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By
Steve Weiss
This month our topic will be equalization (EQ) as it applies
to tube guitar amp tone controls. A working knowledge of how
tone controls operate will aid on your quest for the holy grail
of tone. Almost any guitar amp has some form of tone control.
On inexpensive amps you may see a single control marked tone.
Better amps have bass and treble or bass,mid and treble and some
amps have a plethora of knobs, pull pots,and toggle switches
all relating to tone/ equalization.
Why do we need equalization? Why can't we just amplify the guitar
the way it is? There are a number of reasons to use EQ, including
compensation for room acoustics, optimizing the response of a
speaker cabinet, variations in the sound of different guitars
(a humbucker is different than a single coil) and of course personal
taste.
Let's first look at two terms that will help us to understand
EQ. Those terms are bandwidth and frequency response.
BANDWIDTH
Bandwidth is the range of frequencies from the lowest to the
highest contained in a given sound. Frequencies are measured
in cycles per second and abbreviated as Hz (Hertz). Humans
(and musicians too) can hear frequencies from around 20 Hz
to 20,000 Hz especially when we are younger, but this degrades
in high frequency response as we age and for rock musicians
like myself an additional dip in the mid range occurs caused
by blasting through Marshall stacks for twenty years. Guitar
bandwidth starts at 82 Hz (low E string) on guitar and 41 Hz
on bass and goes up to maybe 15,000 Hz when you factor in harmonics.
FREQUENCY RESPONSE
Frequency response is the balance of frequencies to each other
within the given bandwidth. No sound naturally exists containing
an equal amount of each frequency, for instance a sound may
contain a larger amount of frequencies from the lower part
of its bandwidth than the top.
EQ can be used to reduce the size of the bandwith by using filters
that attenuate out the high and low frequency extremes (called
shelving) or we can use EQ filters to change the shape of the
frequency response by designing them to filter based around a
specific frequency (center frequency control). For example a
bass control that has a center frequency of 100 HZ exhibits the
most effect at that frequency and gradually lessens its effect
on frequencies the further they are away from the center frequency
both above and below it.
How does a tone control
electronically shape the frequency response? To explain
let's look at some basic electronic concepts. Let's take
the signal from your guitar and hook it to a resistor on
its way to your amp. What will happen? A resistor is an
electronic component that opposes the flow of electrical
current in proportion to the value of its resistance. It
does not favor any one band of frequencies. It lets all
frequencies through in their original frequency balance.
The only change is that all frequencies have been equally
attenuated to some smaller value. Obviously, we can not
use a resistor to alter the tonal spectrum.
In the majority of guitar amps this is
done using a capacitor. When we run a signal through a
capacitor something different happens than a signal going
through a resistor. The capacitor will not let all frequencies
through it equally. It is more restrictive to some than
others. You can think of a capacitor as a frequency dependent
resistor. I mean by this that if you were to inject a range
of frequencies one at a time through a capacitor and measure
the difference in each one between its input and output
you could then say that each individual measurement is
the equivalent of running that same frequency through a
resistor of a value that produces the same amount of attenuation. |
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You will also see the
capacitor is the equivalent of a higher resistance at low frequencies
and lower resistance at higher frequencies.The specific results
of course are dependent on the value of the capacitor being measured.
We call this effect capacitive reactance or impedance. Impedance
although it is closely related to resistance, and shares the
same unit of measurement,the ohm, is not an interchangeable term
with resistance. In relation to sound which is alternating current,
impedance can be viewed as frequency dependent resistance. Resistance
is used when talking about direct current which has no frequency
(0 HZ).
Extrapolating upon the above concept we can now construct a
guitar amp tone control circuit. If we want to construct a circuit
that gives us a bass and treble control we can do so using a
handful of inexpensive components. First we need two capacitors.
