Note: this entry was already published in my old Spanish version of this blog, around April / 2011. I am just visiting it here for its possible interest.
When I was trying to learn something about Physics, my mind blew up with the affirmation that said that accelerator, steering wheel and break where all them accelerators. Accelerator is whichever device that creates some force that changes the movement status of a certain object and changes it's acceleration or direction.
Same stupid face I showed when I've realized that the controls of our amps (volume, gain, tones) aren't amplifying nothing but, that they are in fact ATTENUATORS!!!.
In fact, an amplifier is a power attenuator, at the end, because it reduces the total input power (often twice the delivered output power) to amplify that weak guitar signal.
The amp cannot add nothing but, reduce what already exists.
So, if we had a direct signal guitar -> preamp -> power amp -> speaker, we would have the maximum "amplifying" power of the amp (without controls). Every control we add is reducing such a maximum power.
It's true that our guitar signal is being amplified but, this is being achieved be consuming part of the input power to do such a task. So the output power is lower than the input power, even that our output guitar signal is higher than our input guitar signal. Lots of the input energy was wasted for such a task.
To understand where our controls were place and which are their functions, can help us to understand how to better achieve the tone we are after. And, that's all about this blog entry.
Here we go...
Amplifying elements: tubes
The electric signal generated by a guitar is really low. AC voltage goes around 300 mV (mili-volts) and, current level is also low. Guitar' signal by itself is unable to move the coil of a speaker, that is the way we use to translate (transduce) the electric energy into air pressure energy.
That's why we need a guitar amp, to increase guitar signal levels up to an energy level enough to move speakers coils with authority.
The amp gets the signal delivered by the guitar and "sculpts" it (reducing some unwanted frequencies or adding some harmonics or clipping it, by example) and, rises the power of such a signal to those levels needed for speakers.
Therefore, basis function of preamp and power tubes is just to amplify several times the input signal.
To amplify a signal, tube amps need at least a triode. Most of Pre-amp tubes that you will find in a guitar amp have a pair of triodes (family 12AX7 or ECC83 and similar 5751, 7025, etc.). This is the same than to say that each preamp tube has two amplifying stages.
If each triode was able to rise the signal 10 times the input signal, if we link the output of the first triode to the input of the second triode, we will amplify the signal 100 times (10 from input triode 1 to output triode 1 and, will multiply by 10 triode 1 output in triode 2).
In following picture, T1 is the first triode of tube V1 and, T2 is the second triode of same tube.
Each triode has a maximum gain level were the original signal "fits" without distortion. In the same way that you will zoom a picture on your screen up to a limit were part of the picture cannot be shown (crop), tubes clip that exceeding amount of signal (peaks). Probably one of the uglier distortion types that you can find in a tube. Tubes introduce several types of distortion: harmonics of different order, floor noise and other thing that we will not discuss here to don't go lost.
So, every preamp tube is like two independent amplifying stages that can be cascaded (stacked) to multiply the gain level.
Since the guitar' signal level is really weak, to "overflow" the first triode it's really difficult (we can go from 300 mV to 3000 mV = 3 V, by example) but, when crossing the second triode, the signal level can be too much and, therefore it's easier to cross the threshold between clean amplification and distorted amplification.
To better control the signal flowing between the first and the second triode, a regulation element was designed, to attenuate (yes, to attenuate!) the amount of signal entering in the second triode.
A variable resistor (a potentiometer) was provided for that and it was called Volume.
Just inserting that Volume pot there, even if it's totally open, it creates some resistance that drops a bit the voltage in that part of the circuit (once more, we are attenuating the signal).
So, thinks looked like this:
Nice, that Volume pot was acting as a faucet that prevented that the "water stream" from triode 1 overflowed the "sink" of triode 2.
And, since any change in a spot of the chain propagates to following ones, the Power tube was also controlled in hat way.
But, two triodes are in the vintage ballpark, right. We wanted hi-gain amps also. So, why don't be cascade a couple of pre-amp tubes to achieve a higher gain sound?. We will have up to 4 amplifying stages in that way and, that means that we can raise the guitar signal level up to 10^4 (10,000) times , just in the pre-amp section!.
So, the cascaded gain increased:
T1 and T2 are the triodes for tube V1 and, T3 and T4 the triodes for V2.
Adding one more tube, here we are again with same "overflowing" issues we addressed before but now, the max amplifying power is around 10,000 times the original signal. That's a lot for small pre-amp tubes, indeed.
As we are overflowing each triode, the peaks of signal are being clipped and, what remains is the center portion of the signal that "fits" each triode. Result is a thick wall of sound, with poor dynamics and, highly compressed.
To have a better control of such an amount of clipping and power increase, one more pot was introduced between T3 and T4 but.... we had a pot already named volume so... which name can we give to that new pot. Well, it was called Gain, instead of Pepe and, things looked like follows:
While amplifiers where all Class A, single-ended, everything was ok. The last triode was acting also as the Driver, feeding the power amp section, with a controlled power given by that Volume control (between T3 and T4).
