Picture of Quick Test for Electronic Tubes
A friend was at a yard sale and bought a facsimile antique wall telephone that was really an AM tube radio. The radio did not work. I used a simple way to test the tubes and fixed the problem.
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Step 1: Tube Circuitry

Picture of Tube Circuitry
Until the late 1970s many drug stores and electronics shops sold electron tubes. Those outside the USA know them as valves. Each store selling tubes usually had a tube testing machine with several dozen tube sockets and various charts to find the right data for your tube.

Below is the electrical diagram for a typical tube. The filament heats up when current flows through it. When heated by the filament, the cathode emits electrons that are attracted by charge to the anode. Various grids insert signal charges, often fluctuating, to restrict or speed the flow of electrons to the anode.

Although there are a number of things that can go wrong in an electron tube, one of the most common is that the filament burns out.

Step 2: Testing for a Burned Out Filament

Picture of Testing for a Burned Out Filament
As you look at the bottom of a tube, you see a pattern of pins. There is a wider gap between two of the pins. My friend's radio used a couple of different types of tubes, but one was a 50C5, which is a very common tube. The first two pins (red circles) go to the filament. I used my ohmmeter to check all of the tubes for two pins with a few ohms resistance between them, as would be expected in a filament. In one tube, there was no resistance between any two pins, only open circuits. That suggested this was a faulty tube. I happened to have a couple of old tube radios I was using for parts. I scavenged a 50C5 tube from one of them and my friend's radio worked again. She was very pleased, whether she ever plays that radio, or not.
dataphool4 years ago
Although you have described the pins as the first two, the pin numbering system counts the pins in a clock-wise direction, so the pins you are referring to are 6 and 7. What about a 12AU7 which has 9 pins, or 6L6 which has 8?

You have a good i'ble, but I don't think anybody cares about tubes anymore. Except audiophiles, who are a little strange anyway.

Not true many of us Guitarist care about Tubes in the worst way , not to mention 10 and 11 meter band Radio operators . Im currently in the process of converting/restoring a 1968 Sears silvertone into Amp :)

Don't forget guitar players. A lot of us still use tubes in our amps. Although, that is diminishing due to the proliferation of digital SS amp modelers. Still many of us still like the sound of tubes.
Phil B (author)  tunghaichuan1 year ago
I have noticed that in the last few years.
Phil B (author)  dataphool4 years ago
Thank you for the corrective information.
Ernest121 year ago
Thank you for a great instructable, there are a growing number of young people looking for this old school type of information, to rebuild vintage radios, tube amps to guitars. Find a 1940's or 1950 ARRL hand book and it will be full of old school information on tube's.
Phil B (author)  Ernest121 year ago
Thank you for your comment. I have watched with interest the resurgent interest in tube amplifiers for music. I did not really know younger people want to learn vacuum tube circuits, even though I have noticed several Instructables in which people are restoring table top radios with electron tube circuits. I once had an ARRL Handbook I bought new in 1965 and thought I wanted to become an amateur radio operator. This Instructable is rather basic, as you can note from some of the comments by people with more knowledge and experience in vacuum tubes. Still, it is good to check the very simple things first because they are often the problem.

In the late 1980s I was listening to a lot of shortwave radio so I could hear spoken German and so I could improve my ability to understand and to speak it. I had a fairly basic transistor receiver I used at home, but I also wanted a receiver at the office for a couple of German broadcasts that came during the workday. I bought a Zenith Transoceanic receiver that cut out a lot until I had replaced a couple of tubes and all of the paper capacitors. I had improvised a signal tracer from a common diode for a probe and the audio amplifier section of my daughter's discarded radio/tape player. I had also built a capacitor tester using a 555 integrated circuit and plans from an electronics magazine. Both of those were very handy for locating a problem in a tube radio circuit, even though I was and remain a complete novice.

master key6 years ago
(removed by author or community request)
Slam on the brakes for a minute did She replace the paper capacitors with brand new capacitors and don't for get the resistors they tend to float above there resistance but this is a great way to test tubes
Phil B (author) 6 years ago
Thanks. It worked out well for me as a diagnostic tool when I needed it, even if it is not a completely reliable test in all cases.
wierd idiot6 years ago
now sorry to say but this method can be unreliable because you can get mutable pins that have ohms measurement but the filament could still be blown.