Tone Stacks, Response and why amplifiers "sound different"
Why does a Fender amp sound differently than a Marshall?  Why do some amplifiers sound "louder" than others?  
There are many factors, some you might know right off the bat.  Power for one.  Is a 100 watt amplifier twice as loud
as a 50 watt one?  Well, the answer here is no, not even close.  When you double the power of an amplifier, you will
get about 3 db more volume.  1 db is the smallest amount of perceptible loudness change than the human ear can
generally hear.  So, a 100 watt amplifier is just a "hair" louder than his 50 watt brother, but it will have more clean
headroom.

If you want to make your 50 watt amp move four times more air ... and appear to be a lot louder, just double the
speaker radiating area.  That means that changing from a 1 x 12 cabinet to a 2 x 12 cabinet will do the trick, as an
example.

Many people prefer the lower wattage amplifiers.  They allow that magic breakup and output tube stage distortion to
be captured as levels that are not worthy of anything other than huge performing venues.  Eric Clapton and Jimi
Hendrix, are just two folks that prefered 50 watt (or less), versions when they recorded.  Many of the world's most
recognized recorded guitar solos were performed on a Fender Champ amp, with somewhere between 4 1/2 and 6
watts, depending on the year of the amplifier.

A VOX AC30 only has about 30-35 watts, and these amps can be quite loud.  This has to also do with EQ, speakers,
cabinet design, and other factors.

Speakers and speaker cabinet design have a big part in all of this.  JBL speakers are very efficient.  There was a
speaker by JBL years ago that produced 107 db output at 1 meter .  Many speakers today have an output of about
85 db.  Remembering the 3 db = power x 2 formula above, to get the same level at 1 watt from the JBL, out of a
Celestion would require about 128 watts.  EV speakers are also quite efficient, but not up there with some of the JBL
speakers.

A speaker which is less efficient might have other qualities that load the amplifier in a different way that is more to the
taste of the player.  This is one of the reasons the Celestions are so much in favor.  A blues or rock player may prefer
the breakup associated with this style of speaker.  A rhythm player, bass player, or slide steel player may prefer the
clarity of the JBL or EV speaker.

Now, lets say we have the same speakers in the same style cabinet.  It still appears that a 50 watt Marshall seems
louder than a Fender amp of the same general wattage.  An additional factor comes into play now, the
Tone Stack,,
or how the amplifier is voiced for EQ.  There are other factors too, such as type of output tubes (EL34's have a more
pronounded midrange over 6L6's for example).  For now, lets look at some of the basic
Tone Stacks, and see how
the response curves differ.  Remember .... raising a level only 3 db is like doubling your amplifier power.  Hope this is
helpful.
The chart to the left shows the frequency response of a Fender tone stack, a
Marshall tone stack and a VOC AC-30 tone stack.

The upper most trace is the Marshall amplifier.  The lower green trace is the VOX
AC-30, and the lower blue trace is the Fender amplifier.

All tone controls were set to "5", or the mid point.

This does not take into account the speaker or the power amplifier sectiion.  This is
just the pre-amp section.

As you can see, the Marshall has quite a bit of gain in comparison to the other two
in this "mid" state.

The figure below is a similar trace, controls set to mid point again, but with some
reference information related to the guitar as far as what frequencies are in which
ranges.

Remember that these three tone stacks are used in 95% of the world's guitar and
bass amplifiers.
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Amplifier Power - Modeling Amps - and the misunderstanding of amplifier power

The vast majority of the time when I first walk into a venue where I will be listening to music for the night, I can generally tell if the performance will
be a memorable one.  I can generally tell from the equipment setup, and not to brag, I have about a 90% track record.

The 10% of the time I am mistaken, it is generally easy to explain.  I did not know the performer or group, and what to expect.  I was invited as a
guest, and the music was already known to me as to not be of my particular taste, or a few other reasons.

The big tip off, is amplifier compliment ? Amplifier power that is.

There are folks that have blinding fast technique.  Speed metal players, fast articulate players, folks with speed as their underlying strong point.  
Frankly, this is not my personal taste.  I am generally impressed for ten minutes, but then my attention is lost.  I generally ask myself, are these
folks practicing, or just looking for the right note?  One note played with feeling and that has tone, is worth 100 64th note triplets from my point of
taste.

