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 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Thu 25, 2018 8:05 pm 
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Location: Ann Arbor, MI
jdivito wrote:
From the book I referenced (by Robert Tomer, Getting the Most out of Vacuum Tubes), which talks about the degenerative development of internal shorts in vacuum tubes:


...Natural mica has very excellent insulating properties and can be subjected to very high temperatures without break-down. However, in a vacuum tube, there are materials operating at temperatures near their vapor point. When these metallic vapors settle on the smooth mica surfaces, they produce a gradually decreasing resistance. To slow down this process, micas are coated with a finely divided substance that tends to break up the metallic vapor films. This results in a situation similar to that which occurs when raindrops fall on a clean automobile hood. Many small droplets appear first, widely separated from each other. As more rain falls, the drops become larger until finally one or two merge and then suddenly a whole series of drops become connected as a small stream of water starts running down the slope of the hood. In a vacuum tube, that small stream is one leakage path. The more it forms, the lower the resistance between elements becomes...
...
Insulation leakage caused by the deposition of metal films proceeds at a uniform rate in all tubes; however, it is greatly accelerated by such excesses as high heater voltages and high electrode dissipation. Cooling bulbs will often retard its development considerably because the vapors will usually deposit on the coolest surface. If this is the glass envelope, they will generally cause less harm than on the micas. As insulation resistance begins to decrease, it will progress very rapidly even though its initial build-up may be extremely slow. This is because even though the release of metallic vapors is more or less constant, the early effect is to produce islands of metallic film which are not connected with one another. As the islands become larger, the chances of their remaining isolated gets smaller, until finally, many islands connect and form multiple leakage paths almost simultaneously....

This certainly explains the failures, sometimes catastrophic, we see in horizontal output tubes and dampers in TVs. But I expect that it is less common in low power receiving tubes. As I remember it, when I worked in a radio / TV shop in the middle 1960s, open heaters accounted for over half the cases of a radio suddenly going dead. In fact there were companies making so called tube testers that just checked for continuity of the heater.

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Tom


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 Post subject: Re: Vacuum Tube Failure
PostPosted: Jan Fri 26, 2018 3:55 pm 
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Joined: Jun Thu 30, 2011 9:03 pm
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That’s actually part of what that book I quoted implied, high power tubes or high heater tubes (such as a hot running 50L6 or 6V6) would be more prone. But any tube could have it happen, just less likely. It also goes with the recommendation to keep tubes well ventilated to let them naturally run cooler.


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