A 12AX7 is a miniature vacuum tube (called an electronic valve in Britain) that was widely used in audio amplifiers and preamplifiers during the 1960s and 1970s until transistors superseded vacuum tubes. However, vacuum tube equipment has made somewhat of a comeback. The 12AX7 houses two independent triodes in a single small glass envelope that are matched so that they can provide a balanced signal to the amplifier's power section. Its main features are high amplification and low noise. There are a number of equivalent and near-equivalent vacuum tubes.
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Things you need
- Replacement 12AX7 tube
Switch off your apparatus and allow it to cool down. Unlike transistors and integrated circuits, vacuum tubes get hot and can burn your fingers. They also use high voltage, so it is a good idea to give them time to wind down to avoid getting a shock. Unscrew the cover to gain access to the tubes. Find the 12AX7. If it is held in place by metal clips, carefully push them aside.
Remove the 12AX7 by gently rocking it and pulling it at the same time. It has nine pins on the bottom, arranged in a circle with a gap. Carefully insert the replacement tube, paying attention that the gap is lined up with the gap on the tube holder. Do not use excessive force and be careful not to bend any of the pins.
Screw the cover back on. Switch on the equipment and wait for it to warm up so that you can check that everything works fine.
Tips and warnings
- It makes no difference who is the manufacturer of the replacement tube, they are all equivalent.
- You can also use an ECC83, which is the name given to this tube under the European tube naming system.
- 12AT7 (ECC81) and 12AU7 (ECC82) are near equivalents. They have exactly the same pin-outs and voltage ratings as the 12AX7, but they have different electronic characteristics. You will not cause any damage by using one, but the amplifier will not function correctly.
- While the glass envelope housing the tube is very sturdy, the electronics inside it are quite delicate, so be careful not to knock it.
- The tubes get hot when they are working, so be careful not to burn your fingers.
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