#1 · Aug 13, 2008 18:06 UTC
I have been asked to start a small series on how different amp makers produce those "recognizable" sounds. For instance, you know a Marshall when you hear one. You know a Fender, or a MESA, or a VOX when you hear one. So what makes for "that sound" that allows us to identify those particular brands/tones so easily. I want to focus on the more readily reproducable aspects of this, rather than the more subjective qualities that spark an intense amount of debate. If it sparks that much debate, it's my belief (mine only, for now, ;D) that it really doesn't count for much - otherwise folks wouldn't have to debate it so much.
I want to start out this series with a few opening remarks that can be discussed and/or argued and/or questioned as much as necessary till we get into the various amps and how the manufacturers put them together.
My favorite books on how tube amps work come from London Power's, Kevin O'Connor. He has created a series of books called "The Ultimate Tone" (TUT). Of all the books written by "tube amp guru's" these sport the least amount of technical errors, oversights, mistakes and misrepresentations. It's why I like these books, overall.
WHAT CONTRIBUTES TO THE SOUND OF AN AMP?
The "character" or "sound" of a tube amp is created from various component sections all working together. I am going to divide the amp into the following stages:
1) Preamp
2) Tone Stack
3) Power Amp
4) Speakers / Cabinet
Kevin O' Connor makes this statement about the contribution of each to amplifier character in his book "The Ultimate Tone - Modifying and Custom Building Tube Guitar Amps", Chapter 4, Page 4-1:
In most guitar systems, particularly those greater than 100W, at least 50% of the tone will be determined by the preamp, especially when distortion is used. In lower power systems, particularly in the 25W-30W range, the power amp contributes significantly, but the preamp still contributes at least 30% to the sound. These numbers reflect that 50% of teh system tone is determined by the speaker and instrument...
Now, whether you agree on the exact numbers, percents and ratios is, really, neither here nor there. I'll try to show how different input stages, to a guitar amp, may actually mask which guitar you're using making the preamp character more "noticable" than the guitar character and vice-versa. Ultimately, the preamp contributes a great deal to the final tone of the amp, the power amp contributes some - especially in lower powered amps and the guitar and the cabinet contribute quite a bit.
Tone Stacks, or No?
Interestingly enough, you'll note that Kevin does not indicate that the tone controls (a.k.a. tone stack) contributes much, if anything, to the final character of the amplifier sound. All through the TUT series Kevin notes that most tone stacks are "interchangeable" in that you can put a Fender stack on a Marshall, or a VOX stack on a Marshall and it'll still sound distinctly Marshall. I'm going to explain why this occurs, and how it occurs as we continue this series, as well. In the end, the tone stacks contribute FAR LESS than what we think they do.
Preamp Divisions
As we go through this process there are several elements that make up a preamp, that I'm going to discuss. The first, is the "gain stage". A "gain stage" will be defined as a complete group of components that actually produce gain. This will include an "active device" (tube, transistor, op-amp, etc), and the surrounding components which "bias" it to the "operating point". Sometimes, these produce frequency selective networks (i.e. they "shape" the resultant output tonally).
Another component of most preamps is the "interstage coupling". These are the resistors and capacitors that link one preamp gain stage to another preamp gain stage. These parts change the frequency response of the preamp in addition to the frequency changes made in the gain stage itself.
A preamp is made up of one, or more, gain stages. Fender typically uses two, stages. Marshall uses three. Most of the time, preamp gain stages will be in multiples of two as most tubes, used in preamps, have two tube elements inside. A 12AX7, for example, has two functional tubes within the glass bottle. Being economical, if I use two bottles, then my preamp will have 4 stages. If I use a single tube, it'll have two stages, etc. The Peavey 5150 uses three 12AX7's for a total of six gain stages - a stinkin' lot for any guitar amp.
