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How to Test A Vacuum Tube: Effective and Easy Way

How to Test A Vacuum Tube

Did you know that repairing your electronic device could be as easy as replacing the damaged vacuum tube?

But to know that the vacuum tube actually needs to be replaced, you need to test it first. You could either do quick tests like listening to the sound that the tube makes or inspecting it for signs of damage. Or, you can use a test tube tester and a chart, which is a more accurate method.

Now, in this article, we will go deeper into how to test a vacuum tube using easy but reliable methods.

One thing to note is that although vacuum tubes are old technology that is largely outdated, they’re still being used in a number of common devices including microwave ovens and amplifiers.

Methods of Testing a Vacuum Tube

Method 1: Observation Techniques

Visual Inspection

The coating

The tube has a coating at the top, and the normal color of this coating is either black, gray or silver. If you see that the coating is white, that’s an indication that the tube could have a crack, and in that case, it’s about time you replaced the tube.

The glow

To observe the tube’s glow, you need to plug it into a suitable electronic, perhaps your amplifier. Switch on the electronic and the tube.

If the vacuum tube is working properly, the filament should have an orange glow, much like the sun when it’s setting.

Note that some tubes glow a little less, and thus if you don’t see the glow, it doesn’t necessarily mean the tube is defective.

So, if you don’t see the glow, touch it, and while doing so, be careful because some tubes become hot when activated. If the tube is totally cold, that shows it’s no longer working.

Also, note that if the tube isn’t turning on, there might be an issue with the electronic’s fuse. Have an electrician check the fuse and if it’s the problem, have that replaced before testing the tube again.

The internal wires that are visible at the rear side of the glass commonly have a concentrated purple glow around them when the tube has a problem. The normal glow around these wires ought to be blue.

The plating in the tube has turned red?

This can be caused by installing the tube incorrectly on the electronic device, so try removing it and installing it again.

If the plating stays red, that’s an indication that there’s a problem with the electrical current control, and it’s time to replace the tube lest it damages the device.

Sound Inspection

The rattling

Shake the tube and listen to the rattling. A little rattling is okay, but if the sound is loud, that denotes the internal components are broken. You might even see a broken piece moving around in the tube, and in that case, the tube needs replacing.

The ringing

Once you’ve connected the tube to the electronic device, use a pencil or some other wooden stick to tap the tube. A good tube will ring a little, but a bad one will let off a loud sound and probably make the electronic squeal.

Method 2: Tube Tester

Get a tester

You can buy a tube tester from online stores like Amazon, eBay, and so on; they go for an average of 35 US dollars but if you want a high-quality tester, you will likely part with a few hundred dollars.

For instance, an emission tester will tell you if your vacuum tube works or not, but a mutual conductance tester will also tell you how well it is working.

So, go for what fits your budget.

Put the vacuum tube in the tester socket

First, you got to identify the tube. To do this, look at the printed codes at the side of the tubes and check the tester’s chart. These codes (in the form of letters and numbers) will help you determine where to position the tube on the tester.

The code can read something like “10BY9”.

In case your tester did not come with a chart, search online for the specific tester’s manual. You should find the chart in the online manual.

Do This Before You Test the Tube

Before testing the tube, you got to inspect it for any physical signs of damage. Look for cracks on the glass, bent pins, discoloration and any of the signs we talked about earlier.

If there are any visible or audible signs of damage, don’t test the vacuum tube, as that could ruin the tester.

Test it

Switch on the tester and test the tube based on the instructions displayed on the chart.

There are switches on the tester; set these as stipulated in the chart. Once you’ve done that, the electrical connections should be turned on, and the tube will be activated.

There’s also a gauge on the tester and zones that are colored differently, most often green and red. If the pointer moves to the green zone, that normally shows the tube is functioning all right, but if it moves to the red zone, that normally shows there’s a problem with the tube.

Some tubes also display numbers and the results are read by checking the number against the numbers displayed on the chart.

Remember, the testing process and the test results will depend on the sort of tester you’re using.

So, those are the main methods of testing a vacuum tube.

Before you go, please check out the tips below for efficient testing and the FAQs.

Tips for Testing a Vacuum Tube

  • If you see similar codes on different tubes, for instance, 9K7LR and 9K7L, that means there’s only a slight difference between them, and thus the tubes can be interchanged.
  • Some vacuum tubes have two different codes, with one code representing the European system of numbering and the other code representing the American system. You can just check the codes online if you have trouble distinguishing them.


Some vacuum tubes get too hot when activated and can burn your hands. Consider using a pair of gloves when handling them to avoid getting burned.

Frequently Asked Questions (FAQs)

1. What’s a vacuum tube used for?

The primary function of the vacuum tube was to amplify electronic signals. However, today, its duties are largely handled by the field-effect transistor, leaving the vacuum tube obsolete.

2. Which devices still use a vacuum tube?

Desktop display monitors use a specialized vacuum tube known as a CRT or cathode ray tube in full. Other devices that still use the vacuum tube include guitar amplifiers, x-ray machines, and microwave ovens.

3. What led to the vacuum tubes getting replaced?

It the invention of the transistor that led to the vacuum tubes getting replaced. Later, the field effect transistors were invented as a further improvement. These were a lot smaller, faster, and more energy-efficient.

Final Word

If your electronic device uses a vacuum tube, checking this little component could save lots of money for you as replacing a vacuum tube would be much cheaper than replacing the entire machine.

So, before you assume your guitar amp or microwave oven is broken, it would, perhaps, be a great idea to check if the problem is with the vacuum tube using the methods described above.

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