But Rock needed more energy, we love noise, indeed. Class AB amps were way more efficient, generating more energy, moving bigger speakers and creating more sound pressure. What a nice!.
Class AB, usually in pull-push topology, has two rails of power tubes (each rail with one or more tubes).
Each rail takes care about half of the signal (the positive or negtive) during aprox. the time that half takes (slightly more).
Between the pre-amp and power stages, one more small tube, called Phase Inverter, Driver or PI, will split the signal in negative and positive, sending each side to one of the rails, alternating (as pistons in an engine).
But, being the last tube in the pre-amp chain, this tube can be saturated the first and, power tubes could also being saturated with an excess of power delivered by the PI.
Oh, oh, oh. wait there!. I have an idea... one more pot?.
Yes. Sometimes you can find that pot between the PI and Power section or you could even found it before the PI. We already had names for the other two pots (gain and volume) so, we called "Master Volume" to the new one.
It doesn't seem that any of hi-gain designs went further than using up to 4 gain stages in pre-amp. Amplification level at this point is so high that the signal is highly compressed and clipped. Adding one more triode will mean to have the original signal raised 100,000 times in pre-amp section, overflowing any pre-amp headroom.
This is a way the master volume could be added:
But, it could be even used like this, to avoid to overflow the PI tube:
There are amps with several channels, one for clean, one more for crunch and, even one more for high-gain (there are amps with even more channels, differently voiced). Those amps can be designed as having completely independent paths for each channel, with their own tubes or, they can be designed with some switches that just bypass or include some triode or triodes. By example:
But the designer, could decided that the clean channel was just triode T4 or that started in triode T2.
Or he could decided another kind of path, from T1 to T3, from T3 to T2 and from T2 to T4, by example.
Note: All you see here are just examples and, doesn't correspond to the design of any particular amp and, don't show every detail. The goal is just to understand how several gain stages are being chained and which tricks can be used to take some control over the gain and volume levels. Placement of controls and their name, can change from amp to amp, maker to maker.
To give to us better possibilities to sculpt our tone, an array of filters was introduced to remove (yes, to remove, we don't add bass, mids or treble, we remove them!) part of the frequencies of the original signal.
This array of filters was called Tone Stack, because of how it looked in schematics and, its solely existence drops also the signal level (because creates resistance, also).
By example, it could be decided to insert the Tone Stack like follows:
The Presence control helps to recover some of the high frequencies and, it's usually placed between the speaker-amp feedback loop.
The Contour control is often associated to several filters (resistors and caps) that reduce the contents in some frequency bands (always attenuating!), re-EQ'ing the amp and changing its voice.
The only way to ADD something is to use ACTIVE filters, instead of passive but, most of amps are being designed with passive filters so, they just reduce things.
Designs and names
Don't be fooled. Same functions we were describing above can be found in a certain maker/amp with a different name but they are named just differently.
By example, in the VHT Special 6, we will find an Ultra control that works as the Gain Control defined above, a Depth switch that works as a Contour control (together with Texture switch), a Power Selector that works similarly to a Master Volume (but, it's placed before the Power tube, instead).
We can find bizarre names for same function, as we also can find those in pedal effects.
To achieve the tone we are after, at every moment (sometimes, a crystalline clean, a nice crunch, a wall of compressed sound...) it's a good think that we know how controls were implemented in our amp, to better understand what are we doing when setting up the amp.
To draw some basic block-diagram of the several gain stages of our amp and where controls are being placed will help us.
It isn't the same to push the very first triode (the early we start our distortion, the most distortion will be get) or leave that pre-amp clean and push power tubes, instead.
Each knob will recover (instead of to add) some of the drops that will overflow the final amplified sound, releasing one or more of the different distortion types that occur inside an amp. The goal is to achieve the "musical" distortions, while avoiding the ugly ones.
Note: not all pre-amp tubes are being used to amplify the original signal. Some have other specific functions, as to raise the signal level in the FX loop, to drive a reverb can, etc.
Selecting your tubes
To understand how your amp's gain cascade was designed is important to better select the right tube for each particular position, to achieve the exact behavior you wanted.
By example, if an amp has two channels and each channel follows an independent path, you should study the tubes that each channel needs separately.
But if your amp was designed just to bypass / incorporate some of the triodes, each tube can potentially affect to every channel.
The sooner you begin to distort, the higher the distortion level you will achieve. The more headroom you give, the cleaner will remain your amp.
Early distortion will allow you to reach the sweet spot at lower volume levels while, late distortion will allow you to play clean at higher volume levels.
The more you push that Gain control, the more you distort every tube in the chain.
The less you push that Gain control, the more headroom you have for following tubes in the chain.