One of the most common failures in vacuum tubes is the development of microphonic properties. This occurs when the tube begins to act as an antenna and introduce stray sounds into the circuit, which is obviously a problem for amplifiers. Microphonic vacuum tubes not only make the amplifier noisy but also may be a precursor to amplifier damaging electronic failures. Luckily, locating microphonic vacuum tubes is easy.
Instructions
1. Remove the access panel and inspect area around vacuum tubes. If tubes have individual metal covers remove them (they usually twist off). If any tube is broken or cracked, it must be replaced. If the area around any tube looks as if it has been melted or burned, there may be an electrical problem and the unit should be referred to a qualified technician. Connect the amplifier to a speaker if one is not built into the unit.
2. Turn the amplifier's main power switch on, but do not turn on the standby switch if unit is equipped with one. Give the unit three to five minutes to heat the tubes, and look for each tube to illuminate a faint orange glow from its filament wires. If a tube does not glow, it may be bad. Do not touch the metal chassis of the amplifier.
3. Activate standby switch and allow another three to five minutes for the high voltage to fully power up. Adjust settings to a normal volume. If the unit makes crackling noises with volume adjustment, the tubes are probably not the problem.
4. Avoiding the metal chassis, gently tap each tube with the eraser end of a pencil. Each tap should result in a gentle thump, similar to bumping against a microphone, through the speaker. If any tap results in feedback or crackling, the tube is microphonic.
5. Gently wiggle each tube with the pencil eraser (try not to actually move the tube) if no tube appears microphonic after tapping. Any noise suggests the tube is microphonic.
6. Power down the amplifier and write down the numbers on any microphonic tube. You will need these numbers to order a replacement. Replace access panel.
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