If you look at the top of a valve amp, you’ll usually see three large heavy black metal blocks with rounded edges sitting on the top of it

These things are a vital part of the design. They are called transformers; their job is a bit like the transmission in a car. They convert electricity from one voltage to another.

The biggest one, in the middle, is for the power supply.

The two smaller ones take the high voltage signal the valves use and turn it into a lower voltage to drive the speakers.

These transformers need to be large because they work by using a magnetic field. The solid core is made of a highly magnetic material. Vintage transformers just used iron for this, but modern materials are better as they are more strongly magnetic.

Each one has many thousands of turns of very thin wire wrapped around this heavy metal core. As you pass an electric current into one side, it comes out transformed into the shape you need it on the other side.

These transformers need to be the size they are so that they can pass through the required amount of current without adding distortion or overheating.

Because they’re so large, we put them on top of the case rather than inside.

They’re quite safe to touch, though they may get a bit warm.