There are a lot of mysterious components within any guitar amp, many of which remain puzzling even to hobbyists who have built a DIY project or two. In the 12th part of Mojotone’s series What Does This Thing Do? we’re examining the cascading gain stages and their effect on the sound of any amplifier in which they’re found.
Many installments of What Does This Thing Do? have focused on an individual component...
...while others have occasionally examined a portion of the circuit that helps to make up the whole. This time out we are firmly in the latter camp, checking out a portion of the preamp circuit in many amps—and one that might be slightly different each time you encounter it, as used variously by different manufacturers, but which generally aims to achieve the same thing wherever it’s used.
The term cascading gain refers to a method of using multiple gain stages within the preamp stage as a whole to ramp up the gain of the signal higher and higher by passing it along to one preamp-tube stage after another, ultimately creating the singing, sustainful, saturated lead tone that is popular in many forms of modern rock. The result is a lot more preamp-generated distortion than traditional preamp stages are capable of generating, and an easy route to cranked-up lead tones when used in partnership with a master-volume control, which these circuits almost always include.
As seen in any of a number of popular circuits...
...cascading gain is achieved by feeding the signal into a triode from one half of a conventional preamp tube (usually a 12AX7/ECC83), then from there to another triode, and possibly another, and even another, usually with a potentiometer (commonly labeled Volume, Gain, Drive or the like) between each stage to rein in the gain levels as desired. While most amps other than the simplest designs use multiple gain stages anyway to help preserve signal levels through potentially gain-draining extras such as multiple EQ controls, the cascading gain stages do it purely to ramp up the signal level to ultra-high levels, providing such gain or drive controls between them so the player can govern how “hot”—and therefore, how distorted—the end result is, rather than having the signal depleted by another fixed network such as a tone stack.
If you’re already familiar with the term cascading gain, there’s a good chance you heard it in relation to Mesa/Boogie amps, which feature it as a cornerstone of their designs and are generally considered a pioneer of the technique. They were not the first maker to chain several gain stages one into the other, however, and some others employed their versions of cascading in the mid to late ’60s to increase preamp gain in an age when heavier distortion was becoming a major feature of popular music. Hiwatt’s cornerstone circuits of the late ’60s, for example, included three gain stages before the phase inverter. These included a Volume control after the first gain stage, and a Master Volume after the tone pots (which followed the second gain stage) as the signal headed into the third full gain stage.
In many ways, even though their use became prominent as far back as the 1970s...
...cascading-gain circuits are kind of what separates modern amps from vintage: their arrival delineates the effort to generate overdrive within the amp itself and to rein it in at usable volume levels, rather than merely to make the guitar louder. With the Boogie’s popularity ascending in a big way in the early ’70s, Marshall adopted a form of cascading gain in the 100-watt 2203 JMP Master Model of 1975, and the design was made hotter for both the 100-watter and the 50-watt 2204 in 1977. Countless makers joined suit, and later makers like Soldano and Bogner—to name just two of many—would make their names on their own variations of cascading-gain circuits.
Some of the more legendary amps to have used cascading gain are those created by Alexander Dumble. While they obviously have a lot more going on besides, Dumble’s best-known designs, like the Overdrive Special, essentially add a footswitchable two-gain-stage cascading lead circuit between a vaguely Fender-like preamp stage and the phase inverter.
As touched upon previously...
...a cascading-gain circuit within any amp’s preamp can be identified by the chaining of the individual halves of a preamp tube (or several) one into the other, generally using coupling capacitors but without any intervening tone controls to lessen the gain (though those will also come before or after their own gain-make-up stages).
Cascading-gain stages will almost certainly have a volume potentiometer between each stage or two, however, as a means of controlling gain levels from one stage to the next. Frequently, they will also have a resistor coupled from some point in the signal (often attached to such a potentiometer) to tap part of it to ground, as a means of reining in the signal strength to a fixed level at that point to avoid overwhelming the next stage, which could result in harsh, fizzy distortion. Brian Gerhardt of TopHat amplifiers, for one, uses this trick in his Emplexador, which has a lead channel based roughly on the Marshall 2203/2204 platform. A resistor going to ground off the output of the master volume tames the signal a little on its way to the phase inverter, without really reducing the volume noticeably, resulting in a smoother overdrive with less harshness when the amp is cranked up.
If you want to try adding cascading gain to your own existing or DIY amp build of a design that doesn’t usually carry it, it’s usually best to check out the circuits of a few known commercial amp that already us it and see what might be adapted to suit your own design. It’s not the kind of thing that can be walked through in a sentence or two, but if your amp has a spare gain stage and you’d like to try creating a hot lead channel it usually isn’t difficult to fold that into your own design by taking a page out of the approach used by some of the major names discussed above.