The
BLUESBREAKER:
Revisited
tube; because this tube was used in the
Fender Bassman, it was the tube of choice.
However, supplies for the 5881 were scarce
and the KT66 was used as a replacement.
The KT66 provided more bite and distortion, suiting the JTM45 perfectly, and unlike
the EL34, it was not overly saturated and
did not sound mushy. The KT66’s bottom-end was solid and exhibited clarity, while
the trebles were almost “
violin-like” in nature and very smooth.
Much like the Fender Bassman,
the JTM45 became known for its
unique sound, primarily because
of this tube.
A basic style of logo badge was used on
these very first combos, and is commonly
referred to as the “gold block logo.” The
badge was rectangular in shape and was
very simple, having only the Marshall name
in bold block letters. The letters were black
in color on a background of gold, although
silver was sometimes used.
quantities, etc.) a low priority – existing serial numbers are not reliable or even logical,
and production numbers are at best derived
from employee recollections. Therefore,
determining just how many amps were built
exactly like Clapton’s is extremely difficult.
The following list of features, derived from
exhaustive research, best represents the
Clapton’s combo specs.
Four 12AX7 (designated the
ECC83 in Europe) preamp tubes
were supplied in the 1961 and
1962, and both featured a Tremolo
circuit as a standard appointment.
These amps also included tube
rectification, which added to the
sound’s overall smoothness. The
5AR4 rectifier tube (designated
the GZ34 in Europe) was used on
all JTM45 configurations, including models 1961 and 1962, and
was the same rectifier used in the
Fender Bassman and many other
classic American amplifiers.
This particular amp is pictured on the
reverse side of the “Beano” LP. The
Series II cabinet was designed by
Marshall’s chief designer Ken Bran, and
incorporated a more modernistic styling. Its dimensions were 32” wide by
23” high by 10. 5” deep. The cabinet
was sturdy and constructed of 3/4”
Marine Ply, incorporating one slot-style
cooling vent and covered with black
Lavent vinyl. The grill cloth was the
classic gray and white striped variety
later known as “Bluesbreaker” grill
cloth. The cabinet was smartly accented
with gold hardware attachments for the
cabinet handle.
The JTM45 also incorporated two
“can” style filter capacitors; the
number of capacitors used, and
their respective value, was a significant factor in the overall sound
of the amplifier. One LCR Co. 32µf x 32µf
at 450 volts was mounted on top of chassis
near the power transformer, and one LCR
Co. 16µf x 16µf at 450 volts was mounted
inside the chassis just beneath the power
transformer – the standard arrangement for
the JTM45.
KT66 tubes as were used in Clapton’s Model 1962
The Clapton-Spec Combo –
One of a Kind?
The particular Model 1962 amplifier that
Eric Clapton used with John Mayall and the
Bluesbreakers was unfortunately lost, without knowing its exact pedigree. It was definitely an early edition of the Series II combo
and was built during a continuous procession
of production changes and revisions. The
typical production run of any of the Marshall
line could have differing components from
amp to amp – it is not an exaggeration to
say no two amps were exactly alike. This
was primarily due to the inconsistent component suppliers Marshall used at the time,
and partly because booming business made
record keeping (serial numbers, production
Chassis Electronics and Components
Clapton’s amplifier utilized the JTM45
bass guitar amplifier electronics
known as the model 1986 and the
same chassis used in the Series I,
made of 6061 aluminum sheet. One
circuit board screwed inside the
aluminum chassis was used and it
was manufactured by the Heathfield
Company.
Speakers & Logo
The Model 1961 would utilize four 10”
Celestion Alnico speakers, while the
Model 1962 would use two 12” Celestion
G12 speakers, handling 15 watts and
featuring an Alnico magnet. The Alnico
magnet contributed to the amp’s unique
and sparkling sound, referred to as the
“British chime.”
Mounted on top of the chassis was the heart
of the amplifier – the transformer. The de-facto JTM45 output transformer was the
“RS De Luxe;” however, since it was rated
at only 30 watts, it was not ideal. Clapton’s
amp may have utilized the newly acquired
Drake Company transformers. The output
transformer would have been model #784-
103, with an 8K primary Z, and the power
transformer would have been model #1202-
55. Utilizing the Drake transformers would
explain why Clapton’s combo did not sound
like a “typical” JTM45 amplifier.
Clapton’s amplifier utilized two GEC KT66
output vacuum tubes. The preamp circuit
included four Mullard ECC83 tubes, with