The biggest problem is amplifier power.  When I see a 100 watt amp on the stage of a 150 seat venue, I know that I am in for trouble ? Most of the
time.  If it is a jazz player looking for a clean sound, then I am safe.  If it is a speed player, well, then it is what I expect.  If it is a rock or blues band,
then I know I am in for a very one dimensional performance most of the time.  I know with good prospects, a few other things.  The player does not
understand amps or tone, perhaps his main rig is broken and this 100+ watt amp was borrowed, or they are into a hi-fi sort of
sound with little or no dynamics.

A 100 watt amp, or even a 50 watt amp, will not distort in its output section at rational volume levels.  Folks that are known for great tone and to be
great players, even in the largest venues, generally stay at around 50 watts or less.  They are looking for a particular sound, tone and feel.  They let
the stadium sound systems do the rest.  If you cannot turn your amp, and most amps, to at least 6 or so on the volume, you will never tap the soul
of most tube amps.

Folks also do not understand "loudness".  Many think a 100 watt amp is TWICE as loud as a 50 watt amp.  This is not all the case.  Double your
wattage, and all you gain is 3db.  Sure, folks talk about "headroom", and think this can be a big requirement.  Folks that actually need headroom are
clean players, rhythm players, folks who get their tone from pedals, jazz players for some styles, and pedal steel players to name a few.  A amp with
a lot of headroom is a hi-fi amp.  It will be clean, and not have the dimension of touch dynamics of a lower powered amp.  If you want more loudness
out of a 50 watt amp, double your speaker area, or go with a more efficient speaker.  Going from a 83db speaker to an 88db speaker, is almost the
same gain in volume as going from 50 watts to 200 watts in amp power.

Then there are those folks that have the great idea of pulling two tubes out of their 100 watt amp to turn it into a 50 watt amp.  To put it bluntly
and get a lot of argument, this is a stupid idea.  A great amp is made up of many components.  Power transformers, output transformers, capacitors,
and other parts, make up the design.  If one takes a Marshall 100 watt Super Lead, and pulls two of the tubes, and properly sets the impedance
selector, turning the amp into a 50 watt amp, what actually happens?  Well, we have a 100 watt power supply, that is now even less taxed than
before.  The "50 watt" Marshall will now have less dynamics, less feel, less touch sensitivity.  It will be a nice, clean, 50 watt amp.  Its 100 watt
power supply will never reach saturation.  Its output transformer will never be pushed.  It will actually be cleaner than it was as a 100 watt amp at
most settings and have greatly reduced touch dynamics.  The only distortion you will get is when the output tubes are at their limit, and this will be
an unbalanced sound, although some might think this is just to their own tastes.

Modeling amps?  In the past, I have written a lot on solid state amps versus tube amps, and modeling amps versus the amp originals they are
modeling.  Some folks think that modeling amps have some strong points such as a lot of sounds for the dollar or in a given space, or for recording.  
Some think in a live venue, modeling amps can have limitations.  To my way of thinking, this live aspect is sort of a "yes and no" to me.  I see many
folks with 50 watt amps in small clubs, where the soul of the amp is never tapped.   Put an amp like a Line 6 Vetta, Fender Cyber Deluxe or Cyber
Twin, or Vox modeling amp out there, and you may be surprised.  I may still prefer the PROPER wattage tube amp as first choice, but I will take the
modeling amp every time over the wrong tube amp.  Why?  Modeling amps allow a degree of touch dynamics and tonal ranges to be captured at
most any level.  You have all sorts of controls for this ability.  A Fender Tweed Bassman in a small venue will never be able to be cranked to its level
of tone potential for some music styles.  A Line 6 Vetta may pull off the "tweed sound" of the virtual Bassman in a much more convincing and
pleasing manner, at least to my tastes.

I think to end this, all I can suggest is, listen to amps, and play them.  See how they react to your touch.  If this is not a part of your music and style,
such as many folks that start the song at 110db and end it at 100db, then most any amp will work pretty well.  As you develop an ear for different
tone aspects, and fingers and touch that can give you at least two more playing dimensions, then you will move to the next step of being a better
player, and also have a more heightened ability as a listener.