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(if not more) continue to use tube amps
for both recording and touring, and these
little glowing bottles still define the cornerstone tones of rock, blues, and country
guitar.
To get quickly to the heart of tube magic,
stop thinking of them as amplification
devices and start thinking of them as
tone-generating devices. A tube-based
amp makes your guitar louder, sure, but
tubes amplify your electric guitar so beautifully mainly because of the way they
ful amp set extremely low) are still distorting a little, and that distortion creates layers of harmonic depth that sweetens and
fattens up that thing that we call our tone,
even when we’re playing “clean.”
All amplification tubes carry at least four
elements within their vacuum-sealed
glass bottles: a cathode, a grid, a plate
(also called “anode”), and a filament (or
“heater”). The most basic tubes are called
“triodes,” named for the first three of
these elements (a filament is always pres-
signal from the preamp is passed along to
the output stage, where the output tubes
make it even bigger, to carry it on to the
speaker via the output transformer.
(Note: some people refer to the latter
as “power tubes”, but I prefer “output”
tubes because that better defines their
function, whereas “power” might be confused with the power stage within the
amp, AC/DC voltage conversion, and the
work done by rectifier tubes, which is a
different function altogether.)
distort. To put it as briefly and concisely
as possible: push a simple transistor circuit
hard, and it clips (distorts) in a sudden,
harsh, “square wave” way; push a tube
into clipping and it distorts more gradually
and more smoothly—it “rounds off” into
distortion—and slathers on a gorgeous
gravy of harmonics along the way. There
are a lot of other factors involved, of
course, but that gets us to the nut of it.
This is why any decent sounding solid-state amp requires a lot of extra circuitry
to do what a very simple tube amp circuit
can do naturally. And be aware, too, that
when I’m talking about distortion, I’m
also referring to sonic elements that influence your so-called “clean tone.” Most
tube amps, even when set to clean levels
(unless you’ve got the volume of a power-
ent, so it’s ignored in the naming process).
Pentode tubes, which account for most
output tubes and a few preamp tubes,
carry two further grids—a screen grid and
a suppressor grid—that help to overcome
capacitance between the control grid and
the plate.
In simple terms, a tube’s job is to make a
small voltage (guitar signal) into a bigger
one. How do they do this? Pluck a string
on your guitar and the pickup sends a
small voltage to the input of your amplifier, where it is passed along to the grid
of the first preamp tube (think of it as the
“input” of this tube). The increase in voltage at the grid causes electrons to boil
off of the cathode and onto the plate at a
correspondingly increased rate and, voila,
the sound gets bigger. This slightly bigger
Preamp tubes and output tubes do essentially the same thing, just with varying
degrees of bigness, if you will. Tubes are
literally the amplifiers at the heart of your
amplifier: they do the real amplification
work, and everything else inside the box
is there to help them run efficiently and
to help pass along the signal. Of course,
in addition to early amplification duties,
preamp tubes are also used for other
functions within the amp: to drive reverb
or tremolo stages, for example, or to split
the signal and reverse the phases of the
two legs that are fed to the output tubes.
Preamp tubes are easily identified, in
most cases, as the smaller bottles in your
amp, and are usually positioned to correspond to your amp’s inputs and early
