Pedal Stackers’ Guide to the Universe
A guide to understanding how amplifiers affect tone and how you can use guitar pedals to recreate most of these effects…like compression, distortion, etc. Contributions from Donner Rusk and Bjorn Juhl.
Using Guitar Pedals as Building Blocks
Think of guitar pedals as building blocks that recreate something that an amplifier does, but at a controllable level that you can manipulate consistently (and at lower stage/bedroom volumes).
Do you remember the last time you plugged into a really good amp and got to turn it up loud? What a feeling of POWER! The guitar seems to be playing itself. The compression, sustain, treble ’rounding’ and the dynamics turn the guitar and amp into a single responsive tone machine.
Think of an amp you enjoy playing. What sets it apart from the others? Just the right amount of sustain? The EQ? The breakup? The responsiveness? Is it the way the whole circuit responds to your touch? This phenomenon is a wonderful thing!
But, it can be difficult to reproduce this tone and responsiveness at varying volumes, from venue to venue, and as tubes and circuits wear they respond differently. In an instant, that “magic” can be gone just like that.
Recreating Amplifier Gain Stages with Overdrive & Distortion Pedals
One alternative to getting that open/cranked amplifier tone is to run a series of boost, overdrive, distortion and fuzz pedals into a clean amp to simulate the various stages that an amp uses to create these wonderful tones. Or, you can run a partially driven amp that will easily slip into overdrive when pushed correctly via a high quality boost or overdrive pedal.
Understanding the Gain Stages of a Guitar Amplifier
An amp usually has four major sections that affect the performance and tone of the output:
Preamp > Poweramp > Transformers > Speaker
Understanding & Recreating the PREAMP Stage
Preamp tubes usually reach saturation first, and as they do, a few things happen. The tube runs out of headroom and the treble is reduced, and limiting and compression/sustain set in. Also, there is a loss of bass as the tube runs out of power (it takes more power to move bass frequencies), so you can end up with a mid hump, treble/bass loss and added compression, sustain and limiting.
Tonal Qualities & Intricacies of the POWER AMP Section
A typical power amplifier can be divided into three basic stages regardless of design:
- Input Stage
- Voltage Amp
- Output Stage/Current Amplifier
All three of these stages become one stage to comprise a power amplifier. This is most often surrounded by a feedback network. Using feedback networks in the power amp section is one approach that reduces distortion in specific stages inside the power amp.
Amplifiers with Feedback Networks
The most common feedback used is a “global feedback network” that comes from the speaker output and mixes with the input signal. This suppresses distortion created in the power amp. While both do essentially the same thing, the “global” approach also reduces distortion in the speaker array, giving a firmer bass response. This is so, because the power amp output becomes “stale” or less robust.
It’s sort of like a kid’s jump rope: one end is the power amp while the other is the speaker. If the power amp end is not swinging the rope, the motion of the rope will be less. The action of both ends needs to be matched and coordinated to get optimum performance.
This means that the speaker’s effect has less effect on the power amp output. The relationship between speaker and amplifier is called “damping” and is expressed as a factor, hence “damping factor”. The less effect being lost through interaction, the higher the efficiency and the connection is “stale”, or less movement is allowed and the amplifier will “dampen” the backlash effect the speaker coil will have. It will resist changes in voltage by producing an equal voltage so as to maintain status quo.
Amplifiers without Feedback Networks
Amplifiers without global feedback usually (and always with tube amplifiers) are affected by the speaker by a larger measure than those with global feedback. For example, Fender amps have global feedback (more so than Marshall amps.) They will be less picky about what speaker is connected. The VOX ACs do not have global feedback and the speaker choice will be much more critical. This effect is the “swampy” feel in some amps.
Some amps have a damping control and it mostly affects the looseness of the bass response. The purpose of this control is to enable settings from VOX ACs to Fender amps, more or less, but usually it’s not even close to either as the relationship between the design topologies is much more complex.
Other Considerations: Bass Amplifiers, Tubes, etc.
In bass amplifiers, the damping is usually high. It has to be, for the bass to sound intelligible. This is often viewed as less important in guitar amps. Feedback reduces the distortion generated by the three stages.
