How Do Vacuum Tubes Work?

In this video, we'll learn how a tube works. All American Five radios share roughly the same circuit and most have five tubes, hence the name. As we progress, I’ll explain in more detail about what each of the five tubes do, and how they work. Generally, though, radio tubes have three functions: Rectification, mixing, and amplification.

Many believe that vacuum tubes easily fail and that to get an old radio working again, one simply has to replace the tubes. The truth, though, is that there are many common failure points in antique radios, but decades-old vacuum tubes are often in perfectly good working order.

Tubes can be tested with a tube tester, but unfortunately mine couldn’t test three of the five tubes. It’s from 1966, so it lacks the ability to test some of the oldest tubes.

And in 1966 many radios no longer used tubes at all. Transistors were speedily replacing them as they were far smaller, cheaper and more efficient. You’ll often see transistorized electronics of the mid sixties to seventies boastfully adorned with the words “solid-state.” It was the beginning of new era in electronics that led to the personal computer revolution of the 70s, 80s and beyond.

Of the two tubes I could test, the 25L6 had a short and the 6A7 had a broken grid cap. To be on the safe side, I ordered five new tubes.

You may wonder what the tube numbers mean. Let’s use the 25Z5 tube as an example. The first number is the filament voltage. The letters after the voltage are the manufacturer code. And the final number indicates the number of elements. I’ll explain more about filaments and elements in upcoming segments.

You may also wonder why they’re called vacuum tubes.

As with lightbulbs, the inside of a tube is a vacuum because otherwise the filament would burn out as it consumed oxygen from the surrounding air.

The vacuum in a tube also improves a tube’s performance because it allows electrons to move more freely.

A vacuum is created in a tube by sucking the air from the glass envelope during manufacturing. To remove any remaining gasses, a small amount of oxidizing compound, called a getter is placed inside the tube. The getter is heated, and as it oxidizes, it consumes the remaining gasses. The silvery coating you’ll often see inside a tube is from the getter flash. If the coating appears white instead of silver, the vacuum has been compromised and the tube will no longer function properly.

Here are the five new tubes I ordered. It turned out I didn’t need most of them so some were kept for spares.

Here you can see one of the new tubes in its packaging. Actually, this tube isn’t new at all. It’s what’s known as “New Old Stock” or NOS. New old stock describes an item which was manufactured long ago, but is only now being sold for the first time. New old stock tubes are still readily available fortunately, as well as newly manufactured ones from Russia and China.

Broken tubes are sometimes able to be repaired and that was the case with our 6A7. The grid cap had come off, but fortunately the grid wire was still intact. I was able to solder a new wire to it... And then formed some Apoxie Sculpt... To firmly attach the cap in place. I then soldered the wire to the cap... And trimmed the excess.

The 6D6 tube is meant to be operated with a metal tube shield. Tube shields block electromagnetic interference from affecting the tube’s performance. Fortunately, the original tube shield hadn’t gone missing as they often do, and just needed a little polishing.

I started by working in the metal polish with a toothbrush. The metal was so tarnished, though, that this had little effect. So, I brought out the big guns and polished the shield with a dremel and abrasive paste. And as you can see... That really did the trick.

On the next video, we’ll learn about the cathode, filament and plate and how the rectifier tube uses those elements to convert AC to DC. To stay updated, please subscribe and click the bell. And if you like this video, give it a thumbs up. See you soon.

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Видео How Do Vacuum Tubes Work? канала Flux Condenser Vintage Audio Electronics