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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Newbie tube question- Tube failure posted by www.records on March 9, 2007 at 06:41:07:
the most stressful time for a tube is at the moment it is turned on. The filament that heats the tube is cold. Because it is cold it conducts current with very little resistance as compared to when it is at operating temperature. That results in a huge surge of current that very rapidly heats the filament. The thermal shock (rapid expansion from heating) resulting from the rapid heating stresses the filament. It is the same reason why light bulbs rarely fail while in operation, and almost always "blow" at the moment they are turned on.Tube gear that utilizes a tube rectifier (device that turns AC into DC) are kinder on the rest of the tubes. The rectifier itself needs to be heated before it starts to conduct. So, the rectifier delivers juice more slowly at startup than solid state rectification. Most rectifiers are themselves fairly rugged and can withstand the initial turnon surge, though I am sure they would benefit from a slow start circuit too.
Some tube gear employ slow start circuits that gradually increase the juice to make the startup thermal shock more gentle. A device called a thermistor is often employed. It has the property of having a higher resistance when it is cold than when it is hot; as it heats up it slowly allows more and more current to flow.
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Follow Ups
- By far. . . - Larry I 06:50:12 03/09/07 (2)
- This is why I tend to live dangerously ... - Dave Pogue 06:09:46 03/10/07 (0)
- Re: By far. . . - astrostar59 12:09:36 03/09/07 (0)
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