With tubes, an amplifier without feedback can be made and there are quite a few popular amps using this design. A non-feedback amp circuit has narrow bandwidth and the distortion levels can be high, but can sound relatively clean. Such an amplifier will compress and soften overall tone quite early on in the volume dial, and show no drastic increase of distortion, but respond dynamically (yet compressed). This means this amplifier will distort just a little harder with a sudden increase in input level, but below saturation level where it will sing, as opposed to amplifiers that will play relatively cleaner until they hard-clip, giving “bursts” of distortion.
Transformers Tie the Amp Together
The output transformer does many things, but basically it’s an interface. Its job is to “transform” the product of the high-voltage, low-current, high-impedance tube circuit into a low-voltage, high-current, low-impedance product that the speaker can turn to actual sound. Some transformers do this better than others, and this is where losses can occur, especially in smaller trannies. The smaller the transformer, the more loss (bass, treble, compression, etc.).
Most often, the transformer colors the EQ of the amp. The internal feedback network of an amplifier’s power amp tries to correct these losses. The presence control in an amplifier reduces the amount of feedback in the treble frequencies. Upper frequencies are now limited by the output transformer. The treble ceiling is now set by the limits of the transformers.
During light overload the transformer will compress since it’s unable to reproduce transients rapidly. This softens sharp edges, but as overload is lessened, the transformer continues to compress a while. It’s an inductor (a coil) and doesn’t respond to rapid changes but tends to preserve the status quo. It may also distort in other ways, adding overtones.
To summarize, the output transformer compresses and diminishes frequency range and adds some distortion during overload, and is a line driver to the speaker. It is almost pointless to look at the effects of the transformer without looking at the primary source (the power amp section, what comes into the transformer) and the secondary load (the speaker). Both will affect the performance of the transformer.
There are distinct differences between power amp behavior in a solid state amp and tubes near overload. At low levels the differences can be quite small. Imagine that you have a distorted sound and you increase output level of power amp. At a “magic” point the sound is softened. Some of the high treble diminishes and the sound becomes more compact. This is how a transformer acts on the overall sound. You don’t hear this in solid state amps.
Speakers…The End of the Line
A speaker most severely changes frequency response, but also distorts and compresses the signal. By definition, any change from the original signal is “distortion”. Frequency Distortion is EQ change, Amplitude Distortion is compression, etc. So, the choice of speaker and how it matches to the amp can have a drastic effect on the resulting tone.
Stacking Pedals to Imitiate Guitar Amplifiers
So, can the “hot amp” tonal phenomenon be broken into components that could then be imitated? To some extent, YES!!!
Compression, limiting, sustain, distortion, overdrive, fuzz, boost and EQ change are all effects of a tube amp at various stages of operation. Most effect pedals simulate something, whether it’s the compression of a tube nearing saturation (compressors), a trumpet mute (wah), a Leslie cabinet (phasers, flangers…), etc.
Tube Screamers & How Amp Circuits React When Pushed
In a situation where a guitar amplifier distorts (perhaps in a complex way), but does not offer enough depth, there would be a few ways to aid an amplifier’s distortion.
The classic overdrive of a Tube Screamer immediately comes to mind. It is not so much made to emulate a tube, but to add some distortion in the midrange, where the amplifier has lower sensitivity. The purpose of cutting bass in an overdrive is to make the sound clearer. Distortion in the bass gives rise to overtones higher up. This is the nature of distortion. It gives rise to artificial overtones higher up than the original frequency (i.e., harmonics).
For the same reason, treble is cut. You rarely see this kind of ‘Tube Screamer filtering” in real tube amp stages unless they are extreme high gain amplifiers. And this is usually the first stage. During these conditions, the filtering results in clarity and all losses will be made up for by the compression of later stages.
Perhaps the Tube Screamers’ name implies that this pedal will make your tube amp scream. A pedal that would react more like a screaming tube amplifier would be less suited to make a tube amplifier scream. One complex tone into another complex tone can be counter-productive (muddy, too many harmonics). If you have worked with a Master volume amp, you know the difference between turning the preamp up and the Master down (saturated, compressed sound) and turning down the preamp and cranking the Master (loud, clean and dynamic)…or, even putting everything on 11!