One will be a larger value for the bass control and then a smaller
value for the treble control. When signal is passed through any
cap, for the purposes of this discussion, it will pass all highest
frequencies and according to the value of the cap will start
to roll off the lower frequencies by not letting them pass through
the cap. We can therefore construct a circuit that puts a signal
simultaneously into these two caps. The output of the smaller
cap will contain mostly high frequencies, the lows having been
stripped away. The output of the larger cap will contain lower
frequencies down to the roll off frequency of that cap which
will be near or below the lowest guitar frequency. If we hook
a variable resistor (pot) to the output of each cap we can now
vary the output of each cap from fully on to fully off(when the
pot is set to zero all the signal is shunted to ground). It we
mix the outputs of these two pots together we now can get an
output we can adjust for more or less treble and bass. This is
an oversimplification of the circuit to serve getting the basic
concept across, a little more is needed in reality. This is also
called a passive circuit, meaning that no amplification is present
in this circuit so we can only attenuate(cut) frequencies from
the input signal not boost them. We are all accustomed to mixers
and graphic equalizers where the setting of five on a scale of
one to ten is the flat point meaning there is no cut or boost
to the particular frequency that control adjusts. On a passive
guitar amp tone control that goes from one to ten, five is not
the flat point in between boost and cut. In actuality on a passive
tone control system which will only cut frequencies,the closest
you can come to a flat setting is to turn the tone controls all
to ten. But due to bleed through from the caps thru the pots
to ground even when they are set to 10 there is still some alteration
of frequency response.Fortunately you can use you ear to set
the tone. It is not necessary to have a flat point as a reference
as long as somewhere within the rotation of the pots you can
dial in the sound you want.
Another thing that occurs in the passive tone circuit is is
a high degree of interaction between the controls. You will notice
that if you turn one control to a specific setting now the other
doesn't seem to do much when turned. Change the setting of the
first control and now the second control does more. Therefore
as you move one control it changes the setting of the other and
it is needed to go back and forth between the controls until
you get the tone where you want it.
EQ circuit
The EQ circuit due to the fact that the signal
can get pulled down to ground through the tone caps and pots
imposes a heavy insertion loss on the guitar signal path due
to it being generally low impedance in nature. One audio truism
is that to maximize frequency bandwidth and level from one amplifier
stage to the next the input impedance of the receiving stage
should be an impedance of ten times higher than the impedance
of the stage driving it. If not it acts as a load on the previous
stage and will squash out level and tone. This is especially
true of this type of tone circuit because it is generally a low
impedance and the driving tube stage is higher impedance (the
opposite of what is best). There are two common ways the tone
stack gets driven in most tube amps and they are part of both
the inherent sound and differences between Fender and Marshall.
Fender drives their EQ circuit from the plate of the preceding
tube and this is a very high impedance point subject to much
loading by the low impedance tone stack. Marshall drives their
EQ circuit from a tube configured as a cathode follower where
the signal output is a somewhat lower impedance than the plate
drive method helping to drive the tone stack with less loss.What
is the difference in sound between the two? The cathode driven
is more Hi-Fi sounding with a little less control interaction.
Is one of these the right way over the other? Obviously not,
as each company has sold more than a few of their amps. Note
also that the EQ section is only one element of the signal path
in any amp and therefore only part of your tonal equation.
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There are other ways to
create a guitar amp tone circuit. Although there are a
few variations on the passive circuit that will bring you
closer to a cut boost scenario (the Baxandall circuit in
Ampegs comes to mind) if you want a greater adjustment
range including the ability to cut and boost the EQ you
will have to use an active circuit. Most of the time this
is done with solid state devices called op amps because
they are inexpensive and easy to implement. Since an op
amp is an amplifier it is easy to add passive components
to it and make it respond to a specific range of frequencies.
This can also be easily done with tubes by providing a
feedback loop back from the tube output to its input where
the tone controls are also located. Peavey does this on
their XXX and JSX amps. Another notable amp in the sixties
that employed this technique was the Marshall Major. Here is
the problem with this. The tone controls range becomes expanded
when made active. The tone pot within its rotation now includes
both cut and boost so besides there being a wider range to choose
from it is compressed into the same rotational space as the cut
only control making changes to the tone happen within a smaller
amount of rotation. This makes it hard for the guitarist to find
his sweet spot on the tone control because he is so used to a
cut only circuit. This was one of several problems that led to
the demise of the Marshall major. |
The placement of the tone controls within a guitar amps circuitry
also has a large bearing on equalization.This is especially true
in older vintage amps. Did you ever notice that sometimes you
play an amp and get a good bit of response from the tone controls
while you are playing clean, but when you distort the amp it
is hard to tell if the tone controls are even working? Here is
the reason for this. Distortion on many amps is created by the
output tubes adding in harmonics of many different frequencies.
