Output transformer
An output transformer sits between the power tubes of a tube amplifier and the speaker. Tubes work at high voltage and low current, while a speaker needs low voltage and high current, so the transformer converts between the two by matching their impedances. It is usually one of the most expensive and hardest-to-replace parts of a tube amp.
Example: The large iron block at the back of a tube guitar amp chassis, with wires running to the speaker jack, is the output transformer.