There are a lot of mysterious components within any guitar amp, many of which remain puzzling even to hobbyists who have built a DIY project or two. In the eighth part of Mojotone’s new series What Does This Thing Do? we’re taking a look at the phase inverter, the gateway to the output stage.

When an amplifier’s output stage is discussed...

...many novice builders will display a decent understanding of the big output tubes and output transformer, but some might take a while to grasp the component that kicks it all off: the phase inverter (or PI for short). The very nature of push-pull tube amps with two or more output tubes feeding an output transformer requires that the signal entering that stage be split into two, each reverse-phase of the other. That’s how the “push and pull” of the thing happens at all, and the phase inverter—as the name implies—does precisely that job. 

Any amp other than a single-ended design carrying only one output tube (or two running in series in the occasional dual-single-ended type) will use at least one triode—half of a traditional preamp tube—in its phase inverter, and most use two triodes, a full tube, in one way or another. 

In more technical terms, though still kept relatively simple...

...the phase inverter’s job is to take the AC signal from the preamp and split it into two different AC signals of equal strength (current) but in opposite phase to each other. And while this is more of a functional operation than a tone-shaping one, the way in which different phase-inverter designs perform their duties has a significant impact on the overall sound of any amp, and on its distortion content and character in particular.

Phase-inverter circuits evolved through the years as amp designers and manufacturers sought greater efficiency and that long-desired marriage of optimum headroom and increased output power. Some of the earlier topologies have continued to be in use nonetheless, either in amps intending to emulate a more vintage-correct sound, or in smaller or simpler designs that simply work well enough with those more archaic circuits in place.

Many guitar amps of the 1940s and ’50s—what we might call the earlier phase of the “tweed era”—used what’s called a paraphase (aka self-balancing) phase inverter. This PI, which comes in a range of varied but related forms, splits and inverts the signal just fine, but has difficulty delivering a very clean, linear load to the output tubes. Also, it tends to distort within itself at levels far short of what a larger set of output tubes, 6L6s for example, would otherwise be capable of achieving. As such, the use of a paraphase PI kind of self-limits the potential of any amp in which it appears. Sonically, it contributes to a vintage-leaning blues and rock ‘n’ roll tone, characterized by a smooth, early distortion with lots of compression, and some attenuation of both highs and lows, making it sound rather thick and midrangey. It can be a cool sound, for sure, but perhaps not for players demanding a broader vocabulary from their amps.

By the mid 1950s many guitar amps used a more efficient PI called the split-load (aka cathodyne or concertina) inverter.

This one will be far more familiar to players today than the earlier paraphase inverter, since the split-load network appeared in the legendary Fender 5E3 tweed Deluxe of the mid ’50s and onward, as well as its next-step-up siblings like the Pro, Super, Bandmaster, and low-powered Twin. 

Although the phase inverter in the split-load design uses one triode to do this splitting and inverting—sending one leg of the signal from the plate and the other leg from the cathode—it is often coupled with a driver stage in front of it, and as such usually requires a full preamp tube. The split-load PI is capable of producing a sharper signal with a little better fidelity than the earlier paraphase PI, but when pushed hard it still folds into some distortion of its own, and therefore is still unable to push the output tubes to their own maximum potential before, and during, distortion. The distortion heard from the split-load PI is generally considered a little sweeter and richer than the paraphase’s distortion, and is a big part of the mid-sized tweed Fender sound. Even so, it doesn’t enable the full output tube crunch and roar we might be seeking.

For that, makers adopted the long-tailed-pair PI in the latter part of the mid ’50s, and this design has remained the king of inverters ever since.

This phase inverter requires the use of a full traditional preamp tube, two complete triodes. This provides one triode to deliver each half of the split signal, with a more complex network ahead of the tube to help achieve that splitting in the first place. This PI gets its name from the “tail” formed by the two-into-one resistors coming from the linked cathodes, as is visually evident on most schematic diagrams. In any case, the long-tailed-pair PI finally satisfied that quest for fidelity and power that obsessed so many amp designers in the late ’50s—when guitar-driven music was finding its way into bigger and bigger venues—and since then it has generally been considered the most efficient, linear, and balanced-sounding circuit for delivering a healthy signal to the output tubes.

Fender took up the long-tailed pair in 1957 with the arrival of the 5F6 Bassman circuit...

...after using the split-load PI on the previous 5E6 Bassman of 1956, and before. It was also used when the Twin upped its game to the so-called “high-powered” version, delivering about 85 watts from a quartet of output tubes, and somewhat counter-intuitively was adopted for later iterations of the dual-6V6 tweed Tremolux combo. Similarly seeking the most their output stages could deliver, Dick Denney and the folks at Jennings Musical Instruments used the long-tailed pair in all the most legendary early Vox designs of the late ’50s and ’60s. Of course, it also made its way to the Marshall JTM45 a few years later, and the renditions of the plexi that followed, via their emulation of the 5F6-A Bassman circuit. Following these classics, from the early ’60s onward the long-tailed-pair PI has been used in the majority of guitar-amp designs (other than, as before, those intentionally emulating specific vintage models that used other PI types), and is one of the most common elements connecting otherwise vastly different types of amps—found equally in the Mesa/Boogie Mark Series and Matchless Lightning, for example.

For the most part, hobby builders don’t need to understand the minutiae of how every part of a phase-inverter functions. By and large, these days you can simply follow the schematic of the design you seek to emulate, copy what’s there, and run with it. But knowing a little something about the characteristics, degrees of distortion, and general efficiency levels of these main PI topologies can really help you select an amp type that will suit your needs in the first place, and avoid frustrations that just can’t be dialed out once a particular phase inverter is locked into place.