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Much has been written about this topic so I’ll focus on the main issues that relate to guitar amps.
Much has been written about this topic so I’ll focus on the main issues that relate to guitar amps.
See this link http://en.wikipedia.org/wiki/Valve_amplifier for a good background on general tube amplifiers.
See this link http://en.wikipedia.org/wiki/Valve_amplifier for a good background on general tube amplifiers.
While many, including Wikipedia, espouse the linear gain characteristics of tubes, it is precisely their ability to offer non-linear gain characteristics that make them desirable for guitar players. Let’s understand what “linear” means in this context. It means that the output of a circuit is a precise multiple of the input. The multiplication factor is the “gain” of the circuit. Transistor circuits offer far more linear performance today than most any tube systems can achieve. This has to do with a variety of factors, but mostly that they are so cheap to make and therefore solid-state amplifiers can have literally dozens or even hundreds of transistors in an amplifier system. With so many transistors, there is so much gain in the system that extensive negative feedback is used to stabilize the overall behavior of the system, producing very precise amplification. Tube amps, on the other hand, rarely give up gain because tubes are expensive and power hungry (due to those heater filaments), therefore tube systems operate in open-loop mode, or with minimal negative feedback. That means the non-linear characteristics of the tubes within the circuits, however small, lead to imperfect amplification. These non-linear effects matter, even at low signal levels, and they become very pronounced under large signal (hard-drive) conditions.
While many, including Wikipedia, espouse the linear gain characteristics of tubes, it is precisely their ability to offer non-linear gain characteristics that make them desirable for guitar players. Let’s understand what “linear” means in this context. It means that the output of a circuit is a precise multiple of the input. The multiplication factor is the “gain” of the circuit. Transistor circuits offer far more linear performance today than most any tube systems can achieve. This has to do with a variety of factors, but mostly that they are so cheap to make and therefore solid-state amplifiers can have literally dozens or even hundreds of transistors in an amplifier system. With so many transistors, there is so much gain in the system that extensive negative feedback is used to stabilize the overall behavior of the system, producing very precise amplification. Tube amps, on the other hand, rarely give up gain because tubes are expensive and power hungry (due to those heater filaments), therefore tube systems operate in open-loop mode, or with minimal negative feedback. That means the non-linear characteristics of the tubes within the circuits, however small, lead to imperfect amplification. These non-linear effects matter, even at low signal levels, and they become very pronounced under large signal (hard-drive) conditions.
Follow this link (Guitar Amplifier Overdrive) to the first book that focuses specifically on nonlinear and time varying circuit behaviors in tube guitar amplifiers.
Follow this link (Guitar Amplifier Overdrive) to the first book that focuses specifically on nonlinear and time varying circuit behaviors in tube guitar amplifiers.
There are lots of theories about why we seem to like the sound of these non-linear effects. There is much in the way of speculation, but some have done measurements to support their arguments. A seminal paper on analyses of tube amp signals is found here (Hamm73). If you just want some intuition, see the curves below. The diagonal line/curve relates an input signal voltage to an output voltage, therefore the slope of the line indicates the voltage gain of the system. Note this system has a gain of about two. (Actual tube amplifier stages usually have gain factors of 20-100.) Note that the linear gain system has a straight-line gain function. That means that it is very accurate in amplifying the input signal. Another way of saying that is the amplifier has low distortion. The non-linear gain system has a wavy gain function. This means that different parts of the input signal are being amplified by different amounts, leading to the output shown, which is noticeably distorted. This is what tube amplifiers do. Notice also that the wavy line is smooth - it does not have any abrupt kinks. This is what tubes offer that transistors do not – smoothly varying non-linear gain functions. In addition, tube gain functions generally cause asymmetry in the gain function, which means that the top and bottom halves of the input waveform are not equally amplified.
There are lots of theories about why we seem to like the sound of these non-linear effects. There is much in the way of speculation, but some have done measurements to support their arguments. A seminal paper on analyses of tube amp signals is found here (Hamm73). If you just want some intuition, see the curves below. The diagonal line/curve relates an input signal voltage to an output voltage, therefore the slope of the line indicates the voltage gain of the system. Note this system has a gain of about two. (Actual tube amplifier stages usually have gain factors of 20-100.) Note that the linear gain system has a straight-line gain function. That means that it is very accurate in amplifying the input signal. Another way of saying that is the amplifier has low distortion. The non-linear gain system has a wavy gain function. This means that different parts of the input signal are being amplified by different amounts, leading to the output shown, which is noticeably distorted. This is what tube amplifiers do. Notice also that the wavy line is smooth - it does not have any abrupt kinks. This is what tubes offer that transistors do not – smoothly varying non-linear gain functions. In addition, tube gain functions generally cause asymmetry in the gain function, which means that the top and bottom halves of the input waveform are not equally amplified.
