How to Test Vacuum Tubes with a Multimeter

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You might get allured to throw the good old tube as an artifact of the past. After all, how can a few pieces of metal in a light bulb hold up to the integrated circuits and transistors of today? 

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Tubes lost their place in the storefront of consumer electronics. However, they still remain irrelevant use where there’s a need for a huge amount of power at high frequencies, like:

  • electromagnetic weapons
  • radar
  • particle accelerators
  • satellite communications
  • microwave ovens
  • industrial heating
  • television and radio broadcasting
  • and other applications that need lower frequencies and power levels like audiophile amplifiers, X-ray machines, and radiation meters

Two decades ago, many displays utilized a vacuum picture tube. But did you know that there might be some tubes lurking around your home as well? In the core of your microwave oven lays, you will find a magnetron tube. 

Its job is to produce high-frequency RF signals and high power, which are utilized to heat whatever you place inside the oven. Further, another household device with a tube inside is a CRT TV, which highly likely sits in a cardboard box in the attic after being changed with a flat-screen TV. 

You see, many circuits like mixers, amplifiers, and oscillators are simpler to explain with tubes and learn how they work, as classic tubes, particularly triodes, are simple to bias with only a few components and measure their amplification factor, bias, and more. 

What is a Vacuum Tube? 

In case you didn’t know yet, a vacuum tube is one of the oldest signal amplifier components. It is composed of pins connecting the electrodes, several internal electrodes, and an outer glass housing. 

In electronics , an electron tube,a vacuum tube, a valve, or conversationally, a tube is a device controlling electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied. You will find a cathode, a filament, and a plate. The diode is the easiest and simplest vacuum tube. 

A vacuum tube’s structure is composed of:

  • a control grid
  • an electron-producing portion 
  • an anode lead enclosed in a glass container 
  • an acceleration grid

All of these are welded to the tube base. Further, an electric field is utilized to insert an electronic modulation signal into the control grid within the vacuum. Numerous signal data are acquired at the anode through feedback oscillation or signal amplification. 

In a real working condition of an electric tube, it will produce heat, especially when the filament has current. The electrons will get sufficient energy to be emitted when the heated cathode reaches a particular temperature, and the electrons will be absorbed by the plate. 

The direction cannot be reversed, as the cathode or the filament cannot absorb electrons. That one-way conductivity is the basis of the working principle of the vacuum tube. 

In the early stage, these tubes were utilized in electronic products like radio amplifiers and TVs. However, it has been slowly substituted by integrated circuits and amplifiers made of semiconductor materials in the past years. Still, electron tubes with high stability and low noise are still utilized as audio power amplifier components in some high-fidelity audio equipment. 

A Little History

In 1883, Thomas Alba Edison was doing a small test to find the ideal filament material for his light bulb. He mounted a tiny piece of copper wire close to the carbon wire in the vacuum bulb. He does that with the copper wire to avoid the carbon filament from vanishing. Unfortunately, he failed the experiment.

But, he incidentally noticed that there’s no copper wire attached to the circuit, but it produced a weak current because of the hot electron’s reception emitted by the carbon wire. At the moment, Edison was focusing on learning the urban power system and didn’t pay any attention to this marvel. 

He applied for a patent for that exploration and called it the “Edison Effect.” Eventually, it was proven that the current was produced, as the hotel metal could produce electrons to the environment. But the first person to realize the efficient value of that effect, is the British electronic engineer and physicist Fleming. 

The planet’s first electronic diode was created in 1904 by Fleming, and he acquired the patent for that creation. The birth of the first electron tube, which marks the world that it has already entered the electronic era. 

How Do You Test a Vacuum Tube?

Are you wondering how you can test a vacuum tube? There’s a basic and simple way to do that using a digital multimeter. Follow the steps below:

  1. Connect the positive probe of the multimeter to the cathode of the vacuum tube and the negative probe to the anode.
  1. Point the controlling needle of the digital multimeter to the Ampere area of the ring and check if there is any current flow across the vacuum tube. 
  1. Do you suspect a filament burn? Then you can use the meter to check between every two pins with a resistance between them. Keep in mind that that’s what drives a functioning filament. Was there no resistance between the two points? Then the cause might be a broken tube.

Final Thoughts

To make it short, the perfect way to test your vacuum tube is to learn about it and understand how it works. The more you know, the more likely you will explore where to search despite the absence of a tube tester. 

There you have it! We hope you find this article engaging and helpful as well. Keep in mind that other vacuum tube devices include photomultipliers, magnetrons, cathode ray tubes, and X-ray tubes. They have also seen applications and uses even in modern-day microwave technologies utilized for Wi-Fi transmissions, Bluetooth, mobile phones, and even in satellite communication and radar devices. 

So, what about you? Are you ready to test your vacuum tube with the given steps above? Feel free to visit this page again in case you forgot any of the steps we discussed earlier. Have fun and enjoy!

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