Getting Started: From Novice to Expert - How to Pick a Transformer

Transformers are the heart of any tube amplifier, they make all different types of performance and sound quality. Transformer is key, it is what every tube amp is made from — whether you are building a new amp or improving an old one, you must choose transformers right. This article will explain some of the basics in selecting the correct transformer to meet whatever needs your tube amp has including power transformers and output transformers.

A classic tube amplifier showing both power and output transformers

Different Types of Transformer Explained

In the case of tube speakers, you definitely should look to have two transformer types which happen to be the ones rode to fame:

1. Power Transformers

High-quality power transformer for tube amplifiers

Power transformers deliver the necessary voltage and current your amp needs. Whatever the case: whether you're racking up 50-watts, 100-watts or over 200-watts — you will of course require a power transformer capable of handling the amperage requirements NOT ONLY for your high-voltage (B+) but ALSO the filament (heater) circuits needed to power your amplifier.

2. Output Transformers

Output transformer for vacuum tube amplifier

Output transformers are used to couple/connect your amplifier to your speakers, converting the high-voltage signal that tubes generate into a lower-voltage signal suitable for driving speakers. However, absolutely matching the impedance setup between the tubes and speakers is vital here — this mismatch can lead to poor sound quality or ruined equipment.

EI vs. Toroidal Transformers, A Discussion of the Basics

EI Transformers

Classic looking transformers with laminated core found in vintage tube amplifiers. These are thick, inexpensive and provide the "sound" we think of when we think tube. But they generate more EMI then toroidals in your part number.

Toroidal Transformers

Design sense; used in modern audio equipment; better harmonic attenuation but considerably more less EMI!

A Word on Specification

1. Voltage Ratings: Just make sure the transformer fits the criteria for your tube amp in voltage. For instance, a typical 6L6 tube may need about 400 volts of B+.
2. Winding Capacity: The transformer needs to carry enough for all the tubes and components without getting overheated.
3. Quality of Materials: Increased performance and reduced losses with high quality laminations and windings. Some of the popular brands in audio include Hammond and Edcor, for their consistent transformer designs.
4. Frequency Response: Audio should have a much wider frequency response in output transformers so they can translate all frequencies on the audio signal with fidelity from deep bass to high-end.

Practical Example

You want to build a home use 15-watt EL84 push-pull tube amp Suppose your power supply transformer for example rated at 250V before the B+ voltage and you need about 6.3V at 4A for heaters. For the output transformer go with one that has a ratio of 8kΩ primary to 8Ω secondary rated for EL84 and your speaker setup.

Final Thoughts

Selecting a transformer for your tube amp is an art and science, for no other reason that you must have the correct parts for your vintage audio treasures. Learn how to meet the needs of your specific amplifier, voltage, current and even transformer type, will result in some incredible sounding equipment tailored just for you. It doesn't matter if you love EI transformers as much as a vintage sound or scream at the top of your lungs about how toroidal designs are the future, an educated selection will take your audio table in new realms 😉.