Chassis Electronics and Components
The internal chassis that included all the electrical components was made of 6061 aluminum sheet metal. Aluminum has excellent non-sparking and non-magnetic characteristics, making it ideal for electrical shielding purposes and chassis use. It is, however, not as strong as steel and would fatigue and sag under the weight of the heavy power and output transformers. To remedy this, cast aluminum end blocks were riveted inside each end of the chassis to add strength and rigidity. The chassis were 2.5” deep and purchased from a sub-contractor named Smiths.
The JTM45 used one circuit board screwed inside the aluminum chassis, providing a central platform for the various electrical components. Prior to 1965, Marshall used Tufnell perforated circuit boards; from 1965 to 1967 a subcontractor called Heathfield manufactured the circuit boards, which can be recognized by the drilled holes on the solid sheets.
The output transformer was officially known by Marshall as the “RS De Luxe.” It was light gray in color, with horseshoe end bells. The maximum output rating was only 30 watts, and all three transformers used on the JTM45 – the output, power and choke – were purchased from Radiospares, an electrical component clearing house.
The first combos produced in late 1964 were equipped with two GEC (General Electric Co.) KT66 vacuum tubes. The original design of the JTM45 called for the Tung-Sol 5881 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 bottomend was solid and exhibited clarity, while the trebles were almost “violinlike” in nature and very smooth. Much like the Fender Bassman, the JTM45 became known for its unique sound, primarily because of this tube.
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.
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. 32ìf x 32ìf at 450 volts was mounted on top of chassis near the power transformer, and one LCR Co. 16ìf x 16ìf 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 (photo courtesy of Dan Duntze/Beanotone NOS Tubes)
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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.” 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.
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 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. 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.
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.
Mounted on top of the chassis was the heart of the amplifier – the transformer. The defacto 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 one dedicated to the tremolo circuit. The Mullard GZ34 rectifier tube was again used. Two can-type filter capacitors were used: one LCR 32ìf x 32ìf at 600 volts mounted on top of the chassis near the output transformer, and a second LCR 16ìf x 16ìf at 450 volts mounted inside the chassis.