The simplicity of this Komet K29’s point-to-point-wired preamp makes it easy to see the methods used to voice its two gain stages, which employ both halves of a single 12AX7 preamp tube (not shown, but mounted on the underside of the chassis, beneath the two highlighted orange capacitors). Two sets of cathode-bias resistors and bypass capacitors (Box 1) help establish the core voice of each stage, while two .022µF coupling capacitors (Box 2)—one between each gain stage and the level control that follows it—further refine the frequency response.
As mentioned previously, each gain stage requires a small network of capacitors and resistors to deliver the voltage that enables the gain stage to function, and to set its operating bias. The designer’s choices of the types and values of components used plays a big part in shaping the frequency range of the signal at that point in the circuit. Signal capacitors are also used between stages (so-called “coupling caps”) to block unwanted voltages from straying down the line when the audio signal itself is passed along, and the type and size of these capacitors further shapes the core tone of the amplifier. Clever designers very consciously and deliberately calculate the effect that components of different values and types will have on the audio signal at each stage.
In brief, and without getting overly technical, a designer might make a particular gain stage sound cleaner or hotter by adjusting its bias through careful resistor selection, and then make it fatter or brighter by using the appropriate bypass capacitor linked in parallel to that resistor. (Interestingly, the signal doesn’t actually pass through these components: They merely help govern the tube’s handling of the signal.) The designer may further sculpt the amp’s voicing by placing a coupling capacitor of a relatively high value between that gain stage and the next to allow a full, bassy response, or one of a lower value to enhance a higher range of frequencies. Given the types and values of components available, there are countless possible permutations and nearly endless ways to dial in an amp’s tone.
Beyond these means of voicing a gain stage and setting its propensity to distort, the ways in which stages are chained together plays a big part in determining how the amp as a whole behaves. We might split amps into two main categories—low and high gain—but in reality there can be a lot of overlap between the two.
Low-Gain or Vintage-Style Preamps
By today’s standards, the preamps in most amps of the ’50s and early to mid ’60s are considered low-gain. These also tend to be simple and have fewer gain stages. They generally apply just enough preamplification to get your signal to a level that the output stage can handle, since the original objective of most guitar amps was to make the guitar louder while keeping it as clean as possible, rather than to intentionally generate distortion. It’s no surprise, then, that these amps lean toward a cleaner, less distorted sound when the volume control is kept within reason—maybe up to 4 or 5 on the dial, give or take. Many low-gain or vintage-style amps are also known for their juicy overdrive, of course, which is achieved by turning that volume control up higher, at which point it does start to distort and push the output stage harder. In many cases this distortion would have been considered undesirable by the designers—an anomaly caused by pushing the amp beyond its intended operational parameters—but players quickly discovered that overdriven amps produce delectably dynamic tones.
The Fender “tweed” Deluxe schematic shows the first and second gain stages, tone control, phase inverter, and output stage of one channel. Note the way the 12AX7 is used by the second gain stage and phase inverter.
Good examples of this sort of amp are the Fender “tweed” Deluxe and the normal channel of a Vox AC30—as well as the many amps they inspired. Each has just a single gain stage in the preamp, although the tweed Deluxe also has another gain stage—often called a “driver” stage—right in front of the phase inverter, which is the gateway to the output stage.
Others, such as Fender’s “blackface” Deluxe Reverb, Twin Reverb, and similar models, have multiple tone controls (treble, bass, and sometimes middle) in a more complex EQ circuit sandwiched between two tube gain stages. This doesn’t always make the amps a lot “hotter,” though, because that second stage is necessary to recover some signal level that’s lost in the more complex tone network (which, of course, also allows more fine-tuning of the amp’s frequency response). The tweed Deluxe and its ilk, on the other hand, have only a simple tone control that acts as a “treble bleed,” much as the tone control on a guitar, to pass some of the high frequencies to ground, rather than having the entire signal pass through it and lose gain in the process.
Others still, such as larger tweed amps, the Vox “top boost” channel, and the Marshall plexi-style preamp, have a first gain stage followed by another tube that drives the tone network that follows it—something called a “cathode follower.” In the most basic sense, the results are somewhat similar in all of these, in that the ratio of clean to overdrive in these low-gain preamp types correlates fairly directly to their volume levels. Yet each of these amps shapes your guitar signal somewhat differently, leading to variations in tone, distortion, and playing feel. All are technically low-gain circuits, but some players will express distinct preferences for one over the other according to how they perform.