How to Test Vacuum Tubes

by Michelle Raphael
Old vacuum tubes image by Dmitry Rukhlenko from

Old-style TV sets and other electrical products made between the 1930s and 1970s contain vacuum tubes for creating, amplifying and modifying electrical signals. The tubes function by controlling the electron movement in a low pressure environment, such as low pressure gas or in a vacuum. A vacuum tube consists of a glass -- sometimes metal or ceramic -- tubular envelope with electrodes contained in a vacuum atmosphere. To test a vacuum tube use one of the many vacuum testers available on the market.

Step 1

Identify the type of your vacuum tube by looking for printing on the side of the tube. The type of vacuum tube will determine which tube socket and which switch you'll use on the tube tester. Tube designations typically start with a number followed by a series of letters and numbers. Often the first number is the filament voltage required for the tube; for example, "6L6" or "12AX7."

Step 2

Insert the vacuum tube in the proper socket on the tube tester as indicated by the chart that came with the tester. Inspect your vacuum tube for signs of damage to the glass or for bent pins.

Step 3

Turn on the vacuum tube tester. Set the test switch positions for your specific tube according to the tube tester's chart.

Note the "Gm" reading on the vacuum tube tester's meter. Compare it with the acceptable reading for the specific tube as indicated by the tester's chart.


  • Take care when working with vacuum tubes because they tend to get hot.


  • If you don't understand the Gm reading displayed by the tester, consult an online vacuum tube reading table.
  • Vacuum tube testers come in two varieties -- a simple emission tester and a mutual conductance tester. The emission tester will only show if the vacuum tube is usable and working. A mutual conductance test, on the other hand, will measure the plate current and indicate a Gm reading, also known as trans-conductance (mA per volt).


Photo Credits

  • Old vacuum tubes image by Dmitry Rukhlenko from

About the Author

Based in New Jersey, Michelle Raphael has been writing computer and technology articles since 1997. Her work has appeared in “Mac World” magazine and “PC Connections” magazine. Raphael received the George M. Lilly Literary Award in 2000. She holds a Bachelor of Arts in political science from California State University.

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