Vacuum tubes, or thermionic valves, are arrangements of electrodes in a vacuum within an insulating, temperature-resistant envelope.

After the preceding looks of the early 50's (TV front from 1950 to 51/2; wide panel '52–54), Leo Fender changed the cabinet design again, this time opting for no extra wood on the front of the amp, except for the narrow top and bottom panels that hold the baffle board to the cabinet.

The early models of the larger "narrow-panel" tweeds are also remarkable for their refined electronics whose circuit design incorporated dual 5U4 rectifiers in the Twin and Bassman models, another improvement given Fender's quest for a louder, cleaner amplifier.

These were easily the most powerful amplifiers commercially produced back then.

They all had the classic features we're familiar with now: heavy steel chassis, chromed control plates, and heavy pine cases covered with tweed fabric.

Leo soon realised that amplifiers needed to be sturdy to withstand the life on the road, and decided to build his own, to care for the needs of travelling musicians such as his customers.

In 1946, Fender began manufacturing a series of now-legendary amps: the Deluxe, the Professional, the Dual Professional, and the Princeton.Several variations on the amp's original design have been produced through the years, including the Twin Reverb, the Super Twin, the Twin Reverb II, and the Twin Reverb '65 Reissue.The Cyber Twin, which combined a tube amp with a digital processor, was introduced in January 2001.The electrodes are attached to leads which pass through the envelope via an air tight seal.On most tubes, the leads are designed to plug into a tube socket for easy replacement.The simplest vacuum tubes resemble incandescent light bulbs in that they have a filament sealed in a glass envelope which has been evacuated of all air.