However many fancy gizmos the techs
devise to give us “warm, tube-like” guitar
sounds, real tonehounds know it’s only happening
in one place: real tubes! If you want
to get it happening for you, though, you’ve
got to understand what’s going on with different
tube types, and learn the various ways
the wide range of tubes available will affect
your sound.
Click here to read part one: Everything You Ever Wanted to Know About Preamp Tubes
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In the last issue I discussed
preamp tubes,
along with a basic primer on how and why
tubes do what they do. This trip under the
hood we’ll explore output tubes—the big
bottles at the back of your amp that pump
out the serious wattage. Although some
people refer to them as “power” tubes, I feel
“output” is far more fitting: for one thing,
these tubes create your amp’s output; for
another, the term “power” might cause confusion
with the third type of tube in an amp—a
rectifier tube—which lives in what is correctly
described as the power stage of the amplifier.
In any case, you mainly just need to be aware
that the output tubes are the “amplifier within
your amp”: while the preamp tubes ramp up
the signal from the guitar, the output tubes
are the babies that really make it loud.
Output tubes can be recognized as the biggest,
or at least tallest, tubes in the back of
your amp, although a tube rectifier (if your
amp has one) can also be mistaken for one of
several output tube types. Your clue here will
be that there’s usually only one rectifier, but
at least two matching or similar output tubes
in any amp, other than small single-ended
“practice” amps such as a Fender Champ or
a Gibson GA-5. Many, many types of output
tubes were used in the glory days of thermionic
devices, when they appeared not only
in guitar amplifiers, but in radios, stereos,
TVs, and many other applications. Today, only
about half a dozen varieties of output tubes
are regularly used by contemporary
amp manufacturers, and just four of these
are seen in any great numbers. The four
most common output tube types are the
6L6GC, 6V6GT, EL34, and EL84. A handful
of contemporary makers still offer amps
with KT66 and 6550 tubes, and a few even
manufacture unusual designs using more
esoteric tube types, but you’ll see one of
those first four in a good ninety-nine percent
of amps you encounter today.
Other than EL84s, which are the same diameter
as preamp tubes (although taller) and
use the same 9-pin socket, all of the most
common output tube types use large 8-pin
(octal) sockets. While they might appear
interchangeable in terms of socket size,
however, most have different circuit, voltage,
and bias requirements, so they cannot
simply be substituted one for the other
in most amps. There are a few makers
today producing amps that are specifically
designed to let you swap between
output tube varieties for “tube tasting,”
and models such as THD’s UniValve and
BiValve, and Victoria’s Regal II and Two
Stroke can take any of the common 8-pin
types without needing rebiasing or other
adjustment. For the most part, though, a
maker will design an amp with a very specific
tube type in mind, and will work very
specifically to the performance and sonic
characteristics of that tube. Let’s take a
little look at the signature tones and capabilities
of some of these more common
output tube types.
Note: photos courtesy of thetubestore.com
6L6GC. Think “big Fender amp tone”
and you’re thinking 6L6 (also sometimes
substituted for the interchangeable 5881,
essentially a ruggedized 6L6). This is the
big-amp output tube traditionally seen in
American-made amplifiers, and it has a
bold, solid voice with firm lows and prominent
highs, which can be strident in loud,
clean amps, or more silky and rounded in
softer, “tweed” style amps. A pair of these
will generate around forty to fifty watts in
an efficient Class AB amp; a quartet (with
two pairs working in teams on each side of
the phase-inverted signal) can put out up
to one hundred watts. In less efficient, but
juicily toneful, cathode-biased designs (socalled
“Class A” amps) like TopHat’s Super
Deluxe or Carr’s Rambler, or a mid-fifties
tweed Fender 5E5 Pro, a pair of 6L6s will
put out around twenty-five to thirty watts.
This is the tube of anything from the Fender
tweed Bassman and blackface Twin and
Super Reverbs, to early Marshall JTM45
heads and “Bluesbreaker” combos, to the
Mesa/Boogie Mark Series and beyond.
6V6GT. Think small-tweed amp and you’re
hearing the 6V6GT. Smaller American-made
amps of the nineteen-fifties, sixties and seventies
most often carried 6V6 tubes, which
are known for their juicy, well-rounded tone
and smooth, rich distortion, which occasionally
exhibits an element of grittiness
that is not necessarily unappealing. They
produce about half the output of their big
brother, the 6L6, and are therefore more
easily driven into distortion. The 6V6 was
used in many Fender designs—the Champ,
Princeton, and Deluxe lines among them—
some great vintage Gibson amps like the
GA-40 Les Paul Amp of the nineteen-fifties
and early sixties, and countless others. From
the late eighties to late nineties no reliable
current-manufacture 6V6s were available,
so few manufactures designed new
amps around this tube. This is the course
of events that led to the virtually unthinkable
release of smaller Fender amps that
used EL84s, such as the Blues Junior (the
early-sixties Tremolux, which briefly carried
EL84s, being something of an anomaly).
The release of a rugged and reliable 6V6
first from Electro-Harmonix, then from other
contemporary makers, has led to a renewed
popularity for this tube, and it proliferates
again in the twenty-watt-and-under range.