4 Simple Ways to Test Vacuum Tubes

PDF download Download Article
Learn to replace your vacuum tube like a pro
PDF download Download Article

Vacuum tubes are an older technology not often used today, but they still show up in some amplifiers and other devices. Whether you’ve recently found yourself in possession of a collection of vintage tubes or you’re trying to figure out why your amp isn’t as loud as it used to be, we’re here to help you figure out if your tubes are working—with or without a tester.

Quick Steps for Testing Vacuum Tubes

  1. Discard the tube if you see cracks or a white “getter,” or coating inside the glass.
  2. Plug the tube into an amp—a floating purple glow may mean a bad tube.
  3. Turn on the amp and tap the tube with a pencil. Toss the tube if the amp makes noise.
  4. Use a multimeter to check the tube for continuity—if there’s no continuity, it’s bad.
  5. Test the tube with a tube testing machine by following the directions on the tester.
Section 1 of 4:

Visually Inspecting the Tubes

PDF download Download Article
  1. Vacuum tubes contain a “getter”—a small cup or ring that holds a metallic powder. When the tube is sealed, the getter is heated up in a process called “flashing.” During flashing, the powdered metal oxidizes and coats the inside of the glass tube. [1] This coating should be shiny and gray, black, or silver. If the coating is white, it means the tube is cracked and needs to be replaced. [2]
    • It’s a myth that dark spots in the getter indicate a used tube—this occurs during the heating process.
    • Not all tubes contain getters, so don’t worry if you can’t find yours. Just skip this step!
  2. Put the tube into your guitar amplifier, tester, or other tube-using machine. Electrician Ricardo Mitchell says that if the vacuum tube is functioning properly, “it should glow orange, red, or purple.” [3] In most cases, the glow will be a warm orange color. [4]
    • The filament may be hard to see. If you don’t see the glow, that doesn’t necessarily mean the tube is bad. Keep in mind that some tubes glow more than others.
    Advertisement
  3. The wires are inside the tube, easily visible behind the glass. If the wires give off a purplish-pinkish glow, it may indicate that air has gotten into the tube, and the tube needs to be replaced. When electrified, they may give off a purple glow. Purple concentrated around the wires is a sign that the tube is defective. [5]
    • You may notice a blue or purple glow around the glass—this is normal. It’s only when the glow is between the elements that there’s a problem.
    • There are a few rare specialty tubes that are supposed to have this glow. If you’re not sure what color your tube should glow, search online or reach out to an expert.
  4. Sometimes the plating inside the tube turns red. This can be a sign that the tube isn’t installed properly in your electrical device. If the tube stays red, it can’t control the electrical current, which will eventually damage your device. [6]
  5. 5
    Replace the tube if it’s cold to the touch. If the tube doesn’t seem to glow at all, try touching it. Be very careful, since tubes get hot when they’re active. If the tube is turned on but feels cold to the touch, it probably needs to be replaced. [7]
    6. Advertisement
Section 2 of 4:

Using a Multimeter to Test Filament Continuity

PDF download Download Article
  1. 1
    Find the filaments by using the pin layout for your tube. Search online for the exact tube that you want to test, and look for a diagram that shows the pin layout diagram. Once you find the diagram, compare it to your tube to determine which pins are the heaters or filaments—these will usually be marked with an “f.” [8]
    • For many 8-pin tubes , including 6L6, 6V6, EL34, and KT88 series, the filaments are pins 2 and 7. [9]
    • For some 9-pin miniature tubes , including 12AU7, 12AT7, 12AX7, and 12BH7, the filaments are pins 4 and 5 for 12-V supplies and pins 4, 5, and 9 for 6-V supplies. [10]
    • For 4-pin power tubes like the 572B, 300B, and 2A3, the heaters are pins 1 and 4. [11]
  2. 2
    Turn your multimeter to measure continuity. To measure continuity , plug the black cord into the COM port and the red cord into the port that says “mAVΩ” or “AVΩ”. Then, turn your multimeter to the dial setting that looks like soundwaves. [12]
    • Some multimeters have a triangle with a line on the side instead of soundwaves, or both symbols may be present together.
    • Some multimeters only have one port for the red cord—if that’s the case, just use that one.
    • Check your owner’s manual if you’re not sure what this setting is on your multimeter.
  3. 3
    Touch the clips on your multimeter to the filament pins on the tube. If the tube is working properly, the multimeter will buzz or ring. If you don’t hear any sound, there’s no continuity, which means the tube is bad. [13]
    • It’s a good idea to also test the other pins. If any non-filament pins create a continuous circuit, it could be a signal that the tube has a short and should be discarded.
    4. Advertisement
Section 3 of 4:

