This diagram illustrates the cascading gain stages, cathode-follower stage, tone stage, and phase inverter of an early-’80s Marshall JCM800 2204 in lead mode.
Consider the sound of the lead channel in modern channel-switching amps or single-channel amps intended purely for rock overdrive, and what you’re hearing is a high-gain preamp. Although we refer to these as modern, this topology really roared into existence in its popular form in the late ’60s, when Randall Smith introduced his first Mesa/Boogie amps. Shortly thereafter, others also began modifying existing amps’ low-gain preamps to become high gain.
As we’ve touched on already, a single gain stage can be designed to squeeze the maximum amount of gain from that preamp tube, but most of the amps we consider high gain use several gain stages chained together—something called “cascading gain”—to achieve a hotter signal than one or two more vintage-style gain stages are capable of on their own. In amps of this type, the saturated overdrive sound we hear is typically produced by pushing early gain stages to very high levels, often chaining one into the other to continually drive the gain higher and enable desired levels of distortion, and then reining in the signal at the end of the line to create the desired final output level.
There’s a broad range of high-gain designs on the market, and different makers’ amps often do things in quite different ways. It’s also worth noting that they achieve a pretty wide range of gain levels within what we broadly call “high gain.” For example, today’s metal player might not consider a late-’70s Marshall 2204 high-gain at all, whereas it would have sounded extremely hot to the average rocker of its era.
Familiar names that typify the high-gain genre are Bogner, Soldano, Diezel, Fryette, EVH, ENGL, Fuchs, and, of course, good old Marshall and Mesa/Boogie—plus far, far more than we can list here. Most follow some evolution of circuitry that began with the original Mesa/Boogie designs, which chain together several gain stages. By increasing the gain incrementally from stage to stage (usually with controls labeled drive, gain, lead, or even just volume placed between them to govern how much signal from the previous stage is passed along to the next as you ramp up the signal all along the chain), designers can both conjure much higher levels of gain than a one- or two-stage preamp and still provide the player with the ability to dial in anything from the minimum to the maximum of that preamp’s capabilities.
Earlier renditions of cascading-gain amps, such as the Marshall 2204 and its ilk, only chained two gain stages into each other, with a master volume control further down the signal path to govern the overall volume. Modern high-gain amps, on the other hand, might have four or more gain stages.
Most modern high-gain amps also offer two or more footswitchable channels. The clean or rhythm channels are often configured like traditional one- or two-stage vintage preamps. If there’s a channel between the low-gain rhythm and high-gain lead channels, it’s usually configured to produce anything from crunch to a more old-school lead, using a couple of gain stages.
In these amps, a stomp of the footswitch merely selects which of the multiple preamp channels is routed to the output stage, which itself is not reconfigured in any way. (A few more elaborate designs do offer some switchable changes in the back end, too. Fryette’s Sig:X and Mesa/Boogie’s Mark Five amps come to mind, but this is still a rare feature.) Most so-called channel switchers simply incorporate different types of preamp stages that exist in parallel to each other, with only one being active at any time.
On the other hand, some channel switchers with high-gain possibilities introduce their lead modes by enabling an extra gain stage or two after the initial stages that provide the clean/rhythm mode. The original Mesa/Boogie designs were like this, with the guitar signal running through both the clean and lead circuits when switched to lead mode. Similarly, Dumble’s legendary Overdrive Special—and the many amps inspired by it—add a two-stage overdrive circuit to the foundation that you’re already running through for your rhythm tone when you stomp on the lead footswitch.
Another big distinguisher between multi-channel amps is whether the individual channels have their own EQ stages or share a stage. The former allows the player to tailor tone settings to suit the individual channel, but is obviously more complicated to build and requires squeezing more into the circuit. The latter requires finding a compromise in tone settings that works for both the rhythm and lead channels (and possibly a crunch channel)—although in most well-designed amps this isn’t all that difficult to achieve.
This Divided by 13 CCC 9/15 amp uses a post-phase-inverter master volume (PPIMV, highlighted here), with shielded leads running to and from the circuit junction just beyond the phase inverter.
Master Volume vs. No Master Volume
Almost all high-gain amplifiers have a master volume control, which is necessary to achieve the desired overdrive/distortion level at a manageable overall volume. In such cases, the first volume, gain, or drive control (the name can vary) sets the gain of the first preamp stage, while the master volume generally follows any and all other gain stages to govern how much signal is passed on to the output stage.
A master volume can be placed toward the back end of the preamp stage, or in an early part of the output stage, or just about anywhere in between, but will behave somewhat differently in different locations. Many modern high-gain amps with multiple gain stages have corresponding level controls within each preamp channel, in addition to a master control in the output stage to govern overall volume. If your amp has a lead channel with controls labeled gain, lead level, and master, for example, this is most likely what you are seeing: one knob to set initial preamp gain, another to rein it in following a further gain stage, and a final control to set the overall volume level of the amp (which might also determine the final volume level of any clean/rhythm channel that the amp includes).
A late-stage master volume control (also sometimes called a level control, or just volume, if the first-stage control is labeled something like gain or drive) is often configured as a “post-phase-inverter master volume,” which you’ll often see shortened to PPIMV, so-named because it comes after the phase-inverter but before the output tubes, placing it well into the output stage. Such masters are praised by many players for their “transparency”—the way they preserve the fundamental tone and gain settings of the rest of the amp and allow you to achieve your desired sound at lower volumes, rather than changing the core tone when the master is turned down.
In truth, almost any means of lowering overall amp volume will change its sound slightly. The mere act of reducing decibels makes things sound somewhat different to the human ear. But in many cases, reducing the signal level that hits the output tubes also alters tones in other ways, if only slightly. Even so, several designers have gotten pretty close to perfecting the transparent master volume, and this knob performs superbly in many amps.
That said, the master volumes used in highly acclaimed Marshall 2203 and 2204 amps of the late ’70s used potentiometers placed further up the signal chain, right after the tone stack and beforethe phase inverter—yet few players complain about these amps’ legendary overdrive tones.