For whatever reason, we often hear such non-linear distortions as “good” qualities for guitar sounds. The character of this distortion is often referred to as “rich in harmonics” and there are indeed harmonics or multiples of a input frequency in the output signal, but the distortions can also include non-harmonic tones (non-multiples of the input frequency), especially when the tubes are driven hard – that is, with high signal levels. The challenge in designing amplifiers that sound good is to build circuits that allow the tubes to impart non-linear effects that lie in the realm of desirable and “good-sounding” throughout the range of input signal levels and frequencies that a guitar can produce. This is something of an art and different companies have made a living using different rules and guidelines for how to accomplish their results. A circuit designer has some control over the non-linear tube effects, especially for large-signal conditions, but the degree of control is limited, and in the small-signal range the effects are largely caused and controlled by the tube design and physics. The circuit designer can and does exploit the non-linear tube effects to achieve a desired goal - the tone-quality of the amplifier.
For whatever reason, we often hear such non-linear distortions as “good” qualities for guitar sounds. The character of this distortion is often referred to as “rich in harmonics” and there are indeed harmonics or multiples of a input frequency in the output signal, but the distortions can also include non-harmonic tones (non-multiples of the input frequency), especially when the tubes are driven hard – that is, with high signal levels. The challenge in designing amplifiers that sound good is to build circuits that allow the tubes to impart non-linear effects that lie in the realm of desirable and “good-sounding” throughout the range of input signal levels and frequencies that a guitar can produce. This is something of an art and different companies have made a living using different rules and guidelines for how to accomplish their results. A circuit designer has some control over the non-linear tube effects, especially for large-signal conditions, but the degree of control is limited, and in the small-signal range the effects are largely caused and controlled by the tube design and physics. The circuit designer can and does exploit the non-linear tube effects to achieve a desired goal - the tone-quality of the amplifier.
in addition to non-linear gain, there is another major issue that affects how we evaluate amplifiers, and that is their frequency-response curves. Amplifier circuits are designed so that input signals at different frequencies are amplified by different amounts. Note that this variation of amplification (gain) with frequency is different from the non-linear gain described above, and both have a strong impact on how an amplifier “sounds.” The gain variations over frequency are primarily produced by other components in the circuit, the capacitors and resistors, not the tubes or transistors. Many designs emphasize low frequencies and high frequencies, while suppressing middle-range frequencies. Others suppress low-frequencies and emphasize high frequencies. It all comes down to what people like and what companies think musicians want.
in addition to non-linear gain, there is another major issue that affects how we evaluate amplifiers, and that is their frequency-response curves. Amplifier circuits are designed so that input signals at different frequencies are amplified by different amounts. Note that this variation of amplification (gain) with frequency is different from the non-linear gain described above, and both have a strong impact on how an amplifier “sounds.” The gain variations over frequency are primarily produced by other components in the circuit, the capacitors and resistors, not the tubes or transistors. Many designs emphasize low frequencies and high frequencies, while suppressing middle-range frequencies. Others suppress low-frequencies and emphasize high frequencies. It all comes down to what people like and what companies think musicians want.
Tone controls on an amp allow users to alter the response curves to suit their taste, within limits. A great little gadget to display response curves for the tone controls in common amps is available at http://www.duncanamps.com/tsc/ . Everyone’s reaction to any particular amp’s response curve and tone control system is really a matter of personal taste, however, over time a few guidelines have emerged, based on the popularity of commercial amps. Many of the successful amps (built by Fender, Marshall, Vox, etc.) emphasize high and low frequencies, with reduced gain in the middle frequencies (near the middle octave). They vary in degree of emphasis and suppression and they also vary in gain variations between very low frequencies and high frequencies. Each make/model is somewhat different, so it’s inaccurate to over-generalize, but for my purposes, I generally focus on two response curve characteristics, the degree of high- and low-frequency emphasis over the middle frequencies, and the low frequency response from about 250 Hz down to about 70 Hz. These are (in my humble opinion) key variables in amplifier response characteristics. The choices made about these characteristics have a substantial impact on the circuitry, and final tone qualities of the amplifier.
