Vox is a British musical equipment manufacturer. Although the company makes guitars, it is best-known for two iconic amplifiers, the Vox AC15 and the Vox AC30. Artists including The Beatles, The Edge from U2 and Brian May from Queen have famously used these amplifiers both live and on recordings. Both are tube amps and possess a distinctively warm tone. If your amplifier is not functioning at its optimum, troubleshoot it before attempting repairs. Even if you can't perform the fix, knowing the nature, location and severity of the problem before hiring a professional technician can save you money.
Turn off the amplifier. If you have been using it, leave it for ten minutes so that the vacuum tubes cool down. The Vox AC30, for example, has four EL84 tubes, three 12AX7 tubes and a single GZ34 rectifier tube. If hot when moved, the tubes can shatter.
Remove all cables, apart from the power cable.
Place the amplifier on a stable, flat surface for inspection. Place it at a comfortable height so you don't have to stoop.
Flick on the power switch but leave the standby switch off. This sends an attenuated current to warm the tubes so they are heated before the full power is delivered and prevents jolting the tubes into life each time you turn on the amp. If the red power light fails to illuminate when you turn on the power, the problem is located in the power stage. It could be a blown fuse or a faulty power transformer.
Turn off the amp. Twist and push the fuse holder in, so that it pops out. The fuse holder is located next to where the plug cable is attached. Remove the fuse and replace it. The type of fuse required should be shown next to the fuse holder. For example, the AC30 requires a 250-volt fuse.
Turn on the amp again. If it the power light comes on, the problem was a blown fuse. If it still fails to come on, the problem may be a shorted power transformer. Refer this problem to a qualified repair tech.
Remove the speaker cable from the jack in the rear of the cabinet. Reattach it, making sure you push it in fully. These cables are not hard-wired, so a jolt or careless handling can make them come loose. If a speaker cable comes loose, the amp will appear to function normally, but it won't produce sound.
Turn on the amplifier, wait 20 seconds then flick the standby switch.
Look through the grille on the top of the amplifier. If any of the vacuum tubes are not glowing, this is likely the cause of the lack of sound. A blown tube will cause the preamp, or power amp section, depending on which tube is blown, to short out.
Turn off the amp. Wait for the tubes to cool. Unscrew the back panel and remove the non-glowing tube. Replace it with an identical or compatible type. Lift the tube vertically out of the socket, discard it and push the replacement in until it clicks into place.
Wear latex gloves when handling the tubes. This prevents oil from your skin contaminating the glass, causing hot spots. The EL84 is compatible with the 6BQ5 type tube. The 12AX7 is compatible with the ECC83. The GZ34 rectifier has no compatible alternative. If you hear a flapping sound as a note decays, the speaker may be torn. If there is a crackling sound during use, swap the cables for brand new ones. The sound of a dying cable can be mistaken for a failing amplifier. Clean the dials regularly with a plastic-safe contact cleaner.
Typical guitar amplifiers, especially high-powered tube amplifiers like the Vox AC30 and AC15, carry sufficient voltage to be potentially lethal. If you are unsure of anything, refer the problem to a qualified technician.