How to test amp tubes

Written by robert russell
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How to test amp tubes
(Hemera Technologies/ Images)

An amplifier tube displays a number of symptoms when it becomes weak or bad. The quickest way to diagnose a tube is with your eyes and ears and this important skill allows you to locate and replace a bad tube at a gig. Strange crackles, hums and other noises indicate a problem with one or more tubes in your amplifier. Tubes glow when they are turned on and the colour of the glow also provides information about the quality of the tube.

Skill level:
Moderately Easy

Things you need

  • Contact spray
  • Tube tester

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  1. 1

    Turn the amplifier on and locate the bad tube by wriggling the tubes while they are still in the tube sockets. If the crackling noise stops when a tube is wriggled you have isolated the bad tube. If the crackling sound is intermittent, but recreated when you wriggle a tube, this also indicates a bad tube.

  2. 2

    Turn the amplifier off and allow the tubes to cool down. Pull out the bad tube and spray the tube socket with contact cleaner, because dirt and grime may be the source of the trouble. Reinsert the tube and turn the amplifier on, because pulling the tube out and reinserting it may help to make a better contact .

  3. 3

    Turn the amplifier on and leave it in the standby position for a few minutes. The tubes should glow evenly when the amplifier is in the standby position.

  4. 4

    Examine the colour of the tubes when the amplifier is fully turned on. Healthy tubes emit a soft yellow-orange colour. A blue-purple colour is an indication of a weak tube. An intense red colour is symptom of an overpowered tube, which indicates a problem with the amplifier.

  5. 5

    Tap on the tubes with a pencil with the amplifier is on because a tube that emits a pinging or crackle sound while being tapped is bad.

  6. 6

    Remove the tubes from the amplifier and visually inspect them for black burn marks, cracks in the tube itself and red spots on the plate of the valve.

  7. 7

    Purchase a tube tester, which will measure the flow of electrons inside the tube. Identify the tubes by the digits on the tube and insert the tube into the proper socket on the tube tester. Compare the GM reading from the tube to the chart that comes with the tube tester.

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