( A Guide to understanding how amplifiers effect tone and how you can use pedals to recreate some of these effects)
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The Pedalstackers Guide to the Universe
by Donner Rusk and Bjorn Juhl
(last updated - 12/31/03)
The concept of using more than one pedal at a time is nothing new.
But we can take an in depth look at how they can interact.
One way to think of pedals is as building blocks that recreate something that an amplifier does, but at a controllable level that we can manipulate easier.
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 and sustain and treble 'rounding' and the dynamics turn the guitar and amp into a single responsive tone machine.
Think of an amp you enjoy playing... Why ? 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 at lower volumes from venue to venue, and as tubes and circuits wear they respond differently, and that 'magic' can be gone just like that.
One alternative is to run a series of pedals to simulate the
various stages that an amp uses to create these wonderful tones, and run
them into a clean amp (producing no audible distortion of its own) or a
partially driven amp that will easily slip into overdrive when
An amp usually has four major sections that effect the performance and tone of the output.
Preamp > Poweramp > Transformers > Speaker
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 and treble/bass loss and added compression,sustain and limiting .
POWER AMP SECTION:
A typical poweramp can be divided into three basic stages regardless of
1. The inputstage
2. The voltage amp
3. The outputstage or current amplifier.
And all three become one stage.
This is most often surrounded by a feedback network
Using feedback networks in the poweramp section is one approach that reduces distortion in specific stages inside the poweramp. The most common feedback used is a 'global feedback network' that comes from the speaker output and mixes with the input signal - this surpresses distortion created in the poweramp.
While both do essentially the same thing - the 'global' approach also reduces distortion in the speaker array giving firmer bass.
This is so because the poweramp output becomes 'stale' or less robust.
It's sort of like a kids jump rope - one end is the poweramp while the other is the speaker. If poweramp 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.
(Not a perfect analogy - but a useful picture.)
This means that the speakers effect has less effect on the poweramp 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 global feedback usually (and always with tubeamplifiers) are affected by the speaker by a larger measure than those with global feed back.
Example: Fender amps have global feedback (and more than Marshalls.)
They will be less picky about what speaker is connected.
The VOX AC's 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 effect the loosness of the bass response.
The purpose of this control is to enable settings from VOX ACxx to Fender more or less - but usually it's not even close to either as the relationship between the design topologies is many times more complex.
In bass amplifiers the damping is usually high- it has to be for the bass to sound intelligable. This is often viewed as less important in guitaramps.
Feedback reduces the distortion generated by the three
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 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 hardclip -
giving 'bursts' of distortion
The output transformer does many things, but basicly 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 the speaker can turn to 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 colours the EQ.
The internal FEEDBACK network of an amplifiers' 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 its unable to reproduce
transients rapidly. This softens sharp edges but as overload is lessened,the transformer
contiues to compress a while - it's an inductor(a coil) and doesn't
respond to rapid changes but tends to perserve 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 the primary source ( what comes INTO the transformer- the Power amp section) and the secondary load (the Speaker). Both will effect the performance of the transformer.
There are distinct differences between Poweramp behaviour in a solid state amp and tubes near overload. At low levels the differences can be quite small.
Picture this: you have a distorted sound and you increase output level of
power amp, and 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)
A Speaker most severly 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.....
Pedals as Amplifier Imitators
So, Can the 'hot amp' tonal phenomenon be broken into components that
could then be imitated ? To some extent: YES !!!
Compression / limiting / sustain /distortion /drive / fuzz / boost / EQ change
are all effects of a tube amp at various stages of operation...
Most effect pedals simulate something whether its the compression of a
tube nearing saturation (compressors).... or a trumpet mute (wah) or a Leslie
cabinet (Phasers,flangers etc..)
How do the different parts of the amp circuit react when
An amplifier distorts, perhaps in a complex way, but not
enough depth. There would be a few ways to aid an amplifiers' distortion.
The classic overdrive of a Tube Screamer 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 (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 !!
SO, taking all this into consideration - ( and your Guitar/Amp combination will obviously effect the end tone) - we can use a set or 'stack' of pedals to emulate the various products of the various amp stages
and come up with a variety of useful tones.
First we can consider what type of amp tones we want to recreate:
A small Supro running at near max volume (lots of natural compression, treble roll off, mid increase and complex distortion)
Or a 100 watt Hiwatt with lots of clean head room, and dynamics simple distortion, or anything in between.
The BJFE line of pedals has a variety of distortion types and sources to pick from.
BBOD - Baby Blue Overdrive
DRD - Dyna Red Distortion
PPF - Pink Purple Fuzz
CAF - Candy Apple Fuzz
LGW - Little Green Wonder Overdrive
HB - Honey Bee Overdrive
EGD - Emerald Green Distortion Machine
RRB - Red Rooster Booster
BPB - Baby Pink Booster
LRT - Little Red Trebler
LGE - Little Green Emphaser
PGC - Pale Green Compressor
Any one of these pedals by itself can be set to simulate an amp at different stages of operation
The BBOD for example - you can set the drive higher and the level lower and get a master volume sort of tone or set the drive low and the level high for a more non-master setting.....
Lets Start with a simple stack:
BBOD > DRD > BPB
Using the Master Volume amp analogy - we can set the BBOD to a setting similar to preamp over drive (saturated drive at low level)
and the DRD at lower distortion but raised level so it has more punch and dynamics like a high headroom power amp....
and then use the BPB at the end to simply raise the level of either or both drive pedals......giving us the BBOD by itself , the DRD by itself and the combination of the two and then a straight level boost of any of these or the straight clean signal from the BPB.
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