Output transformers are one of the most important factors in the sound of a tube guitar amp, but they don't get discussed as often as other components for a few reasons: output transformers, or OPT's, are fairly expensive, they're inconvenient to swap out, and there aren't many transformer options available for most amps.
How They Work
The output transformer couples the signal from the output tube or tubes to the speaker. The tubes feed one coil, which is magnetically coupled to another coil through literally a bunch of iron, and the speaker is connected to that second coil. Most of the signal makes it through the transformer without becoming too distorted, and the rest becomes heat.
Why is a transformer needed at all? It's the same reason a car's engine isn't connected directly to its wheels: the engine turns optimally within one speed range, and the tires at another. Think of the output transformer as an amp's transmission – a one-speed, fluid-coupled transmission. The tubes are like an engine that's most content going a few thousand RPM, and the speaker is like the wheels turning at only a few RPM. The important thing to remember is that the load on the transformer's output – i.e. the speaker – is “felt” by its input, or primary.
To illustrate this, think of a fixie, or fixed-gear bike – one where the rear hub can't coast or freewheel. If the bike is moving, so are its pedals. When you climb up a hill, it's harder to pedal. If you're going down a steep hill, the pedals might be turning as fast as you can keep up with. The load on the bike's wheels (gravity) is reflected back to the driving force (your legs). A tube amp's output transformer works the very same way: it reflects the speaker load across its secondary back to the tubes via the transformer's ratio. At the risk of belaboring the bicycle analogy, just as a multigear bike lets you choose how hard you have to push the pedals to go up a hill, output tubes can function into a range of loads, affecting power output as well as distortion magnitude and character.
You'll often see a specification say that an output transformer has something like an 8000 ohm primary and an 8 ohm secondary. It would be more accurate to say that that transformer will present an 8000-ohm load to the output tube(s) if there's an 8-ohm load connected to its secondary. In other words - and this is a more useful spec – that transformer has a 1000:1 impedance ratio.
So what happens if the speaker load is 4 ohms instead of 8? That OPT with its impedance ratio of 1000:1 will now present those output tubes with a reflected load of 4,000 ohms. They may or may not like it. In any case, they'll operate differently, deliver a different power level, and a different sound. Or they could melt and cost someone a lot of money.
In the same way, removing two output tubes from an amp that has four of them doubles the impedance reflected to the remaining tubes. Of course, this can be compensated for by halving the speaker load, e.g. from 16 to 8 ohms, or from 8 to 4.
It's important to realize an 8-ohm speaker doesn't present a pure 8-ohm load to the output transformer the way an 8-ohm resistor would. A speaker's impedance rating is its average resistance to an AC signal. It'll be 8 ohms at one or two frequencies, and more or less than that at all others. So that changing load, which is being affected by which notes are being played and how that speaker reacts to them, is always being reflected back to the amp's output tubes.
This touches on why it's generally a bad idea to operate a tube amp with the speakers disconnected: that's an infinitely light load being reflected back to the tubes. They don't like that. For that reason, the output jacks on many amps actually short the OPT's secondary when the speakers are unplugged, offering a bit of short-term protection from tube damage. With tubes, a heavy load can be better than none at all.
I mentioned that the output transformer is a little like a fluid-coupled transmission. That's because output load variations like high or low frequencies or fast transients are not all reflected purely back to the output tubes. The output transformer can be seen as a non-phase-linear bandpass filter, and its inherent non-linearity can compromise the effectiveness of negative feedback loops that include it.
The output transformer in a tube guitar amp is an iron-core transformer. It's essentially two coils wound around the same block of iron. That iron is sliced up into thin layers, or laminations, so that currents don't form in the iron that would interfere with its ability to transfer signals efficiently from one coil to the other.
The coils are wrapped one on top of the other onto the same form or bobbin, which is then slipped over the iron core's center shaft. In practice, part of the primary is wound onto the form, then some or all of the secondary, then more primary, and so on. This is called interleaving. In high-end audio transformers for tube hi-fi amps there can be a lot of interleaving, which helps the magnetic coupling of the two coils, reduces losses, and improves frequency response.
From a purely technical standpoint, the output transformers in a lot of great sounding vintage amps were actually a weak link. Transformer iron is expensive, so some amps' transformers were undersized and would saturate at high signal levels. They also had a minimum of interleaving, which also helped keep the costs down. Frequency response is not critical for electric guitar: the low E is about 80 hertz, and typical guitar speakers were only functional up to about 5kHz tops, so why bother with a high-quality output transformer?
Some people believe that old transformers have some sort of special “mojo”, owing to the type of construction and materials used, like the paper or the formula of their varnish. It would be interesting to conduct a double-blind listening test with all other parameters held constant to see whether these things actually make a detectable difference. Although they might be present in an amp that sounds good, the reasons for that sound probably lie elsewhere. Insulating materials in a transformer don't behave like the dielectrics in capacitors, which are electrostatic devices. What matters most in a tube guitar amp output transformer is that its materials can stand up to several thousand volts and possible high temperatures without melting, burning, or becoming perforated.
So if the guitar amp output transformer is a lo-fi piece of iron, how can it have so much impact on the amp's sound? We've discussed how output tube efficiency and distortion characteritics will depend on the speaker load that's reflected upstream via the transformer. Low-efficiency coupling means less volume and more heat, so amplifier designers typically want those tubes to be seeing a load that allows them to deliver the most clean power. But we guitarists like a little dirt, preferably from several places at once. The way to find the sweet spot where the output tubes are distorting in a way we like is by being willing to sacrifice a little bit of efficiency. This is a worthwhile trade-off, as many tube amps don't sound their best until they're impractically loud anyway. In general, a lighter than optimum load favors 3rd harmonic distortion, whereas a heavier load favors 2nd harmonic distortion.
To a limited extent, it's possible to experiment with tweaking the load on the output tubes by changing the speaker load and/or the amp's output impedance selector. But don't do this indiscriminately, as it can permanently damage your amp's costliest components. Plan your experiment and calculate the new load on the output tubes, checking it against the dissipation limits on the data sheet.
A Helpful Tip
There's a way to roughly measure the impedance ratio of an unknown output transformer. Two important facts make this possible. First, the impedance ratio is the square of the voltage ratio. This means a transformer with a measured voltage ratio of 30 to 1 has an impedance ratio of 30 x 30, or 900:1. Second, a transformer's primary and secondary are interchangeable. This means you can apply an AC voltage to the secondary, and read a higher voltage on the primary, which makes for more accurate readings. Connect a signal generator capable of driving a low impedance (like a tuner app on your phone fed through a small signal transformer), and connect it to the mystery transformer's secondary. Measure that AC voltage, then measure the voltage across the entire unloaded primary. Divide the two voltages and square the result to get an approximation of the impedance ratio. As we did before, multiply the chosen speaker impedance by that impedance ratio to get the primary impedance, or load presented to the output tube or tubes.
Wrapping it Up
Like pickups and speakers, a tube amp's output transformer is an electromagnetic device that plays a crucial (and often overlooked) role in forming one's overall tone. Keeping in mind how the transformer reflects (“transforms”) speaker impedance back to the output tube or tubes can be the basis for some informative experimentation. It might even save those precious output tubes from a spectacular demise.