has the effect of making the plate appear even more positive. Second, although the screen attracts electrons, its mesh structure prevents there from being much screen current (the electrons pass through the screen and contact the plate instead), which reduces the fluctuation in the screen voltage as compared to the plate voltage (which fluctuates with plate current). As a result, the screen reduces the non-linearity (distortion) of the output voltage. When electrons flow to the plate they can dislodge electrons on the plate causing what it known as secondary emission. When those dislodged electrons return to the plate, there is no net change in current. However, because the screen is at such a high positive voltage, it attracts some of the secondary emission, reducing the current flowing from cathode to anode. The suppressor grid, which is tied to ground (thus a much lower voltage than the screen ), functions to shield the screen from the plate and minimize the loss due to secondary emission. Now, looking back to our schematic of the power amp, the cathodes of the EL34s are tied to the suppressor grids, all of which are tied to ground; the screens are fed a high voltage from the power supply and are additionally connected to a pair of screen resistors (470 ohm, 1 watt resistors), which are intended to keep the screen current within the limits of the tube; the grids are connected to the push-pull outputs from the phase inverter; and the plates are connected to the primary of the output transformer and also are fed a high voltage from the power supply. Based on our discussion of the triode based voltage amplifiers above, it should be apparent that the EL34s operate in the same manner, only with the additional assistance of the screen grids and suppressor grids, which both improve the ability of the tube to amplify the input signal. The Output Transformer A critical portion of the power amp output section is the output transformer. An output transformer ( Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 The Power Supply You Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 Thanks to all that have contributed to my understanding of amplifiers and their operation, particularly, the Navy Electricity and Electronics Training Series and the writings of Link to comment Share on other sites More sharing options...
Members bassman1956 Posted February 17, 2009 Members Share Posted February 17, 2009 Thanx Patrick! Link to comment Share on other sites More sharing options...
Members der oxenrig Posted February 17, 2009 Members Share Posted February 17, 2009 Build me a pair of JTM45-100s and a pair of 8x12s!!!! Link to comment Share on other sites More sharing options...
Members Super_Donut_Man Posted February 17, 2009 Members Share Posted February 17, 2009 Wow:thu: Link to comment Share on other sites More sharing options...
Members Dave Campbell Posted February 17, 2009 Members Share Posted February 17, 2009 Impressive thread. Kindness, you rock. Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 Build me a pair of JTM45-100s and a pair of 8x12s!!!! Send the parts to me! Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 Impressive thread. Kindness, you rock. You guys actually read this? Snooze central. Link to comment Share on other sites More sharing options...
Members Dave Campbell Posted February 17, 2009 Members Share Posted February 17, 2009 You're welcome. Hopefully it'll make even more sense when you have the amp in front of you. You guys actually read this? Snooze central. Come on, man, give your self some credit Link to comment Share on other sites More sharing options...
Members Jugghaid Posted February 17, 2009 Members Share Posted February 17, 2009 Great thread Kindness. Now I'm having flashbacks to when I actually did electronics for a living. Link to comment Share on other sites More sharing options...
Members BeeTL Posted February 17, 2009 Members Share Posted February 17, 2009 Have you considered publishing your work? This is good stuff... Link to comment Share on other sites More sharing options...
Members bnyswonger Posted February 17, 2009 Members Share Posted February 17, 2009 Dang - that is so well written and logical, you oughtta publish that and collect some dough, bro. I actually read most of it, and I'm not enough building one! Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 This is good stuff... This is the thread I talked about in my build thread. This should help you sit down and start to make sense of the electronics in an amp. Once you make it through this material, you'll be ready to start asking the questions that you don't yet have formulated. Link to comment Share on other sites More sharing options...
Members BeeTL Posted February 17, 2009 Members Share Posted February 17, 2009 What would you call what I just did? I might be interested in writing more and more formally, but I'd likely keep it in open forums/websites. That's more my style. This is the thread I talked about in my build thread. This should help you sit down and start to make sense of the electronics in an amp. Once you make it through this material, you'll be ready to start asking the questions that you don't yet have formulated. I think what I meant by "publishing" was a more formalized, indexed and cross-referenced kind of format, but I think you knew that. As far as the content goes, this is exactly what I was hoping it would be. I think if I ever do buid an amp, it will be something along the lines of a black face Vibro-Champ like we discussed before I picked up the Tweed Deluxe. Of course, if the Tele sounds as good through the Tweed as I hope it will, that may be a moot point. Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 As far as the content goes, this is exactly what I was hoping it would be. I think if I ever do buid an amp, it will be something along the lines of a black face Vibro-Champ like we discussed before I picked up the Tweed Deluxe. Of course, if the Tele sounds as good through the Tweed as I hope it will, that may be a moot point. For what you are doing, that Deluxe is where it's at. Link to comment Share on other sites More sharing options...
Members lokidecat Posted February 17, 2009 Members Share Posted February 17, 2009 While I gloss over the electronics (that's way above my head) I like reading how things work. I can pick up on a functional level pretty fast, so I do enjoy reading the intricacies of how it does work. Not like popcorn-gulping suspenseful, but nerdy interesting. Then again, I read tech manuals for fun. Link to comment Share on other sites More sharing options...
Members Goofball Jones Posted February 17, 2009 Members Share Posted February 17, 2009 So...where do you plug in your bass at again? And you just flip the switch to "on"? But really, very nice and detailed write-up! As always. Link to comment Share on other sites More sharing options...
Members Pickdust Posted February 17, 2009 Members Share Posted February 17, 2009 Nicely done K! Looking at this project as a whole is daunting. Eating the elephant a bite at a time is really taking the fear out of it for me! Thanks mang for taking the time to logically explain things! PD Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 Eating the elephant a bite at a time is really taking the fear out of it for me! Yum! I have a few more threads planned to break down some of the concepts we need for the build (you technically don't need any of the information in this thread, but I know some people are interested in it) and hopefully, by the time the kits arrive, the fear will be gone and you'll have a good game plan for putting it together. Link to comment Share on other sites More sharing options...
Members groutt Posted February 17, 2009 Members Share Posted February 17, 2009 Excellent work. Link to comment Share on other sites More sharing options...
Moderators Kindness Posted February 17, 2009 Author Moderators Share Posted February 17, 2009 Excellent work. Thanks for pointing out my mistake in the preamp section. I welcome anyone's/everyone's input/corrections as I have not taken the time to proof read this thread to ensure every word is correct. If I allowed myself to the time revise, it'd be weeks or moths before I posted as I rewrote the entire thing over and over and over... It can always be better! Link to comment Share on other sites More sharing options...
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