Tone Stacks
Tone stacks come in a few different types. The first is a "high cut", typical of original Fender Deluxe. When the tone control is "wide open" all frequencies are passed. When the tone control is set to anything else, then the high frequencies will start to be "cut", "dulling" the sound a bit. The second type of tone control is a Bass / Treble control. There are various implementations of this, including those in Fender designs (i.e. '65 Deluxe Reverb), VOX Designs (ala AC30), etc. The third type of control is the standard Bass / Middle / Treble control seen on Fenders (Twin, Bassman, etc), Marshalls and various other amps. These controls have a "treble coupling cap" and the Treble control acts as a "mixer" mixing the amount of bass and midrange to the treble frequencies. The ratio of bass/mids to treble is controlled by what is called a "slope resistor". We can get into this a bit more, later.
The Power Amp
I include the splitter stage, power amp tubes and negative feedback loop in the power stage. Class A amps (a.k.a Fender Champs and the like) don't have a splitter and, typically, don't have a feedback loop "controlling" the power amp. Class AB, and AB1 amps typically do have negative feedback and a splitter as these are built in a push-pull configuration (more on this later). Examples of this include VOX AC30 (yes, it's Class AB1 NOT Class A as is commonly stated), Marshalls, most Fenders, etc.
Speakers and Cabinets
As you're aware, speakers (Greenback, Vintage 30, G12T-75, Jensen P12R, P10R, etc) all have different sounds. Rather than engage in a long discussion about how various speakers work we're going to talk about general charactaristics of each, when it's necessary but, more importantly, discuss how the power amplifier interacts with speakers to create "that sound". This is, typically, referred to as "damping" and is also, many times, discussed as "loose", "tight", "flubby", "bouncy" or "responsive". Negative feedback, coupled with the rest of the output components creates this "feel" in power amp response.
I'm going to shy away from discussions of cabinet construction, whether 11-ply or 13-ply plywood is better and that sort of thing. We can delve into some of it, but, quite frankly, we want to keep "under the hood" and relatively objective about most of this stuff. A lot can be argued in favor of, and against, any of those types of things.
That should be about it, for now. My "Remaining characters" indicator shows I only have 353 characters left to this post, as of the start of the number "353." Let me know what you think about this, and if you feel it's useful. If it is, we'll start to delve into topologies and differences between them in the next posting. We can chat this one out, first, before we move on.
Enjoy...
Dar
I want to start out this series with a few opening remarks that can be discussed and/or argued and/or questioned as much as necessary till we get into the various amps and how the manufacturers put them together.
My favorite books on how tube amps work come from London Power's, Kevin O'Connor. He has created a series of books called "The Ultimate Tone" (TUT). Of all the books written by "tube amp guru's" these sport the least amount of technical errors, oversights, mistakes and misrepresentations. It's why I like these books, overall.
WHAT CONTRIBUTES TO THE SOUND OF AN AMP?
The "character" or "sound" of a tube amp is created from various component sections all working together. I am going to divide the amp into the following stages:
1) Preamp
2) Tone Stack
3) Power Amp
4) Speakers / Cabinet
Kevin O' Connor makes this statement about the contribution of each to amplifier character in his book "The Ultimate Tone - Modifying and Custom Building Tube Guitar Amps", Chapter 4, Page 4-1:
In most guitar systems, particularly those greater than 100W, at least 50% of the tone will be determined by the preamp, especially when distortion is used. In lower power systems, particularly in the 25W-30W range, the power amp contributes significantly, but the preamp still contributes at least 30% to the sound. These numbers reflect that 50% of teh system tone is determined by the speaker and instrument...
Now, whether you agree on the exact numbers, percents and ratios is, really, neither here nor there. I'll try to show how different input stages, to a guitar amp, may actually mask which guitar you're using making the preamp character more "noticable" than the guitar character and vice-versa. Ultimately, the preamp contributes a great deal to the final tone of the amp, the power amp contributes some - especially in lower powered amps and the guitar and the cabinet contribute quite a bit.
Tone Stacks, or No?
