Authors Note: While this article will explain the characteristics of tubes and how to check for signs of wear, it will not go in-depth on how to change or modify amplifiers. Working with electrical equipment is dangerous and should be left to professionals, and due to the nature of high-voltage amplifier components, this article will not recommend or endorse any modifications or maintenance.
All players know that the tone produced by tubes (or valves, in British English) is most often considered to be the best type of tone. Warm, full, and rich, tube-driven tone is the stuff of professionals. The simple pieces of 20th century technology are found in radios, televisions, and even early computers. And of course, these vintage components can really make a guitar scream. And while many players swear by tubes, few actually understand what they are and how they operate. In fact, many players don't know how to take care of tube amps. After reading this article, one should understand how their tubes work, and in turn, appreciate these precious inventions.
Vacuum tubes were invented in the early 1900s. John Ambrose Fleming, a British scientist, placed an electrode in an incandescent light bulb and studied how an input alternating current would be converted to a direct current against the positive electrode while the light was on. Fleming's design was adapted for use in telegraphy and communications, and is still used to convert currents in modern electronics. In 1907, New York native Lee de Forest found that by adding another electrode grid to the bulb and applying the telegraph's signal to the grid, a more sensitive device could be conceived.
A vacuum tube in the simplest form uses a heated filament, such as one found in a light bulb, to amplify or strengthen a signal. As the filament is superheated, electrons are released off the filament, and the current is channeled one way towards a wire grid. The grid is a sensitive detector of the signal, and can essentially allow the signal to be amplified to a more usable, more intense signal. Of course, the key to the signature tube sound can be attributed to many factors. The signal naturally flows through less parts than in a solid state amp, it gets a considerably noticeable aural quality when passing through tubes and an output transmitter, and some even say the way that the signal is clipped when the tubes are overdriven is something that solid state amps just cannot replicate.
So how does a tube's operation affect an amplifier? Of course they impact tone, but how is their usage different than that of solid state components? Why do tube amps require extra care and maintenance?
A tube's filament must warm up, just like a light bulb's. Due to the size and complexity of the tubes, however, it's not instantaneous like when a light switch is flicked. Larger tube amplifiers often have both an on/off switch and a standby switch. Before playing, it is better to turn on the standby switch for a few minutes to allow the tubes so warm up. A cold filament will be pushed harder, reducing the lifespan of the tube.
And just like a light bulb, a hot tube is fragile. The filament is superheated and is not very shock-absorbent, so great care is required when moving a hot amp. In fact, it's recommended to just let the tubes cool down to room temperature (taking approximately 10 minutes) before moving them. Glass of course becomes more brittle with heat applied, so caution must also be taken so that the glass does not break. A single, miniscule crack can let in air, ruining the vacuum and the tube.
And (again with the light bulb analogies!) of course, tubes can burn out. There are a few warning signs that a tube is reaching the end of its life span that players can look for, however, which can save them the pain of having a tube fail during a show. Most amps have fuses in them as a safety precaution. A dying tube draws more current to run, and repeated blown fuses can signify a tube problem. Another common problem is a high pitched, whiny noise (pre-amp tube) or a low rumbling noise (power-amp tubes) that is produced when the amplifier is turned on. This is often a side-effect of an inefficient, worn tube. In general, tubes last about one and a half to two years with average use.
Hopefully, the history, science, and care of tubes is a little more clear. A little appreciation should be shown for this sacred piece of vintage technology; which outdated, it can never be replaced, and will be coveted by players for quite some time.
Barbour, Eric. "How Vacuum Tubes Work."
Vaccum Tubes.net. 12 Jul 2009.