Mid 70’s Super Bass Circuit Info and Schematics

So what makes this Super Bass schematic specific to 1974? Let’s dive in.

I’m sure you have seen the 1992 circuit schematic floating around from 1970. That one is accurate but if you’ve had the experience of working on many of these made throughout the ’70s you’ll notice many variations over the years. I chose ’74 as this was one of the models where I first saw a large 680n capacitor on the presence control accompanied with a resistor which is not shown on the 1970 schematic. Marshall used a lot of different presence control configurations and different NFB arrangements and documentation on this is scarce at best.

In the mid ’70s Marshall amps are described as “more aggressive” sounding than the earlier years and much of this is because of the presence and NFB circuit. The large 680n presence cap reduces the amount of treble and mid-range frequencies present in the negative feedback loop. In turn this allows those frequencies to amplify and distort in the power amp. More distortion in the mid-range equals more of that classic Marshall crunch sound. The smaller 100n cap is reducing only higher treble frequencies in the negative feedback loop. Since it may be counterintuitive to rationalize “less treble equals more treble” let’s disregard how the circuit actually works for a moment and pretend the presence control is a high pass filter. The higher you turn the presence knob, the more treble frequencies are amplified. I hope that makes sense…

The smaller presence cap sounds very bright and many players do prefer this because it can add high frequency information to the signal without adding a bunch of distortion. For someone like myself who loves aggressive hard rock tones like Motorhead, the larger presence cap is absolutely the way to go.

The other piece of the puzzle is the NFB resistor and output tap. A 100K NFB resistor tied to the 4ohm tap yields a lot of power amp distortion and can sound glorious but too much distortion can lessen the punchiness of each note. The NFB tap can be moved to the 8ohm tap to tighten things up. Some older Super Bass amps had a 47K NFB resistor which allows more NFB into the power amp resulting in a cleaner sound. These configurations can be changed to match the style of the player and still be considered “stock” in my opinion since you could find multiple circuit variations within the same year.

If you really wanted to take it one step further you could add a large bright cap across the bright channel volume control. Many Super Bass amps have come from the factory with a bright cap and if you like playing these amps fully cranked for heavier sounds then a bright cap can work wonders. A large value like a 4700pF or 5000pF will allow a large amount of midrange to pass over the volume control so now the control acts more like a bass-cut when rolled down. Cleaner players will likely prefer a very small bright cap or no bright cap at all. That’s fine too but we don’t listen to children’s music around here.

The key points surrounding a Super Bass are generally:

• no bright cap
• 56K tone slope resistor
• large 100n coupling caps on the PI
• 22n coupling cap on bright channel
• shared cathode on V1
• 250pF treble cap in the tone stack

Despite these things being part of a Super Bass they are not exclusive. There are hundreds, if not thousands, of Super Leads that came from the factory with 100n PI and 22n bright channel couplers. Early Super Leads and some reissues have the shared cathode as well. I’ve seen everything from 33K, 47K, and 56K tone slope resistors in Super Leads and master volume circuits alike. The only thing consistent is inconsistency. These factors along with transformers and potentiometer tapers are major contributing factors as to why there is a tonal difference between Marshall amps of the same model. Cheers.