(Part
Two)
This
month we continue our discussion on tubes. I invite all
who missed last
months article to click on the Metro Music Mayhem archive
link to read it as it will get you up to speed on
what we will cover this month.You can also access any other
of my past articles if you keep scrolling down the page.
I promised this month to talk about those magic buzzwords,
Class A and Bias, so here we go.....
First a reminder
of how a speaker operates will aid in understanding
what a Class A tube amplifier output stage is actually
doing. Every speaker has a coil of wire (voice coil)
that is suspended inside a circular gap in a magnet.
Putting a signal from an amplifier into this coil
causes it to move around inside the magnet gap due
to the signal inducing a magnetic charge on the coil
causing it to attract or repel the magnet. The voice
coil is attached to a paper cone and moves in step
with it pushing and pulling air and creating sound.
When there is no signal present the voice coil is
adjusted to sit at a point allowing it the freedom
to move equally in either direction. If the voice
coil couldn't do this it would create a bad sounding
distortion since an audio waveform contains complementary
parts that push or pull the speaker cone equally
in either direction from its resting point. If the
speaker cone can't follow the waveform exactly distortion
is created since what is being heard is different
than the signal going into the speaker.
Moving on let's talk about an amplifiers
power output stage. This is where we come up against
the first of the magic buzzwords "Class A".
I hear many guitar players speak of Class A with
much reverence. Problem is, I never meet any who
can explain to me what it means. An amplifier class
refers to how the signal sent to the speaker is created.
Class A single ended amplifiers contains one output
tube that creates the power going to the speaker
all by itself. To do this the tube must be amplifying
100% of the time.This is an inefficient process that
creates a lot of waste heat and doesn't allow a lot
of audio power to be generated. You won't see more
than 15-20 watts at the most coming from a Class
A power stage.So the people telling you that Vox
AC30's and old Plexi Marshalls are class A are very
wrong. |
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So if Class A is so inefficient why do we even use it? The
answer is Tone. Each amplifier Class imparts a different
type of harmonics on the guitar. Class A accentuates warm
sounding even ordered harmonics. Additionally the lower
powered Class A power stages can be pushed into distortion
without creating ear-splitting volume so the guitar player
can more easily get output stage distortion(please refer
to my earlier articles in this series for an explanation
of this).
In order to create a more efficient way of producing power
we can utilize Class AB. In this class we use two or more
tubes in what is called a Push-Pull circuit. Here, for
every cycle of audio one tube turns on and moves the speaker
forward from its center point and back to center and then
the other tube turns on and causes the speaker to move
backwards and then back to center. If more than two output
tubes are being used they are arranged in even multiples
on each side of the Push-Pull. This is a much more efficient
way to create power and allows us to easily hit 50 watts
for two tubes or 100 watts for four as each tube is not
required to handle the entire signal cycle.
If you desire a loud and clean sound especially when playing
with other musicians Class AB will play louder than Class
A. Class AB when pushed into distortion is certainly louder
than Class A and for a large stage where a louder sound
is required it is usually the way to go.When it comes to
tone Class AB contains a different set of harmonics than
what is created in Class A. Class AB amps can also be adjusted
as to where during the signal cycle one tube turns on and
the other turns off. This can really effect the distortion
sound of the circuit. How this is adjusted is where our
other buzzword comes in- Biasing.
BIAS-What does this magical adjustment really do? To begin
with it is a measurement of how much power a tube is dissipating
when it is at a no signal condition called idle. Let's
look first at Class A. We said before that to make sound
the amplifier must both push and pull the speaker to create
sound. Therefore, we need opposite electronic signals input
into the speaker to both push and pull it. Audio is alternating
current (A/C) and changes polarity with each cycle of sound.
Obviously this is perfect to drive a speaker. But the voltage
inside a tube guitar amp is positive Direct Current. So
how do we take this one positive voltage and convert it
to varying A/C? I suggest you look back at last month's
column for the explanation of how a tube works because
we are going to build on that concept now. Remember that
a signal on the control grid of a tube will control the
flow of electrons from the cathode to the plate. Make the
grid more positive and more electrons get though, more
negative and less electrons get through. Suppose instead
of a signal we cause the tube grid to stay at a specific
DC voltage.
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It will cause the
tube to have a specific non varying current flow
through it. As we said before current times the voltage
in the tube equals power, so this static condition
of the tube causes a specific amount of power to
be dissipated by the tube. This static voltage is
called the bias voltage. Now if we input an A/C audio
signal it will add and subtract to this dc bias and
cause the tube to conduct more or less current in
step with the input signal. This is input to the
output transformer and since the transformer blocks
DC from appearing on its output, all that appears
at the output going to the speaker is an A/C audio
signal.
