The Pedalstackers Guide to the  Universe (of BJFE)

 

by Bjorn Juhl and  Donner Rusk

(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
pushed correctly.

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 .

 

 

 

A typical poweramp can be divided into three basic stages regardless of
design
 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.

 

 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 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.....

 

 

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
pushed?

 

 

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

 

(many  more  'stacks'  to come)