Playing guitar is an exercise in discovery. The more you do it, presumably, the better you get, and the more you learn about the instrument’s possibilities coupled with your own potential to advance. People often travel repeatedly to the same places. Not out of lack of imagination, but as an extension of the discovery process. Once you’ve seen the Eiffel Tower or Grand Canyon, you can go back and discover a nice bistro on a side street you’d never noticed, or a trail that may have been hidden previously. With that in mind, here’s a question that deserves repeated exploration: How do preamps and power amps—and rack systems in general—differ in dynamic behavior from self-contained guitar amps? And why? This was a hot topic during the heyday of rack systems. Technology has come a long way since then, so it’s worth revisiting the subject.
We’ve discussed the dynamic loop created by pick attack and playing volume, and how current demand from the power amp stage affects the preamp stage [“The Big Bang” and “The Sonic Legacy of Tube Amplification”]. To recap, a self-contained amplifier, consisting of a preamp stage and a power amp stage, gets its operating current from a common power supply (Fig. 1). When you play clean and at low volume, both the preamp and power amp benefit from a healthy reserve of available current. Once the volume goes up and pick attack intensifies, the power amp’s demand on the power supply increases exponentially, leaving whatever crumbs are left to the preamp. Now, to be fair, an amplifier’s power supply is normally designed to deliver sufficient juice to the preamp even under demanding conditions. But the truth is, a lot of what differentiates amplifier personality can be directly traced to the designer’s ideas about how much preamp voltage variability is acceptable or even desirable. This gives rise to terms ranging from “spongy” and “forgiving” to “dry” and “stiff,” and—sin of all sins—“unforgiving.”
So, let’s look at the rackmount preamp and power amp. In a system made up of separate components, the defining feature is that they each have their own dedicated power supply (Fig. 2). Right off the bat, this precludes that beast of a power amp from hogging all the juice to the preamp. The first thing we discover in playing this rig is that the dynamic feel is noticeably stiffer and absent some familiar gooeyness and bloom. In early rack systems, getting a group of components to feel like playing a normal amp usually took a back seat to the benefits of extensive signal switching and processing. My personal feeling was, and still is, that a well-executed rack system demands a power amp with a good range of control over frequency response and dynamic feel. Most of the time power output is less important than the kind of tube character you like, especially in a stereo power amp. I bring up stereo because this is a feature of rack power amps that always gets overlooked.
In a stereo power amp, the power supply is usually common to both channels, and therefore has to be sufficient to provide full output for them. It rarely occurs to players that if you only use one 50-watt channel of a stereo 100-watt power amp, you still have a 100-watt energy reservoir on tap. Naturally, that’s going to feel extra stiff compared to a single power-amp stage of a 50-watt head. If you expect to arrive at reliable conclusions about different power amps in A/B comparison tests, it’s important to run both channels to really understand what you’re hearing.
Fig. 2
Some stereo power amps use a single 12AX7 for the input stage of both channels, while others have a separate tube for each channel. The reason this is important is not immediately obvious, but it certainly bears scrutiny. Crosstalk, or signal bleeding from channel A into channel B, often occurs in a power amp with non-isolated triodes, as opposed to isolated triodes. If you use a stereo FX processor to A/B test each design type with both channels operating, the amp with non-isolated triodes will sound practically mono due to signal bleed at the first preamp stage, while the amp with isolated triodes will deliver a markedly superior stereo image. If you compare these two design styles side by side, one channel at a time, you’d totally miss the significance of isolated triodes on the input stages.
Those two simple subjects—power supply capacity and crosstalk—play a major role in the difference between rack gear and self-contained amps. Here, a seemingly benign question offers an opportunity to understand nuances of amplifier design that are rarely discussed.
What happens from the moment you pick a guitar string to that instant you’re engulfed in amplified sound? It’s all about energy transfer: When you pick a string, the energy from the velocity of a pickstroke becomes energy in the form of string vibration. How hard you pick determines how much energy the string will absorb and then transmit to the pickup, which it does by cutting through the pickup’s magnetic lines of force. How much voltage is generated by the pickup is proportional to how much skin or fingernail you’re willing to sacrifice for the glory of an astounding power chord. Even though there’s a practical limit to the voltage generated by the pickup, as determined by the magnet size, wire diameter, and number of turns, there’s still a respectable amount of potential to work with, and this gives a typical guitar pickup an insanely wide range of sensitivity. You can move from pounding the strings to delicately brushing them to producing barely measurable voltage output by literally blowing on them.
Once you connect your guitar cord to the amplifier, the electrical impulses from the pickup become transformed. Normally, the amplifier’s gain will be more than sufficient to accommodate a wide range of signal levels, and this allows us to explore a full spectrum of dynamic interplay. But it takes a special kind of amp to respond musically to such a vast range of signal input levels at any given control setting.
Guitar amplifier gain stages are capable of high sensitivity to very small signals. When an amplifier’s preamp stages are set to reproduce small signals, a very large transient attack will readily transition into distortion and overdrive territory. And the same goes for the power amplifier stage. The beauty of this phenomenon is that we can set the amp to respond to subtleties in string attack with the guitar volume set low, and then instantly conjure up a whirlwind of sonic fury on demand.
With a tube amplifier, we usually perceive distortion as pleasant, musical, and inspiring—even essential—and we’re often barely aware that we’ve crossed the threshold from clean to distorted sound. In most tube amps, amplifier distortion is so seamlessly associated with the amp’s signature sound that it’s not really considered distortion until you’re in over-the-top territory. This is because our brain identifies certain flavors or “orders” of distortion harmonics as part of the musical structure, whereas it treats others as alien and filters them out to the extent it can. In extreme cases, this filtering process is inadequate and thus leaves unwelcome harmonics exposed. When this happens, we consider the sound to be harsh, intrusive, and unrelated to the musical content.
In Photo 2, the bottom trace shows the same chord, but slammed. This generates a very large difference in level between initial string attack (big hump in the middle of the wave) and string decay (smaller humps to the left and right). Here, the top trace shows how the amp with the same control settings as the clean example reacts to the dynamic attack: It flattens the waveform at maximum pickup level and remains in saturation mode, even though pickup output has decreased significantly.
There’s an elemental difference between a tube guitar amp and other amplifier topologies: In a tube amp, most of the ways it departs from accurate reproduction of the guitar signal are perceived to be additive—they contribute color and complexity to the sonic tapestry. In simple terms, most vintage amps stay cleaner in the preamp, which makes power-amp saturation responsible for the amp’s dominant overdriven voice. Modern amps can produce an avalanche of preamp overdrive. We tend to want to amplify this overdrive with clarity in the power-amp stage, and this results in an amp capable of infinite sustain with a high level of detail and articulation. Of course, a myriad of amp designs stake out territory in the infinite space between these two extremes.
And while tube amplifiers are distinguished from one another in large part by how each stage of amplification distorts or colors the sound relative to every other stage, it’s too simplistic to say that’s all there is to it. Next time, we’ll dive deeper into what differentiates tube amps from other topologies, what gives an amplifier it’s signature personality, and what gives certain guitar-and-amp combinations their timeless appeal.
