What Does This Thing Do? -- Output Transformers

Written by
Dave Hunter
Published on
December 20, 2021 at 2:05:33 PM PST December 20, 2021 at 2:05:33 PM PSTth, December 20, 2021 at 2:05:33 PM PST

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 seventh part of Mojotone’s series What Does This Thing Do? we’re examining output transformers and their impact on any guitar amplifier’s sound.

Most tube-amp enthusiasts who at least know their way around the major components will recognize an output transformer (OT), and many might have a basic understanding of the role it plays at the end of the signal chain. Few, however, are likely to have thought through the significance of this chunk of iron, wire and paper in shaping your amp’s overall voice.

In most guitar amplifiers the OT will be the second-largest transformer hanging from the underside of the chassis.

Many amps—a traditional tweed Deluxe, for example—will only have two transformers, making the OT the smaller of the two. In others that carry a choke and/or a reverb transformer, the OT will be next down in size from the power (aka mains) transformer. In many amps (though not all), the OT will be mounted close to the output tubes and output jacks; it functions right between these two links in the chain and it makes sense to keep their connections relatively short.

In simple terms, the OT converts the output tube or tubes’ high-voltage, low-current output signal to a low-voltage, high-current signal, which also makes the tubes’ high-impedance output into a low-impedance signal that can drive a speaker. In short, this is where your guitar signal becomes wattage, and wattage of the sort that can become sound in air through a traditional speaker. The OT’s input, known as the primary, will be designed to receive a signal in the general range of 2,000 to 10,000 ohms—depending on the number and type of output tubes it is designed to work with—and will convert that to a signal of a certain strength (again, depending on the tubes and the OT’s own maximum wattage capabilities) that’s also in the very low 2-ohm to 16-ohm range, as desired to suit the amp’s speaker complement.

All of this might pretty matter of fact, a mere mathematical process—and it is, in one sense.

After all, this is the “transforming” that an output transformer does. As with almost everything else inside a tube amp, though, the way different makes and designs of OTs perform this transformation can have a major impact on any amp’s overall sound.  

In very general terms, and in most cases, the size of an OT correlates fairly directly with its wattage-generating potential and therefore the volume it helps to produce, in conjunction with a set of output tubes with which it was designed to work. The rule of thumb is “more iron = more volume and better low-end reproduction,” but clever designers can use that equation to tweak their results in either direction. If a large, weighty OT might be able to help a pair of 6L6 output tubes reach their maximum potential of about 50 watts (in push-pull, class A/B), that might be something a designer will take advantage of in a large amp intended for big-stage use, but the same might work against an effort to produce the best-sounding mid-sized amp, or one with smaller speakers that can’t take the power or the bass content.

Fender, for example, did exactly this in selecting its OTs for several classic models: the black-panel Super Reverb, Tremolux, and Vibrolux Reverb of the mid ’60s all used a pair of 6L6s in the output stage, but these were configured with a larger OT in the Super Reverb to generate 45 watts through a 4x10" speaker cab, and a relatively smaller OT in the latter two to generate around 30 to 35 watts through a smaller 2x10" speaker complement. Why produce all that power and low-end rumble if the speakers can’t handle it and the end user doesn’t need it? Leo Fender and his team clearly saw they could make the OT selection work to the design’s favor in either direction, and they used that potential wisely.

So if size does matter in OT selection, that’s not to say that bigger is always better. A big, heavy OT will roar with a lot of volume and, eventually, thick, meaty overdrive when you get a large amp cranked up to where it can deliver the goods. But if you’re hoping to achieve amp-based overdrive at relatively lower volumes—even from the same types of tubes—you might want a smaller OT that will saturate more quickly, at a volume level where the heftier OT remains sharp and clean. (Obviously, the reverse of this applies if you’re sound requires maximum headroom at loud volumes.) 

In addition to these considerations regarding OT selection, it’s worth noting that—all else being equal—larger OTs generally cost more than smaller, so this might be a factor in the parts chosen by more economical manufacturers. If the OT in a lower-budget or bargain-basement tube amp with a pair of 6L6s or EL34s in the output stage appears to be significantly smaller than that in a more expensive hand-wired make carrying the same tubes, chances are it was selected partly out of cost… and even if they both advertise putting out “50 watts,” you can probably guess which one will be louder.

Note that output transformers are designed to work with specific output tubes, so you should check the manufacturer’s specs carefully when selecting and OT for a new amp build or a replacement for an existing amp. Most guitar-amp transformer manufacturers and suppliers will understand the points discussed above, and many can be great resources when you’re looking for a part to help dial in the overall sound of your amp. In any case, arming yourself with a little more knowledge of what an output transformer does and how it’s composition will affect your sound can certainly help you achieve your sonic ends more successfully, and adds a potentially powerful new dimension to your tone quest.