So it should be apparent that if
we bias the idle of the tube so that it sets the
tube's current flow at idle somewhere about halfway
between its maximum and minimum possible current
flow we will be able to amplify both sides of the
waveform equally without running out of headroom
on either side. If we slide the bias away from this
optimum point either up or down it will cause one
side of the signal to distort before the other. This
asymmetry when the power stage is pushed into distortion
sets up a different set of harmonics. You'll hear
a different tonality between the negative side distorting
first, the positive side distorting first or both
sides distorting equally. This asymmetry is sometimes
desirable. Harp players are examples of players known
to favor asymmetric sounding distortion. |
Moving on to the Class AB push-pull circuit we now are
going to split the signal into two halves so in the case
of a two tube output stage one tube pushes the speaker
and one pulls it.In this setup the last stage of the pre
amp is configured as a phase splitter. This tube outputs
two identical signals that are the opposite of each other
(out of phase). When they are input to the output tubes
it causes one of the output tubes to conduct for the positive
half of the waveform and the other tube for the negative.
But it is not that cut and dry. There is not an instantaneous
transfer of the signal where one tube turns on and the
other tube turns off. There is some amount of each cycle
of the waveform where both tubes are conducting. This is
where the A comes from in Class AB. As I mentioned before
the transition between tubes can be varied. Looked at another
way it is possible to vary the amount of output there is
before each tube transitions from class A to AB. This can
be easily exemplified by pulling one output tube out of
a two tube push pull amp. If you play loud, it will sound
distorted because the speaker is moving much more in one
direction than the other. But if you bring the volume down
you'll hear the full undistorted waveform. This is because
up to a certain point of amplification each of the tubes
does amplify the entire waveform. Where this transition
is set can make an amplifier sound different as the distortion
is different sounding based upon where the transition from
Class A to AB is set on the amp's power scale. This is
one reason that people favor 50 watt Marshalls over 100
watt. The distortion point is closer to the class A transition
point at lower volumes.
We can liken biasing a Class AB stage to the idle on your
car. When it is set too low the engine could stall. If
it idles too high the engine wastes a lot of gas. With
tube biasing we find an optimum point below which your
sound will be very brittle (stall) and above which will
cause your tubes to overheat(waste) and prematurely age.
So now let's talk
about how much you will actually hear the effect
of bias in your sound. There is no one answer to
this. It is dependent on the combination of many
variables. These include amplifier power, volume
amp is played at, genre of music, amount of pre amp
distortion used etc. I can give some general examples
so you can get an idea of where your needs fall in
regards to bias. If you are a player using a heavy
amount of pre amp distortion and are also not pushing
the amp's power stage to its limits(master volume
low), you can turn the bias control back and forth
all day long and probably never hear a difference.
This is for two reasons. Firstly the copious amount
of distortion coming from the pre amp masks any difference
in power amp sound change by bias. Secondly, if you
are not pushing the amp into distortion any harmonic
distortion influence by bias settings is nonexistent.
In this type of sound it is better to set the bias
a little cold and lengthen the life of the tubes
by running them cooler. Just not so cold that when
you play clean the amp sounds brittle. Biasing the
stage really hot will do nothing for you except bring
me more business in tube sales. It's the blues and
classic rock players that may benefit more from bias.
When your tone is built around distorting your output
tubes as we previously mentioned, bias can affect
the type of distortion you achieve, although I believe
that in most cases and with most amps slight variations
in bias do not have as much tonal influence as people
seem to believe. |
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Another term we see affiliated with bias is matching.When
output tubes are matched it means the tube vendor has tested
many tubes and grouped them into sets whose performance
characteristics are similar to each other. One benefit
of this is that at least the tubes are being tested before
you buy them so the really bad ones can be rejected. Of
course what performance levels of working tubes should
be accepted comes into question. Groove Tubes position
was to take all the tubes and rate them on a scale of one
to ten. This allowed them to have a lower rejection rate,
although It is the opinion of many techs that anything
outside of their 4 or 5 rating should of been rejected
and that the claims of different ratings for different
sounds were just so much snake oil. How important is matching?
Again as we said above, it depends on the type of playing
style the tubes are used in. Remember also that tubes are
not entities unto themselves. The circuit they operate
in contains many components, so if the component tolerances
have drifted between the two halves of a push-pull circuit
your matched tubes can become unmatched very fast. Some
amps contain a bias balance control in addition to its
bias adjustment control. This will let you slightly vary
the bias to each half of the Push-Pull circuit setting
an optimum match point for the tubes installed and their
related circuitry. When the output stage is matched it
helps with bass response, makes the distortion on power
chords more lush and improves the picking dynamics for
lead playing (the sound will feel spongier and different
picking intensities will vary your distortion sound.
Next month we will pick up from here and make some points
about different bias schemes (fixed, cathode etc) and talk
about the amp's power supply. See you then.