Electron Tube Testing Visual Inspection of the Vacuum

OddMix.com - Technology Note - TN1201 - Karl Nagy

Picture 1. Assorted Old Vacuum Tubes [13 KB]
Picture 1. Assorted Old Vacuum Tubes

Vacuum tubes are occasionally found in old attics basements, and other unexpected places, or in the latest, high priced electron tube type sound amplifiers. Since vacuum tubes are not often encountered, often times the new owner have a hard time to recognize them. Vacuum tubes were invented, and then perfected to their nearly current form, in the beginning of the 1900's.

Since proper vacuum tube manufacture is neither totally simple, nor easy, tubes always used to cost their owners plenty. Because of their rarity, nowadays, tubes are especially worth preserving. Regardless of their shape, size and form, these surviving examples of a bygone era are well worthy of respect, display and use - Picture 1.

Many of these newly found electronic tubes are often useful for the restoration of vintage collectible radios and many tube type electronics instruments. Additionally, many electronic circuits are simpler and more reliable when designed and made with vacuum tubes.

Contrary to popular belief, tubes are still used in many unexpected places as many times they are the only solution to many difficult technological problems. RF heating of steel bars, all higher powered radio and television transmitters are all using vacuum tubes. So far, semiconductor devices were not completely able to displace the tubes from all their current applications. And quite possibly they never will.

Once a tube is surfaced, the first step is to find out if there are any good at all. In the bygone era when most pharmacies were outfitted with tube testers, the testing was a bit easier. But before the tube testing part comes, the first step with a newly found tube is to determine if any further bother is warranted. That is done by giving the tube a good visual inspection. There are no expensive instruments and other high tech devices required for this effort.
Picture 2. Vacuum Tube Internal Assembly [10 KB]
Picture 2. Vacuum Tube Internal Assembly

Naturally, if the glass envelope is visibly cracked or broken anywhere, there is no further hope for the device. Once the glass is found solid, or it appears to be, and the tube is otherwise in a good physical condition, the next step is to inspect the getter of the device.

Two same type electron tubes are shown side by side on Picture 2. The tube on the right side, that has a whitish gray coating, has lost its vacuum. The one on the left has a vacuum and its getter - that is the dark shiny coating on the inside bottom - is dark and mirror like. That is the main visible difference between the two type 2X2 vacuum tube high voltage diodes on Picture 2.

Not all vacuum tubes can be inspected so easily. If the tube does not have a glass envelope, visual detection is not possible. On the lower right side of Picture 1 they are two examples of metal tubes. The other three are glass tubes with their visible shiny dark getter.

Even in the era of the satellite radios, many of you would be simply awestruck by the rich quality sound an old timer radio set can produce. Without a doubt vacuum tube amplifiers sound better than the sterile digital sound of semiconductors. The reason tube types equipment sounds better, because vacuum tube amplifiers had a lot less gain then transistor and IC types and they most often could not afford to use negative feedback. Negative feedback has its uses, but it sure not improves the sound of a radio.

                  Steps of the visual vacuum tube inspection process:

                   1 - Carefully clean the tube, all over, with a wet paper
                   2 - Inspect if the tube is glass or metal type
                   3 - Inspect the glass carefully outside for cracks
                   4 - Inspect the integrity of the tube's getter carefully
                   5 - Inspect the tubes inside and check its connection pins
                   6 - If the getter looks good - tube is ready for an electronic test

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