EL34. Take your aural imagination across
the pond, conjure up that big, British crunch
tone, and your mind’s ear is hearing the
EL34. The classic Marshall tube, the EL34
was the big boy of British amplification from
the late nineteen-sixties onward. It can be
driven at higher voltages to produce a little
more output than the 6L6GC, and it sounds
somewhat different, too: characterized by
a fat and juicy but softer low end, sizzling
highs, and a midrange that exhibits a classic
crispy-crunchy tone when driven into distortion.
This is the tube of post-1967 Marshalls
like the JMP50 “plexi” and “metal” panel
amps, the JCM800, and the majority of
modern models. It also appears in the classic
Hiwatt models, and plenty of modern amps
seeking a big Brit-rock sound. Many contemporary
American makers, such as Rivera
and VHT, have also used EL34s for high-gain
amp designs, and plenty of boutique makers
also employ this output tube.
EL84. Sometimes described as “a baby
EL34” because it is another classic British
output tube, the EL84 really has a tone
all its own. This tall, narrow, 9-pin output
tube is best known for its appearance in
classic Vox amps such as the AC15 and
AC30, and is most often used in “Class A”
circuits, which seek to achieve a sweeter,
more harmonically saturated sound at the
expense of a little output efficiency. The
EL84 can still exhibit a pretty firm, chunky
low end in the right amp, but is most
known for its chimey, sparkling highs and
a midrange that is crunchy and aggressive
when pushed. A pair in a cathode-biased
output stage (a la Vox) will put out around
fifteen to eighteen watts, and a quartet
double that. These tubes also appear
in many modern amps that emulate the
“Class A tone,” including models from
Matchless, TopHat, Dr Z and others.
KT66. Rarely seen for many years other than
in vintage amps that carried them (notably
early-sixties Marshall JTM45s, following
their brief use of 5881/6L6s originally), the
KT66 (pictured at right) is a direct substitute for the 6L6, but
really has a character all its own. This tube of
European origin is a little bolder, firmer, and
fatter than its American cousin, and can put
out a little more volume. A few good recent
reissues of this tube type have led some
amp makers to design around it again, and
Dr Z’s Route 66 is one example of a popular
boutique amp that takes advantage of the
KT66’s potential.
6550. Marshall amps exported to the USA
from around the mid-seventies to the mideighties
were modified to use 6550
output tubes instead of the EL34s they
were originally designed for, apparently for
reasons of availability and reliability. The
change altered their character somewhat,
as the 6550 doesn’t sound especially like an
EL34, but more like a bigger, louder 6L6 (in
approximate terms). That’s not to say it’s a
bad thing, just different. Many other makers
have designed amps around this lesser-seen
output tube, such as Alessandro and ENGL.
The 6550 is perhaps more commonly seen
as an output tube in big bass amps, and was
used for a time in Ampeg’s SVT, and currently
appears in models by Traynor and others.
While each of these output tube types has
its own characteristic tone, different makes
of tubes of the same type can sound quite
different, too. Take six different pairs of
6L6GCs from different manufacturers,
for example, some old and some new,
and each will sound just a little different
in your amp (sometimes a lot different).
Tube connoisseurs rave about NOS tubes
(new old stock), meaning tubes that were
manufactured in the USA or Europe many
years ago, but have never been used—and
certainly these can represent the pinnacle
of output tube quality, provided you can
find a good, tested pair that is genuinely
NOS and not just used, pulled from an old
hi-fi, and polished up a little. You’ll hear
tubeheads go all gooey over black-plate
RCA 6L6s, Mullard EL84s, Brimar 6V6GTs,
GEC KT66s and many other types that carry
the great brand names of old, and certainly
there’s a lot to be said for them. Any pair
you can find in genuinely good condition,
tested, and guaranteed will also be very
expensive these days. Grab some if you can
(and certainly if you find some going cheap
from an old supplier who is selling out; but
also be aware of fakes and forgeries being
sold online these days as NOS—there are
plenty of them around). But there are also
many, many excellent current-manufacture
tubes today that are very good—with better
quality and better selection than was available
even ten years ago—and these exhibit
different sonic characteristics too. Read
about their respective pros and cons online
(there isn’t room to go into full detail here),
or try a few different pairs of the types that
seem like they’ll suit you, and see how they
change the sound of your amp. When you
locate a pair that’s just right, you can always
keep the others for back up.
As discussed briefly in Part 1, distortion
occurs in all stages of a tube amp, but
the resultant overdrive tones sound a
little different depending on which type
of distortion is generated where. Preamp
tube distortion, as much fun as it can be,
will sound a little more fizzy and gritty,
while output tube distortion will sound
comparatively thick, rich, and dynamic (in
broad terms). Old-school tone freaks tend
to enjoy the distortion tones generated
at the output stage, which is why you see
many such players going for vintage—or
vintage-styled—amps with simple circuits,
no master volume (or one that’s bypassable), and a minimum of bells and whistles
such as channel switching and added gain
stages. Such amps aim to drive the output
tubes more than the preamp tubes, and
to generate that creamy, harmonically
saturated overdrive tone when cranked
up. This love of output-tube distortion is
also what’s leading a lot of players, touring
pros included, to use smaller amps
on stage. Few players really need a big
double-stack to be heard on stage these
days, and it’s harder to push such amps
into overdrive without incurring the wrath
of the soundman and blowing your band
mates off the stage. Use a smaller fifteen
to thirty watt combo or mini-stack, however,
and you can hit the sweet spot and
still (hopefully) dodge the tinnitus until
well into late-middle age.
Be aware that many types of amps also
need to be rebiased when output tubes
are changed. This is something you can
do yourself with the help of a kit (several
types are available), or have done for you
by a qualified tech for a nominal charge.
An amp’s bias is like a car’s idle speed: it
needs to be set correctly for the amp to
operate efficiently, and an incorrect bias
setting will also seriously impede your
tone. Confusingly enough, “fixed bias”
amps are the ones that generally have
adjustable bias levels that need to be
checked and reset when you change tubes.
Cathode-biased amps, on the other hand,
which are often billed as “Class A amps,”
have a bias level that is set at the factory
with a fixed resistor. With these, you just
pop in a good, matched pair of new tubes
and away you go.
It’s also worth knowing that any new set
of output tubes, whether NOS or new
manufacture, will need some playing-in
time. They won’t sound their best until
you have put a few hours on them, and
maybe as many as forty or eighty hours
of playing time to get them into the tone
zone. Not unlike a vintage bottle of wine,
output tubes need to “breathe” a little
before they will be at their peak. Similarly,
once you uncork that prized NOS pair that
has rested on the shelf for three decades
and start playing them, they won’t last
forever. Hopefully the tonal payoff will live
up to the anticipation. Test, taste, sample,
enjoy—there’s gold in them thar tubes!