Tone controls on an amp allow users to alter the response curves to suit their taste, within limits. A great little gadget to display response curves for the tone controls in common amps is available at http://www.duncanamps.com/tsc/ . Everyone’s reaction to any particular amp’s response curve and tone control system is really a matter of personal taste, however, over time a few guidelines have emerged, based on the popularity of commercial amps. Many of the successful amps (built by Fender, Marshall, Vox, etc.) emphasize high and low frequencies, with reduced gain in the middle frequencies (near the middle octave). They vary in degree of emphasis and suppression and they also vary in gain variations between very low frequencies and high frequencies. Each make/model is somewhat different, so it’s inaccurate to over-generalize, but for my purposes, I generally focus on two response curve characteristics, the degree of high- and low-frequency emphasis over the middle frequencies, and the low frequency response from about 250 Hz down to about 70 Hz. These are (in my humble opinion) key variables in amplifier response characteristics. The choices made about these characteristics have a substantial impact on the circuitry, and final tone qualities of the amplifier.
As stated above, the circuit design doesn’t have a large impact on small-signal non-linearity, which is provided by the physics of tube operation. However, we can modify a circuit so that regardless of the volume you hear from the speaker, the tubes are operated with large-signals, which means we can control the degree of non-linearity in the output. Also, since the circuit design completely determines the frequency response curve, that affords us the ability to modify an amplifier circuit to produce a different more-desirable response curve, which impacts both the small- and large-signal characteristics of an amp. Therefore we can obtain both clean or richly distorted (harmonic) tube tones at high and low volumes, as well as wide-range full-sounding tones from altered frequency-response curves.
As stated above, the circuit design doesn’t have a large impact on small-signal non-linearity, which is provided by the physics of tube operation. However, we can modify a circuit so that regardless of the volume you hear from the speaker, the tubes are operated with large-signals, which means we can control the degree of non-linearity in the output. Also, since the circuit design completely determines the frequency response curve, that affords us the ability to modify an amplifier circuit to produce a different more-desirable response curve, which impacts both the small- and large-signal characteristics of an amp. Therefore we can obtain both clean or richly distorted (harmonic) tube tones at high and low volumes, as well as wide-range full-sounding tones from altered frequency-response curves.
Example cases are the amplifiers discussed on these pages. All of of these have response curves that suit my personal sense of the tone ideal. This part gets a bit touchy-feely, but my expectation is that amps played loud or soft should produce clean sparkling tones or nice fat tones.
Example cases are the amplifiers discussed on these pages. All of of these have response curves that suit my personal sense of the tone ideal. This part gets a bit touchy-feely, but my expectation is that amps played loud or soft should produce clean sparkling tones or nice fat tones.
Any modified amp may become your favorite amp to play. In fact, price may not even make a difference. There are expensive amps that sound pretty bad to some people. Individual tastes matter, but there are some general preferences for a wide range of players/listeners. It's also desirable that amps sound great at a range of settings. Most musicians don't want to fiddle with knobs trying to find the "magic" settings that work well. However, when you do change settings, you should find new tonal character that surprises you and keeps you playing and exploring their impacts. Much of amp modification is simply tailoring amps to personal tastes... so in case your tastes are different from mine, I try to point out where you can vary part values to suit your own tastes in the amps on these pages. There is no one magic tone for all people and playing styles.
Any modified amp may become your favorite amp to play. In fact, price may not even make a difference. There are expensive amps that sound pretty bad to some people. Individual tastes matter, but there are some general preferences for a wide range of players/listeners. It's also desirable that amps sound great at a range of settings. Most musicians don't want to fiddle with knobs trying to find the "magic" settings that work well. However, when you do change settings, you should find new tonal character that surprises you and keeps you playing and exploring their impacts. Much of amp modification is simply tailoring amps to personal tastes... so in case your tastes are different from mine, I try to point out where you can vary part values to suit your own tastes in the amps on these pages. There is no one magic tone for all people and playing styles.
