Following the circuit of Marshall’s 2204 master-volume amps of the late ’70s, this MGL20 has a preamp gain control for its first gain stage (highlighted at left), and a master control that immediately follows its tone stage (right) in the signal chain, meaning it comes beforethe phase inverter.
No Master Volume Control
In the early days of the “boutique” amp craze, and somewhat as a backlash to the high-gain, channel-switching monsters of the ’80s and early ’90s, there was a lot of buzz about non-master-volume amps—amps built more to vintage-inspired standards and designs.
If your amp has no master volume and just a single volume control (or one volume control per channel), it’s probably a low-gain preamp. That doesn’t mean you can’t push it into overdrive, but you will usually need to crank the volume up to get there—and many players may still want an overdrive or distortion pedal out front for more saturated lead tones. However, not all non-master-volume amps are low gain. The highly prized Trainwreck amps made by the late Ken Fischer (and now being made under license), as well as models by builders such as Komet and Dr. Z—some of which were also co-designed by Fischer—have relatively high-gain preamps. Either way, if your amp lacks a master volume and you can only get your favorite tones at impractical volumes, you can purchase an output-attenuator unit to insert in the signal path between your amp’s output and speaker(s). This lets you turn up the amp to achieve the desired level of dirt, then rein in the volume via the attenuator’s level control.
Low-Gain with Master
These days, many amps with low-gain preamps also have a master volume. This configuration doesn’t usually yield true high-gain tones, but it can frequently allow decent crunch or even vintage-level lead tones by turning up the initial volume and turning down the master volume.
The Output Stage
An amp’s output stage comprises everything from the input side of its phase inverter to the jack on the back of the amp that sends the signal to the speakers, including the output tubes and output transformer in between, plus a bunch of capacitors and resistors connecting it all.
The output stage takes the relatively low-level electrical signal that the preamp has already increased in voltage and increases the voltage further, ultimately converting it to a high-wattage, low-impedance signal that will drive a speaker. The output stage begins with the phase inverter, which includes yet another preamp tube that is configured with a network of resistors and capacitors to split the audio signal into two strands, while flipping one strand to the reverse of the other’s phase in order to pass along two mirror-image signals to the two sides of the output stage for final amplification.
The split, inverted signal is then passed to two output tubes (or two parallel-wired pairs in larger amps), which act in a “push-pull” configuration—one tube “pushing” one side of the split signal while the other tube “pulls” its reverse-phase partner—to further increase the signal’s strength and send it along to the output transformer. The output transformer then converts the signal to one that will power a speaker. (Note that small “single-ended” amps with just one output tube, such as the Fender Champ or Vox AC4, don’t work in this push-pull manner, and therefore don’t require a phase inverter.)
This MGL AmpWorks Lead Master 50’s output transformer is the larger of the silver transformers, at the center of the row of three transformers behind the row of tubes. To its right is the power transformer, and to its left is the choke.
Power and Distortion Capabilities
It’s also important to know that the output stage is where an amp’s overall power capability is determined: The combination of the type (and number) of output tubes and output transformer used are what make it a 15-watt amp or a 100-watt amp. A pair of 6V6s or EL84s and a relatively small output transformer deliver the former, for example, while four 6L6s or EL34s and a large transformer yield the latter. Any type of preamp stage we’ve discussed here, high gain or low, can essentially be partnered with any type of output stage.
In addition to determining output level, the output stage plays a big part in shaping the character and degree of the distortion induced when the amp is driven hard. Although in most amps the majority of distortion is generated in the preamp, this signal will drive smaller, lower-powered output stages harder than it will larger, higher-powered stages, thereby inducing more output-stage distortion in smaller amps, which can dramatically change the character of your overdrive sound in some cases.
Output-tube biasing might seem a rather esoteric and technical subject, but it’s worth knowing a little about because the method by which any given amp is biased can affect its sound and performance. Further, knowing how an amp is biased should tell you a little something about the nature of its playing feel and harmonic content.
All tubes need to be biased—that is, have some control method applied to set their operating level at idle (much the way a car’s carburetor is adjusted to set its idle)—but it is most significant with regard to output tubes. Bias is a very involved subject, but you mainly need to know that most amps’ output tubes are biased in one of two main ways—by connecting their cathodes to ground via a large resistor of a value that determines this bias, or by applying a low negative voltage to their grids, as supplied by a small network of components connected to a tap on the power transformer. The former method is called “cathode bias,” and the latter “fixed bias”—rather confusingly, perhaps, because the bias level on most fixed-biased amps made from the early ’60s onward can actually be adjusted, whereas the bias level on cathode-biased amps is preset and cannot be adjusted (not without physically changing the bias resistor, at least).
Bias methods are significant because they help determine an amp’s character and efficiency. Fixed-bias amps make somewhat more efficient use of their output tubes, in most cases, and provide a means of squeezing the maximum output wattage from any given design, while also generally sounding a little tighter and firmer, in the low-end in particular. Fender’s Twin, Deluxe Reverb, and Bassman, and Marshall’s JTM45 and plexi amps are classic examples of fixed-bias amps. Cathode-biased amps, on the other hand, tend to be less efficient, wattage-wise, while being characterized by a somewhat greater level of harmonic overtones when they begin to distort, along with what might be perceived as a softer bass response—and sometimes a slightly more tactile playing feel, too. Classic cathode-biased amps include the Fender tweed Deluxe, Vox AC15 and AC30, Matchless DC30, and Carr Mercury.
In the vast majority of tube guitar amps, the output transformer (OT) is the largest component in the signal chain. It converts the high-impedance signal from the output tubes to a high-wattage, low-impedance signal. The OT is usually the second largest transformer hanging from the chassis’ underside—the largest being the power transformer. Given that this component transforms the electrical signal from the output tubes to one that the speaker can pump through the air and into your ears, the OT plays a significant part in tone shaping, and its size, design, and build quality all factor into the way it does its job.
Roughly speaking, the bigger the OT relative to the output tubes, the bolder the sound and firmer the bass response. OT size also tends to equate to maximum wattage capabilities, although the OT can only translate what the output tubes provide. There are many other design parameters involved, of course, and these are just basic rules of thumb.
None of this means, however, that bigger is always better. An OT needs to be appropriately sized for the tubes that feed it, and appropriate to the designer’s overall goals, too. For example, many smaller or mid-sized amps owe some of their juicy, succulent overdrive character to the fact that their output tubes are saturating a relatively small OT. Install a bigger, supposedly “higher quality” OT, and they might sound colder and less characterful.
The Tip of the Iceberg
If you want to learn more about the minutiae of how amplifier gain stages work, there are options out there for a tech deep-dive. But it should be easy enough already to see what a major mix-and-match puzzle any guitar amp is, and how much even a relatively minor change of tubes or components or values or topologies within one little stage somewhere between input and output might change its tone.
In the end, you really don’t have to know how every little link in the signal chain functions to find the amp that will work best for you. But a good grounding in their basic operations—and more importantly, how different elements equate to different sound and feel—should help you narrow the search for the amp(s) that will best help you achieve your musical goals.