The Cerulean Amp Overdrive is an updated version of the Marshall Bluesbreaker pedal. The Bluesbreaker is the source circuit for a number of expensive boutique pedals, including the Analogman King of Tone (which is two Bluesbreakers in the same box with a few modifications).
The Cerulean project has integrated two of the modifications from the King of Tone: the Presence control (an internal trimmer) and the ability to control both the soft clipping (stock) and hard clipping diodes. The clipping switches have been expanded further to allow two different diode combinations for both hard and soft clipping, whereas the King of Tone only has “on” and “off” settings for each.
Another notable clone of the Bluesbreaker is the JHS Morning Glory, which added a JFET gain stage at the end for increased output volume. The Cerulean project provides the option of using the output stage of the Morning Glory.
During the development of this new version of the Cerulean, it was determined that the “Bright Cut” switch of the Morning Glory was not very useful, especially when there was already a Tone control and an internal Presence control, both of which affect the high end. This option has been removed in favor of simplifying the decision-making process when deciding how to build it.
However, if you do want to make an exact clone of the Morning Glory including the Bright switch, you can use the earlier version of the Cerulean which will continue to be available for the foreseeable future.
After the success of the Guv’nor (1988), Marshall decided to branch out a bit and released three pedals in 1991—the Drivemaster (a renamed Guv’nor), the Shredmaster (a higher-gain variant) and the Bluesbreaker. The latter was named after the nickname of the classic Model 1961 and 1962 amplifiers, called the “Bluesbreakers” after being used by Eric Clapton in his legendary recording sessions with John Mayall and the Bluesbreakers for the Blues Breakers with Eric Clapton album. (What a convoluted mess!)
It shares some topology with the Tube Screamer such as the op-amp feedback clipping diodes, but it’s an original pedal design. While it was regarded favorably, it never really got popular on its own.
The real reason the Bluesbreaker became a notable pedal is because of the pedals that were based off it, most notably the Analogman King of Tone which puts two Bluesbreakers in a single box allowing them to stack with each other. Another well-known pedal based on the Bluesbreaker is the JHS Morning Glory, which adds a volume boost to the end to give more available output.
The original Bluesbreaker was discontinued in the mid-90’s and was replaced in 1998 by the Bluesbreaker II, which has an added boost mode. By most accounts, the BB2 is not a worthy successor.
Adjustable gain stage & filter → Clipping stage → Tone & volume controls
The first circuit block of the Bluesbreaker is comprised of two op-amp stages: an adjustable op-amp boost & filter, set up as a non-inverting stage with two separate frequency roll-offs at different gain levels, followed by an inverting stage with clipping diodes. Since the gain of these two stages is interactive, I’ll include the second op amp in this section when talking about gain, but discuss the clipping diodes in the next section.
One note about the Bluesbreaker before we get into the calculations. In the earliest version, the two resistors coming off the first op-amp stage were much larger: 27k for R2 (changed to 3k3) and 33k for R3 (changed to 4k7). This has a pretty drastic effect on the available gain, with the second version being capable of much more gain than the first. Some people prefer the subtlety of the first set of values to the second (and in fact the King of Tone uses these original values). However, since the second version was much more common and is considered the “standard” version of the circuit, the below discussion uses those values.
First, a bit about op-amp gain: in a non-inverting op-amp configuration, the gain is a factor of the feedback resistor divided by the ground resistor, plus 1. In an inverting op-amp configuration, the total gain is a factor of the feedback resistor divided by the input resistor.
In this circuit, the Gain pot is actually set up as a voltage divider rather than a variable resistor. When resistance is added to the feedback loop of the non-inverting op-amp, it’s also subtracted from the input of the inverting op-amp—which in both stages increases the gain.
So the gain is calculated like this:
Even though the pot is linear taper, in this setup it behaves somewhat logarithmically, so it’s very usable across the range.
The second op-amp stage is interactive with the first and we already covered much of its inner workings as far as gain is concerned. However, I’ll talk about the actual clipping diodes. The Bluesbreaker uses two 1N914s in series in both directions (four in total) in the feedback loop, but in contrast to the Tube Screamer, it’s inside an inverting op-amp configuration.
By its nature, this method produces a much harder clipping, even harder than diodes to ground—but in the Bluesbreaker, this is offset by the high clipping threshold (two 1N914s in series is 1.2V) plus the 6k8 series resistor which softens the clipping. So the end result is not harsh, though it is sometimes described as “grainy” on higher settings.
The Bluesbreaker’s tone control is pretty basic—it’s a passive control that rolls off the highs as it’s turned down. Nothing much to see here. The signal then goes through a standard volume control and then out.