Testing a Tube


  • Turn on tube tester and set the line adjust.
  • Find the tube to be tested on the roll chart and make settings for that tube. Some older style and European tubes are found in supplemental tube guides.
  • Verify your settings, especially the filament voltage and Bias settings.
  • Set the Range setting to a range that is well above the expected micromhos for that type of tube. For example, a typical 6L6 will be in the range of 5000 to 6000. Be sure you are on the 6000 or 15000 range.
  • If your tube tester has a “Life Test” switch, be sure it is set to “Normal”.
  • If your tube tester has “Plate Current”, “Self Bias”, or “Heater Current” posts, be sure the shorting links are installed.
  • Plug in tube and allow it to warm up for several minutes.
  • Set the line adjust again.
  • Check shorts and/or leakage.
  • If the tube shows a short or leakage, do not test the tube. Re-check your tube settings to be sure you have them set right. If your settings are correct, discard the tube. It is bad.
  • Check the tube. Depending on your tube tester, you will either do a good/bad check or a mutual conductance test. Some tube testers allow you to check either. On testers with a “English” setting on the range switch, this is a good/bad check and the “English” pot is the adjustment for this test.
  • If you have a tube tester with Bias and/or Line Voltage meters (539A/B/C), adjust them again when the tube is being tested.


  • Turn the Bias control to 100.
  • Set the range to 3000.
  • Press and hold the “Gas 1” button and turn the Bias control to show 500 on the 3000 range.
  • Hold “Gas 1” and press “Gas 2”. The meter should not go up more that one small division. If it does, the tube is gassy.


  • After a tube has been tested and you remove it, turn down the line adjust. Don’t leave the line adjust turned up too high. The tube tester is not made for a high a voltage on the transformer primary winding.
  • Some tube testers like the 600/A, 800/A, and 6000/A have an “English” control for the good/bad and mutual conductance tests. You set the “English” to the roll chart setting for the good/bad test, and then set it to the particular dot for the mutual conductance range. Don’t expect to get highly accurate and repeatable readings using the dots. It is very difficult to set the dot using this control.
  • When testing rectifier tubes (such as 5Y3) or diodes (such as 6H6), set the range to “Diodes/Rectifiers”. If there is no “Diodes/Rectifiers” setting, set to the “English” position. Be sure to use the button (normally P3) for testing diodes and rectifiers, and not the Gm button (normally P4).
  • A tube testing as weak may still be good and the ultimate test is to see if it operates properly in the circuit. This is especially true of vintage tube radios. They are pretty tolerant to tubes that are not the highest quality.
  • Tubes can test as weak on one tube tester and still be a good tube. The same tube on another tester might test as good.