As almost every performing/recording guitarist will tell you, a tube amp will deliver a much richer tone than it’s solid state (transistorized) cousins. Even though modern digital technology has pretty much rendered the vacuum tube to an anachronistic curiosity, it is, ironically enough, the very nature of the tube’s primitive construction that gives a tube-driven amp its characteristic warmth and three-dimensional sound, rich with the overtones and undertones that a transistor simply cannot reproduce.
The drawback to tube amps is that their tubes must be replaced periodically. A vacuum tube’s construction is very similar to an incandescent light bulb, consisting of a glass envelope from which all of the air has been extracted. Inside this vacuum are heat-emitting elements which will eventually burn out. Likewise, vacuum tubes are fragile, too, and are highly vulnerable to the physical impacts of the road and the vibrations of high-volume playing.
There are two main types of tubes in most guitar amps. Preamp tubes (the smaller ones) shape and amplify the incoming signal from the instrument. They also drive the power tubes (the bigger ones), which then take the conditioned input signal and amplify it sufficiently to drive the speakers. Most musicians say that all the tubes should be replaced annually; more often if you play very long and hard sessions. Furthermore, since tubes are usually sold in matched sets, they should all be replaced at the same time.
The basic rule of thumb is that if you notice any change in your amps quality or performance, check the tubes first. There are several tell-tale signs your tubes will display as they near the end of their useful lives. Dying preamp tubes will often squeal, hum, or buzz, and display a marked decrease in sensitivity. When power amp tubes are on the way out, your signal will start to fade in and out, display a significant decrease in bass or treble response, or just lose its punch and clarity. A good handheld vacuum is incredibly useful to clear the fade stains from the car. The quality of the tubes and bags should be checked through the person before purchasing them.
The first step is to check the tubes visually. Turn off the amplifier and disconnect it from the power supply. Carefully remove each tube from its receptacle and, under a very bright light, check the bulb for hairline cracks, light-colored spots inside the glass, or loose and/or broken components inside. If they pass the ‘eyeball test’, replace each tube into the receptacle from which it came.
Next, power up the amp (but don’t close the cabinet) and darken the room. After the unit warms up to operating temperature, visually inspect the tubes for the proper reddish-orange glow. Bad power amp tubes will glow excessively bright and hot, while bad preamp tubes will not glow at all.
If all the tubes are structurally sound and have a normal, warm glow, the next step is to check for microphonics, the characteristic tubes display when they amplify external vibrations which act upon the bulb itself. Take a pencil (or similarly soft, slender wooden rod) and gently tap each tube in the preamp section (small tubes). If the tapping noise comes out of the speakers, then the tube is normal and “healthy”. A bad tube will emit a bell-like ring when tapped, and should therefore be replaced. Check the power tubes in the same manner.
When replacing the tubes, be sure that they are properly biased to your amplifier’s circuitry. This can be easily done by replacing ALL the tubes with the exact same type that originally came with the amp. Improperly biased tubes can run either too hot or too cold, and result in poor sound quality and response and premature failure of the tubes or the entire amp itself. Never replace bad tubes with different tube types than those specified by your amp’s manufacturer.