Vacuum tubes (called “valves” in British English) were by far the dominant active electronic components in most instrument amplifier applications until the 1970s when solid-state semiconductors (transistors) started taking over.
Transistor amplifiers are less expensive to build and maintain. They also reduce the weight and heat of the amplifier and tend to be more reliable and shock-resistant. Tubes, on the other hand, are fragile and we must replace and maintain them periodically. Also, serious problems with the tubes can render an amplifier inoperable until the issue is resolved.
While vacuum tube-based circuitry is technologically outdated, tube amps remain popular since many guitarists prefer their sound. Tube enthusiasts believe that tube amps produce a “warmer” sound and a more natural “overdrive”.
How do vacuum tubes operate?
Whichever might be the case, its operation consists of controlling electrons. One way to picture it would be: In the center of a tube’s glass envelope is a cathode. It carries just a small positive charge and is ready to release a gazillion electrons. It’s especially ready if it’s been heated. Surrounding the cathode is the anode—although in the guitar universe it is typically called the plate. The plate carries a high positive charge that’s ready to pull those negative electrons toward it. To the highly charged positive plate, the cathode’s small positive charge appears negative.
When placed in a vacuum and powered up, electrons will fly relentlessly towards the plate. Then, by adding a third element, the grid, between the two, it is possible to control that flow of electrons. When the grid is close to the cathode and connected through the plate to it, relatively tiny voltages will come from your guitar pickups, and something interesting happens: The tiny signal unleashes a flood of electrons, allowing them to fly freely to the plate. That rush of electrons from the cathode to the plate mirrors the signal from the guitar, amplifying it many times.
Vacuum Tubes: The bigger the better?
Another point to make about vacuum tube amps is that bigger is not always better. You will get the most distinctive tube sound when the amp is cranked up enough so that the tubes saturate or nearly saturate. That is what creates the overdriven sound revered by tube amp fans.
For this reason, it is often better to choose a lower wattage amp over a higher wattage amp. By the time you crank up your 60-watt amp enough to saturate the tubes, you could be blowing your audience out the back door! It might have been better to choose a 20W amp that lets you get your saturated tone without ear-killing decibels.
Many professional guitarists prefer this approach both for recording and performance situations. They use close-miking to capture the overdriven sound of smaller tube amps, sending that signal to the recording console or the PA mixer.
https://robrobinette.com/images/Guitar/Overdrive/Rutt_Overdrive_Paper.pdf
https://en.wikipedia.org/wiki/Guitar_amplifier#Vacuum_tube
