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Every component of a guitar amplifier affects tone. This includes an amp's wiring, capacitors, resistors, tubes, chassis, and most definitely; transformers. There may be some debate upon how much a power and/or output transformer affect an amps tone, but regardless - transformers are an extremely important part of an amplifier's performance.
In a tube-driven guitar amplifier, there is a power transformer and an output transformer. The power transformer converts the AC voltage coming into the amp into the appropriate voltage(s) to run the amplifier. Since an amplifier is dependant on its power supply, the component supplying that voltage is a critical part of the amp's performance. It has a great affect on the response of an amplifier to the player's touch. Generally speaking, better quality power transformers can contribute to better note separation, clearer definition, and a more dynamic response when compared to lower quality power transformers.
The output transformer is the last major component in the signal chain of an amplifier before the speaker. It's job is to convert the high-voltage/low-current signal from the amplifier’s power tubes into a low-voltage/high-current signal to power the speakers at a low impedance. It also blocks hundreds of volts (DC) from being passed on to your speaker. The output transformer has a huge impact on the overall tone of an amplifier. When compared to lower quality output transformers, the higher quality transformer can offer an amplifier more sonic overtones, clearer notes, and an overall more three-dimensional tone. Under heavier demands, the amp will be better suited to reproduce a tight bottom and better sustain.
Transformers that are physically larger in size in a guitar amplifier help reproduce a “larger” sound. The reason for this is the energy carrying the guitar tone information through the transformer is seeing less power-robbing resistances. This helps the transformer have a larger capacity for tone to flow through. Larger output transformers have the ability to produce extended headroom and a larger frequency range, a larger power transformer is able to supply more energy faster to the amps circuit. The result is better low end and improved note dynamics in the form of note attack and separation.
Some amp builders may specifically use small transformers in their amp designs. These transformers in such designs can help achieve an underpowered-type sound and aid in capturing the tone that the amp designer is striving for. However, many amp builders will use smaller and/or lesser quality transformers in their designs for the simple fact that more iron and larger transformers not only cost more, but also weigh more. Large transformers increase shipping costs. Another reason a manufacturer may use a smaller transformer could be the transformer's physical size fits with the design of the amp.
It is important to remember that the purpose of this article is not to tell a player a higher quality transformer is better. “Better” is a relative term. What is important is to know what tone you are after as a player and being informed enough to make decisions on how to achieve that tone. No transformer will reinvent an amplifier to be something that it is not. Transformers are merely components — very important components — in a chain of parts in an amplifier that will hopefully help inspire the player to create music.
An amplifier is part of the voice of the musician. Transformers are part of that voice.
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