Tapping the Tubes to Test the Sound

PDF download Download Article
  1. Be gentle to avoid damaging the components. A little rattling is normal. If it sounds very loud, or you notice a loose piece moving around inside the tube, then your tube is broken and needs to be replaced. [14]
  2. Plug the tube into your amplifier or other machine. Bring along a pencil, chopsticks, or another wooden or plastic implement. Turn up the volume on your amplifier and gently tap each tube with a pencil, chopstick, or similar object. If the tube is damaged, you may hear a “microphonic” noise, or a squealing, ringing, or growling noise coming from the speaker. [15]
    • Be sure to remove the tube covers or shields if there are any.
    • A good tube will make a dull thudding noise when you tap it, but you shouldn’t hear unusual noises coming from the speaker.
  3. Turn off the machine and let the tubes cool down, then swap out the bad tube for a different one. Tap the tubes again and listen for the ringing. A bad tube will sound loud no matter what channel it’s on. [16]
    • Swapping the old tube with a new one is also a good test. If the old tube is broken, the new one won’t ring as much.
  4. Slip an oven mitt over your hand. Hold onto the suspect tube as you use your device, such as by playing a note on the guitar hooked up to the vacuum tube amplifier. You will notice much less rattling if the tube is broken. [17]
    5. Advertisement
Section 4 of 4:

Using a Tube Tester

PDF download Download Article
  1. You can find tube testers by searching for them online. Check online tube stores and auction sites to find a variety of testers. Select one that fits your budget. Used ones start at about $35 USD, but higher-quality models may cost a couple hundred dollars. [18]
    • Emission testers only show whether or not a tube is working. Mutual conductance testers show how well a tube is working.
    • Popular testers include the Nobsound 8-Pin Dual Bias
  2. A series of numbers and letters is printed on the side of every tube. This code is what you use to figure out where to place the tube in the tester. [19]
    • For example, the lettering on the tube reads something like “12AX7.”
  3. To find out which socket the tube goes in, use the chart that came with the tester. Look up the number and letter code in the chart to select the correct socket. If you don’t have a chart, try searching for the tester brand online to find a manual. [20]
    • Do a visual inspection of the tube first—if it looks broken, don’t test it or you could damage the tester.
  4. Using the code on the vacuum tube, look at the testing chart again. Set the tester’s switches according to the chart’s instructions. This turns on the electrical connection, causing the tube to function. [21]
  5. The testing results depend on what kind of tester you have. First, look for a red and green gauge with a needle. If the needle moves into the green zone, the tube still works. If the tester doesn’t have this gauge, it will give you a number look up in the chart. [22]
    • Look up a vacuum tube reading table for more information on how to read the Gm number and what it means.
    6. Advertisement

Expert Q&A

Search
Add New Question
  • Question
    What colors should the wires glow in a vacuum tube?
    Ricardo Mitchell
    Electrician & Construction Professional, CN Coterie
    Ricardo Mitchell is the CEO of CN Coterie, a fully licensed and insured Lead EPA (Environmental Protection Agency) Certified construction company located in Manhattan, New York. CN Coterie specializes in full home renovation, electrical, plumbing, carpentry, cabinetry, furniture restoration, OATH/ECB (Office of Administrative Trials and Hearings/Environmental Control Board) violations removal, and DOB (Department of Buildings) violations removal. Ricardo has over 10 years of electrical and construction experience and his partners have over 30 years of relevant experience.
    Ricardo Mitchell
    Electrician & Construction Professional, CN Coterie
    Expert Answer
    If you're checking a gas-filled vacuum tube, it should be orange, red, or purple.
    Thanks! We're glad this was helpful.
    Thank you for your feedback.
    If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow
    Yes No
    Not Helpful 1 Helpful 3
Ask a Question
200 characters left
Include your email address to get a message when this question is answered.
Submit
      Advertisement