Interestingly enough, you'll note that Kevin does not indicate that the tone controls (a.k.a. tone stack) contributes much, if anything, to the final character of the amplifier sound. All through the TUT series Kevin notes that most tone stacks are "interchangeable" in that you can put a Fender stack on a Marshall, or a VOX stack on a Marshall and it'll still sound distinctly Marshall. I'm going to explain why this occurs, and how it occurs as we continue this series, as well. In the end, the tone stacks contribute FAR LESS than what we think they do.
Preamp Divisions
As we go through this process there are several elements that make up a preamp, that I'm going to discuss. The first, is the "gain stage". A "gain stage" will be defined as a complete group of components that actually produce gain. This will include an "active device" (tube, transistor, op-amp, etc), and the surrounding components which "bias" it to the "operating point". Sometimes, these produce frequency selective networks (i.e. they "shape" the resultant output tonally).
Another component of most preamps is the "interstage coupling". These are the resistors and capacitors that link one preamp gain stage to another preamp gain stage. These parts change the frequency response of the preamp in addition to the frequency changes made in the gain stage itself.
A preamp is made up of one, or more, gain stages. Fender typically uses two, stages. Marshall uses three. Most of the time, preamp gain stages will be in multiples of two as most tubes, used in preamps, have two tube elements inside. A 12AX7, for example, has two functional tubes within the glass bottle. Being economical, if I use two bottles, then my preamp will have 4 stages. If I use a single tube, it'll have two stages, etc. The Peavey 5150 uses three 12AX7's for a total of six gain stages - a stinkin' lot for any guitar amp.
Tone Stacks
Tone stacks come in a few different types. The first is a "high cut", typical of original Fender Deluxe. When the tone control is "wide open" all frequencies are passed. When the tone control is set to anything else, then the high frequencies will start to be "cut", "dulling" the sound a bit. The second type of tone control is a Bass / Treble control. There are various implementations of this, including those in Fender designs (i.e. '65 Deluxe Reverb), VOX Designs (ala AC30), etc. The third type of control is the standard Bass / Middle / Treble control seen on Fenders (Twin, Bassman, etc), Marshalls and various other amps. These controls have a "treble coupling cap" and the Treble control acts as a "mixer" mixing the amount of bass and midrange to the treble frequencies. The ratio of bass/mids to treble is controlled by what is called a "slope resistor". We can get into this a bit more, later.
The Power Amp
I include the splitter stage, power amp tubes and negative feedback loop in the power stage. Class A amps (a.k.a Fender Champs and the like) don't have a splitter and, typically, don't have a feedback loop "controlling" the power amp. Class AB, and AB1 amps typically do have negative feedback and a splitter as these are built in a push-pull configuration (more on this later). Examples of this include VOX AC30 (yes, it's Class AB1 NOT Class A as is commonly stated), Marshalls, most Fenders, etc.
Speakers and Cabinets
As you're aware, speakers (Greenback, Vintage 30, G12T-75, Jensen P12R, P10R, etc) all have different sounds. Rather than engage in a long discussion about how various speakers work we're going to talk about general charactaristics of each, when it's necessary but, more importantly, discuss how the power amplifier interacts with speakers to create "that sound". This is, typically, referred to as "damping" and is also, many times, discussed as "loose", "tight", "flubby", "bouncy" or "responsive". Negative feedback, coupled with the rest of the output components creates this "feel" in power amp response.
I'm going to shy away from discussions of cabinet construction, whether 11-ply or 13-ply plywood is better and that sort of thing. We can delve into some of it, but, quite frankly, we want to keep "under the hood" and relatively objective about most of this stuff. A lot can be argued in favor of, and against, any of those types of things.
That should be about it, for now. My "Remaining characters" indicator shows I only have 353 characters left to this post, as of the start of the number "353." Let me know what you think about this, and if you feel it's useful. If it is, we'll start to delve into topologies and differences between them in the next posting. We can chat this one out, first, before we move on.
Enjoy...
Dar