Everything You
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KT66 6550. Marshall amps exported to the USA
from around the mid-seventies to the mid-eighties were modified to use 6550
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
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 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.
EL84 Photos courtesy of thetubestore.com
amp designs, and plenty of boutique makers
also employ this output tube.
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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
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