      Video

      Tips

      • Tubes with similar number codes, such as “6J5” and “6J5GT,” mean the tubes are made slightly differently but are typically interchangeable
        Thanks
        Helpful 0 Not Helpful 0
      • Some tubes have 2 different codes printed on them. 1 is the American numbering system and the other is the European numbering system. Look these codes up online to figure out which is which.
        Thanks
        Helpful 0 Not Helpful 0
      Submit a Tip
      All tip submissions are carefully reviewed before being published
      Name
      Please provide your name and last initial
      Submit
      Thanks for submitting a tip for review!
      Advertisement

      Warnings

      Advertisement

      You Might Also Like

      How to
      Use a Ratchet Strap
      Homemade Dawn Powerwash: Easy Recipe for Refills
      How to
      Make a Smokeless Fire Pit
      Moving a Vehicle’s Electric Power Seat Forward or Back Manually
      How to
      Fix a Leaky Sink Trap
      How to
      Disassemble and Clean a Honeywell Quietset Tower Fan
      How to
      Make a Drone
      5 Ways to Reset Any Maytag Washing Machine
      How to
      Mix Gas for a Weed Wacker
      How to
      Clean a USB C Charging Port
      How to
      Use Instant Hem Tape
      How to
      Fix the SC Error Code on Your Samsung Washer
      How to
      Shrink Rubber
      How to
      Fix a Lazy Susan Corner Cabinet: Simple DIY Steps
      Advertisement

      References

      1. ↑ https://www.pcmag.com/encyclopedia/term/getter
      2. ↑ http://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm
      3. ↑ Ricardo Mitchell. Electrician & Construction Professional, CN Coterie. Expert Interview. 6 May 2020.
      4. ↑ https://blog.thetubestore.com/how-to-tell-if-your-tube-is-bad/
      5. ↑ https://www.vacuumtubes.net/How_Vacuum_Tubes_Work.htm
      6. ↑ http://blog.thetubestore.com/how-to-tell-if-your-tube-is-bad/
      7. ↑ https://blog.thetubestore.com/how-to-tell-if-your-tube-is-bad/
      8. ↑ https://youtu.be/r4FNP5rVnTw?t=6
      9. ↑ https://youtu.be/r4FNP5rVnTw?t=45
      1. ↑ https://youtu.be/r4FNP5rVnTw?t=80
      2. ↑ https://youtu.be/r4FNP5rVnTw?t=108
      3. ↑ https://youtu.be/r4FNP5rVnTw?t=34
      4. ↑ https://youtu.be/r4FNP5rVnTw?t=6
      5. ↑ http://blog.thetubestore.com/how-to-tell-if-your-tube-is-bad/
      6. ↑ https://youtu.be/7r-8cAa-2rg?t=105
      7. ↑ http://blog.thetubestore.com/how-to-tell-if-your-tube-is-bad/
      8. ↑ http://blog.thetubestore.com/how-to-tell-if-your-tube-is-bad/
      9. ↑ https://www.electronicproducts.com/whats-it-worth-simple-vacuum-tube-testers/
      10. ↑ https://itstillworks.com/test-vacuum-tubes-6400721.html
      11. ↑ https://itstillworks.com/test-vacuum-tubes-6400721.html
      12. ↑ https://itstillworks.com/test-vacuum-tubes-6400721.html
      13. ↑ https://itstillworks.com/test-vacuum-tubes-6400721.html

      About This Article

      In other languages
      Thanks to all authors for creating a page that has been read 97,838 times.

      Reader Success Stories

      • Josh Wolfcale

        Feb 1, 2021

        "I have recently acquired a rather sizable lot of mostly new old stock tubes. Was immediately overwhelmed by the ..." more
      Share your story
      If you buy through links on our site, we may earn a commission .

      Did this article help you?

      Advertisement