The Marshall Bluesbreaker – A Short Analysis
The Bluesbreaker, or more accurately the Model 1962, was Marshall's first combo amp. It supposedly came about through a request from Eric Clapton for an amp loud enough for gigging that would nonetheless fit into the trunk of his car. To look at this beast of a combo, you have to wonder what Eric was driving in those days.
Standing 2 feet tall and 2 1/2 feet wide, the Bluesbreaker sports a single handle (as did the “portable” 19” black & white TV my family had back in the 1960's... where did Electrohome think we'd be taking it?) At 30kg / 66 lbs., Marshall's Model 1962 could just as well have been called the back-breaker. The heft comes courtesy of two 12” Celestion greenbacks, a JTM45 chassis lying on its back, and of course the large cabinet itself. The chassis orientation faces the nomenclature away from the player, as with some early Vox amps. From the front, the two toggle switches on the right thus read “NO NO”.
There are countless sound clips and audio reviews for the Model 1962 available online, so we'll skip the subjective descriptions and move on to a look at the amp from a design perspective. Suffice it to say that it can be a great sounding guitar amplifier.
It's well documented that Marshall's first amplifiers sought to provide British musicians with a more affordable interpretation of Fender's successful Bassman. As such, Marshall's circuit was very closely based on the Bassman, and incorporated some circuit quirks Marshall retained long after Fender had abandoned them. These include the direct-coupled cathode follower stage to drive the tone stack, and the presence control. Let's take a quick look at each of these.
The cathode follower is a stage with no voltage gain and a low output impedance. If there's a spare triode kicking around in your circuit, using it as a cathode follower is a great way to buffer the high-impedance output of a preamp triode from the tone controls that follow it. However, there are two problems with this circuit as it's usually implemented in guitar amps. First, the cathode will be biased at about half the preamp rail voltage, which puts the cathode a little too close to the 180 volt maximum filament-to-cathode voltage spec for the ECC83/12AX7. For almost all other twin triode types the average tube roller would reach for, that spec is a mere 90 volts – clearly exceeded by this circuit. That means installing a 12AT7, 12AY7, or any of the other common lower-gain twin triodes into that V2 socket could result in the tube's premature (and possibly spectacular) demise.
The other issue with that cathode follower circuit is that the grid-to-cathode voltage could go through a very unfriendly swing as the amp warms up and everything settles into its design bias state. It's like a bridge: the structure can be stable when completed, but getting it there can pass through some dangerously unstable construction phases. Tubes are thankfully forgiving of momentary abuse, but you might want to think twice before stuffing a coveted rare NOS tube into that circuit position.
The presence control is the problem child of tone-shaping controls. Unlike the passive tone stack typically located between preamp stages, the presence control is part of the negative feedback circuit. By variably reducing the amount of negative feedback at upper freqencies, the resulting bright glassiness can impart some lovely harmonics and sparkle. Or a ton of finger noise.
The dark side of the presence control is how it's implemented. The fixed element of the presence potentiometer carries all the phase inverter's cathode current to ground, breaking two cardinal rules: 1) never run DC through an audio control, and 2) never run DC through an audio control. Why? The answer is in that inescapable rushing sound heard whenever a classic presence control is operated. (Who knows how many perfectly good pots and cans of control spray have been wasted trying to correct this? I'm guessing Leo Fender did).
After tubes, potentiometers are perhaps the most maintenance-intensive devices in a guitar amp. They get handled constantly and are often vulnerable to accidental impact. When the presence control wears out or takes a hit, it can make the amp noisy or inoperable in a way that that's difficult to troubleshoot. Incidentally, the schematic diagram for at least one of the Bluesbreaker reissues contains errors and omissions, including how the presence circuit is drawn: with no DC path for the phase inverter's cathode current.
To remove the chassis from a Bluesbreaker cabinet, you have to lie the amp face-down (preferably on something to protect it), and remove the six wood screws that hold the chassis board to the rest of the cabinet. The awkward distribution of weight on the chassis requires caution when lifting, flipping, and moving it, which is held to its backboard with six bolts that are significantly stronger than the little wood screws that fasten the whole subassembly to the cabinet.
The original model 1962 had an aluminium chassis; steel is used for the reissues, ostensibly for strength, though this is questionable. An aluminium chassis can be as strong as a steel one – it just takes more of it. The cost difference is usually what nudges amp manufacturers towards choosing steel.
Steel is often credited with providing better shielding because of how it conducts low-frequency electromagnetic fields, but this is precisely why it can contribue to hum problems in guitar amps, and the Bluesbreaker is a good example of this. When you first turn the amp on, you hear a low hum through the speakers, even though the amp is still on standby. How is this possible? The line frequency field radiates from the power transformer and is inductively coupled to the output transformer, partly through the air, and partly through the chassis. A component layout that's hum-free on an aluminum chassis can be quite hummy on a steel one. Aluminum is also a better electrical (and thermal) conductor than steel, so the common practice of using various locations on the chassis as circuit grounding points can create more noisy ground loop problems with a steel chassis.
The problem of hum being conducted between power and output transformers is why they're often separated on an amp chassis, and/or positioned on different axes. It's also one of the reasons classic amp architecture favors a long, narrow chassis (the other being to provide more room for an impressive row of controls). This in turn places the two speakers in almost all twin combos side-by-side, despite the acoustic shortcomings of that arrangement. The Model 1962 follows this pattern, and the semi-open-back cabinet provides the usual unofficial storage area for cables and footswitches.
Having achieved legendary status, an original Marshall Model 1962 is now valued more highly than is justified by its function as an amplifier alone. And given its size, the Bluesbreaker isn't a practical choice for many applications. But despite its shortcomings, this thundering combo is likely to remain a favorite among guitarists looking to capture that 1960's British electric blues vibe.