Decades ago, vacuum tubes, also known as electron tubes, were the only devices available for use as amplifiers, oscillators, detectors, and other electronic circuits and systems. A typical tube (called a valve in England) ranged from the size of your thumb to the size of your fist. They are still used in some power amplifiers, microwave oscillators, and video display units.
A power-amplifier transistor (at left) is much smaller than a vacuum tube of comparable powerhandling capacity (right).
Tubes generally require high voltage. Even in modest radio receivers, 100 V to 200 V dc was required when tubes were employed. This mandated bulky power supplies, and created an electrical shock hazard. Nowadays, a transistor of microscopic dimensions can perform the functions of a tube in most situations. The power supply can be a couple of AA cells or a 9-V transistor battery. Even in high-power applications, transistors are smaller and lighter than tubes. Above figure is a size comparison drawing between a transistor and a vacuum tube for use in an AF or RF power amplifier.
Integrated circuits (ICs), hardly larger than individual transistors, can do the work of hundreds or even thousands of vacuum tubes. An excellent example of this technology is found in personal computers and the peripheral devices used with them.