Like many of its “blackface” kin, the Fender Super Reverb has a second gain stage that’s essentially a gain make-up stage. The highlighted areas in this photo of a Super circuit show (Box 1) the first channel’s first gain stage, gain make-up stage, and related circuitry, and (Box 2) the inputs and volume, treble, and bass controls related to those two gain stages.
A Word About Tone StagesTone stages (aka EQ stages) are also part of the preamp, but are generally considered apart from gain stages per se, although some types of tone stages do contain, employ, or rely upon gain stages provided by preamp tubes.
In smaller or more basic amps, like the archetypal Fender “tweed” Deluxe, a single tone control might not be a stage unto itself at all, but rather a simple treble-bleed network formed by a potentiometer and a capacitor or two that determine how much high-frequency content is tapped out of the signal before it exits the preamp. More complex EQ stages have separate bass and treble controls, and many add a midrange control, too, frequently using a preamp tube either to drive those controls (to avoid signal loss) or as a gain make-up stage following the tone controls to get the signal back up to where it needs to be prior to hitting the output stage.
Although most tone stages’ controls feature similar names, they can execute their functions via very different electrical means—and they can have vastly different levels of interactivity and frequency controllability.
Other than the one-knob tone control found on tweed Deluxe-style amps, there are two common tone-stage topologies. In one camp are those found on the Fender tweed Bassman, Marshall JTM45 and plexi amps, the Vox AC30’s “top boost” channel, and similar models. In the other are those found in “blackface” and “silverface” Fenders like the Deluxe Reverb and Twin Reverb. The former group uses an entire preamp tube positioned before the tone controls as a driver and cathode-follower (that is, the signal comes out of the tube’s cathode, rather than its anode). The latter places the tone controls between the channel’s first gain stage and a second traditional gain stage (called a gain make-up stage) that replaces the gain lost by the signal while travelling through the controls and related circuitry. In addition to having controls that interact with each other slightly differently, each tone-stage topology also imparts a slightly different playing feel to the amp, best defined as a crisp and snappy response in the blackface/silverface tone stack, and a tactile, somewhat creamier, touchy-feely response in the tweed/Marshall cathode-follower topology.