It's worth discussing what makes for "good sound." In my opinion, the instrument (guitar and pickups and your fingers) produce signals and it's the amp's job to get them to your ears in a way that's both pleasing and interesting. Both of those qualities are important. Most amps can deliver a simple note clearly and cleanly, but it becomes boring after you hear it for a while, because the harmonic content of the notes is minimal and has little variation over note pitch or amplitude. That leads people to crank up volume to add distortion and add effects to keep it interesting. While that's an option, I think the amp should do more on its own - and there are amps that prove this is possible. I find the key to both "pleasing and interesting" starts with the harmonic content of a simple note. A plucked note has a timbre and a range of harmonics. Your instrument and musicianship can control these to some extent; however, the amp must bring them out and make them heard. This requires wide frequency range and emphasis or additions to the inherent harmonic content of the input. The tubes and circuits should preserve and enhance the harmonic content. However, it’s easy to go too far and wind up with an unpleasing cacophony of noise. This happens when effects boxes are overused or when amps are highly over-driven. Then the harmonic content of each note is indeed rich, but so rich that it becomes difficult to distinguish the notes themselves and a listener/player is often bored before long since everything sounds the same -- a sonic mush. The desired output is one where each note sounds rich, yet multiple notes played together remain distinctive. At the same time, your playing technique, as well as the musical qualities of your instrument, should impact the tone and timbre of each note to some degree, thereby keeping it interesting.
It's worth discussing what makes for "good sound." In my opinion, the instrument (guitar and pickups and your fingers) produce signals and it's the amp's job to get them to your ears in a way that's both pleasing and interesting. Both of those qualities are important. Most amps can deliver a simple note clearly and cleanly, but it becomes boring after you hear it for a while, because the harmonic content of the notes is minimal and has little variation over note pitch or amplitude. That leads people to crank up volume to add distortion and add effects to keep it interesting. While that's an option, I think the amp should do more on its own - and there are amps that prove this is possible. I find the key to both "pleasing and interesting" starts with the harmonic content of a simple note. A plucked note has a timbre and a range of harmonics. Your instrument and musicianship can control these to some extent; however, the amp must bring them out and make them heard. This requires wide frequency range and emphasis or additions to the inherent harmonic content of the input. The tubes and circuits should preserve and enhance the harmonic content. However, it’s easy to go too far and wind up with an unpleasing cacophony of noise. This happens when effects boxes are overused or when amps are highly over-driven. Then the harmonic content of each note is indeed rich, but so rich that it becomes difficult to distinguish the notes themselves and a listener/player is often bored before long since everything sounds the same -- a sonic mush. The desired output is one where each note sounds rich, yet multiple notes played together remain distinctive. At the same time, your playing technique, as well as the musical qualities of your instrument, should impact the tone and timbre of each note to some degree, thereby keeping it interesting.
So, the key goals are rich harmonic content and variation from note to note. It is indeed an art to achieve these goals in an amplifier. Ultimately, the results are subject to the perception and assessment of the player and listener. There is no one answer or one result that is pleasing to everyone, nor should there be. That’s why music and sounds evolve over time – we all crave new experiences.
So, the key goals are rich harmonic content and variation from note to note. It is indeed an art to achieve these goals in an amplifier. Ultimately, the results are subject to the perception and assessment of the player and listener. There is no one answer or one result that is pleasing to everyone, nor should there be. That’s why music and sounds evolve over time – we all crave new experiences.
If you are interested in a deeper understanding of all of this, I have written two two books to fill what I see as a gap in the literature about guitar amplifiers. Guitar Amplifier Overdrive to the first book that focuses specifically on nonlinear and time varying circuit behaviors in tube guitar amplifiers. These are the circuit behaviors unique to tube amplifiers and their distinctive tone and feel. The second book, Guitar Amplifier Design, describes my thoughts and decision making process during the design of a 100 watt guitar amplifier using both tubes and semiconductors. Both books present new material that isn't found in other books.
If you are interested in a deeper understanding of all of this, I have written two two books to fill what I see as a gap in the literature about guitar amplifiers. Guitar Amplifier Overdrive to the first book that focuses specifically on nonlinear and time varying circuit behaviors in tube guitar amplifiers. These are the circuit behaviors unique to tube amplifiers and their distinctive tone and feel. The second book, Guitar Amplifier Design, describes my thoughts and decision making process during the design of a 100 watt guitar amplifier using both tubes and semiconductors. Both books present new material that isn't found in other books.
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The bar stays stocked and open...
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