The output tubes are further down the audio line after the tone
controls which function best equalizing sound that comes before
not after them. Even on many early amps with high gain pre amps
and a master volume (a Boogie Mark II for instance) the pre amp
gain stages that create distortion come after the tone controls
which become unresponsive when the gain is cranked. Note that
the graphic EQ on an early Boogie was located after the master
pot at the input to the power amp section and would let you easily
EQ the distorted sound. Most modern Hi gain amps now put the
EQ at the output of the pre amp (post distortion) right at the
master volume control. This is why there is an easily equalized
distortion sound in many modern amps..
One other type of EQ circuit is the presence control. This is
a bit different than the EQ controls we talked about above. It
is actually part of the power amp circuit. Most tube power amps
feed a small portion of the speaker output back to the driver
tube and mix it with the signal going in to the driver tube..
This serves to eliminate hum, oscillation and linearize the frequency
response of the amp. Since the speaker output is out of phase(
out of phase signals cancel each other) with the input signal
to the driver tube there is a certain amount of cancellation
to all frequencies of the signal attenuating it, the amount of
attenuation depending on the amount of feedback employed. If
a cap is connected between ground and the feedback lines, higher
frequencies from the feedback line (which pass easily through
the cap) are removed and shunted to ground. Therefore since they
don't make it back to mix with the original sound and only the
lower frequencies do, there is no cancellation of the highs and
they remain at the same level at the speaker output as if the
amp had no feedback circuit at all. Since the low frequencies
do make it back they are reduced and the perceived effect is
that there are more highs present (presence). Obviously, if you
add a pot to control the cap shunting the highs to ground, the
level of presence can be controlled. You can achieve a similar
effect on the lows by putting a cap and pot in series with the
feedback, stripping out the low frequency feedback and causing
an increase in low frequency response (resonance). Changing the
amount of low frequency negative feedback with the resonance
controls also changes the amplifier's damping factor which is
the amplifier's ability to control the inertia of the speaker
cone.This can also significantly change the feel of the amplifier
as it can make the amplifier's low end feel looser or tighter.One
last thought on presence and resonance is that since these controls
are situated in the power amp, when distortion is created in
the power amp presence and resonance controls can have a direct
effect on shaping it.
Another tone control worthy of note is called the cut control.
The most known example of this was on the Vox AC30. The original
Vox AC30 did not have any tone controls in the pre amp. This
is one of the many interesting differences this amp had with
its competitors. Lack of a tone circuit meant no insertion loss
leaving more gain to overdrive the output stage. As a solution
to remove brightness/ harshness what was done was to insert a
cap and pot in between the plates of the driver tube. The driver
tube takes the signal from the previous stages and splits it
into two identical but out of phase signals on its plates in
order to drive the push-pull output stage. If you would connect
the two tube plates together with a wire they would cancel and
no sound at all would be heard. But if you connect the two plates
together through a capacitor only the high frequencies coming
through the cap will be cancelled and of course the level of
cut can be adjusted with the pot.
The last tone control I will cover is the bright switch. There
is a bright switch on the front panel of almost every vintage
Fender amp. It is a very simple circuit consisting of only a
cap and a switch. The switch hooks the cap into the volume pot
circuit and lets you bypass the high frequencies past the pot
so they are not attenuated as much as the low frequencies as
the pot is turned down. The bright control is more effective
in the lower range of the pot and diminishes in its effect as
the pot is turn up. When the pot is on ten the bright control
is completely ineffective. But since we need it most to add highs
at the lower volume settings it is a very useful feature.
In closing, the EQ of a guitar signal is extremely subjective
in regards to how people perceive it. I hope I was able to give
some insight into it's inner workings to aid in the search for
your tone.
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Steve Weiss is the owner and main technician
of Steve Weiss Electronics Inc. He is experienced in
the repair of analog and digital musical equipment.
This includes everything from Vintage Tube Amps and
Pro Audio equipment to Digital Keyboards There is also
a guitar repair shop staffed by some of the areas top
guitar repair techs. He is authorized for warranty
work on most major brands. Steve Weiss Electronics
is located inside of Sam Ash Music at 5460 West Sample
Road Margate, FL 33073 954-975-3390 Ext 272. Steve
has also spent 25 years on the road as a performing
guitarist and is the designer of Primal Guitar amps
that can be seen at Primal
Audio.com Steve can also be reached at stevew@metromusicmayhem.com |
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