Guitar Amplifier Output Transformer

How an electric guitar amplifier output transformer functions in a tube amp: If you've every looked inside an electric guitar amp, or simply in the back of a semi open Fender tube amp, you'll notice there is more than one transformer. In fact some amps will have three or even four components that look like transformers. One of these components, typically the second largest, furthest from the power transformer, is an output transformer

Electric guitar vacuum tubes, or vacuum tubes in general are very high voltage, low current devices. Inside a typical tube amp voltages can be upwards of 700volts AC from the power transformer secondary leads, and voltages upward of 500volts DC from the rectifier.

Speakers on the other hand are low voltage high current devices, for good reason. It's probably a bad idea to have 500 volts at a speaker jack. Touching such a high voltage, regardless of the current, could kill somebody.

Power is defined by the voltage multiplied by the current, or P=V*A, so we can easily get the same power with low voltage and high current, or high voltage and low current, and this is where the amplifier output transformer comes into play.

The output transformer converts a high voltage low current signal into a high current low voltage signal. Satisfying the needs of the speaker, while simultaneously keeping the user alive.

Output transformers come in primarily two varieties. The transformer schematic at the top of the page indicates a design used for a push pull amp, like a Fender Deluxe Reverb, or a Vox AC30. These amplifiers utilize the center tap of the transformer, and have two or more power tubes. The transformer schematic above is a design for a single ended amp, which typically has one power tube, but can have two tubes or more running in parallel. Note this transformer has multiple output impedance's, for a wide range of speaker combination's. This is not unique to the single ended transformer, push pull designs also come with multiple secondary taps as well.

The important thing to know is that the amplifier output transformer has a large role in the tone the amp makes. A fender champ has a very small transformer which limits the amount of Bass the amp can make. A larger transformer can result in more frequency range, and volume for a given amp. Transformers, being the most expensive part of a tube amp, is where manufacturers often cut corners. So replacing the output transformer can change the tone of the amp in more ways than different tubes, or other modifications.

There are many transformer manufacturer's out there, many have reasonable prices, with great technical support. Consider changing your output transformer as a way to change or enhance your sound. Just remember tube amps have lethal voltages, so if you're not sure what you're doing bring it to a professional for the swap. Thanks for reading, J.B. 3-22-11

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