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Old 17th March 2010, 05:10 AM   #1
SDB777 is offline SDB777  United States
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Question How long can I expect a tube to last

Did a search, but I came up empty-handed....so maybe someone else is thinking this and just hasn't asked.


Maybe I should have re-worded the topic title?
I am wanting to know about the life of a tube.....does sonic quality diminish over a period of time?

And is there a 'lifetime' for a tube where it should be replaced? 5,000hrs, 7,500hrs, 10,000hr, or the thing is going to out-last me! I listen to my Adcom 555's(they are bi-amping my little Polk Monitor 40's) no less then 40hr per week, and when the weather is nasty....that number can be doubled!


Just wanting to know.... Still going to build one, but I'd like to be prepared for the 'if-n-when'!!

Thanks much!





Scott (there's a vaccuum between my ears too) B
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Old 17th March 2010, 10:58 AM   #2
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My experience has been small signal tubes lest about 5-7yrs when run for about 20hrs a day (6x356x20=42Khrs) power valves under the same conditions last between 1-2yrs (10K hrs).
Of course it all depends on how you heat them (I like to underheat to extend life) and how hard you push them (light duty usually).

Shoog
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Old 17th March 2010, 11:08 AM   #3
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Like he said.

Some can go longer and some shorter. I have seen "Red October" tubes go far longer than the projected life. It just depends on the quality control and the operating conditions.
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Old 17th March 2010, 11:32 AM   #4
SDB777 is offline SDB777  United States
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Sounds(no pun intended) like I'll get just enough hours out of a set of tubes to keep me happy until the next build!!



Scott (once you starting building you're never done) B
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Old 17th March 2010, 12:21 PM   #5
ksporry is offline ksporry  China
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To be honest, I don't see why this info appears to be difficult to obtain. Lightbulb manufacturers usually provide an expected lifespan in hours, under nominal conditions. It should be pretty standard for valve manufacturers to provide a lifespan under nominal conditions. Considering that valves have been used in industrial as well as military applications, this data must exist for main stream valve brands (for practical reasons I will make an exception for Chinese brand valves).
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Old 17th March 2010, 12:26 PM   #6
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Quote:
Originally Posted by ksporry View Post
To be honest, I don't see why this info appears to be difficult to obtain.
It's because the information just isn't there. Just as there is no answer to the question, "How long is a piece of string?", there is no one answer to "How long will a tube last?" The variables that affect it's lifespan are various and several, some which, at least, are in play in any given application.
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Old 17th March 2010, 12:37 PM   #7
Merlinb is offline Merlinb  United Kingdom
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Datasheets sometimes give an expected lifespan, and is usually about 5000 to 10000 hours. Of course, these numbers are related to the manufacturer's idea of "when if the valve no longer fit for purpose", which was usually in computing circuits. Of course, this figure does not necessarily correspond to when the valves stops working completely, and it could be perfectly good for analogue audio purposes long after the 10000 hours are up.

Back in the 1950s, life tests on ordinary receiving tubes showed that there is a burn-in period of about 1000-5000 hours, where failure rate was usually higher than normal (but also sometimes lower than normal!), and after that the failure rate was exactly exponential, that is, the causes of failure were entirely random. The average life is therefore the time taken for the first 63% of a batch to fail (100%-100%/e), and was about 30,000 hours for common receiving tubs in the 1950s, and is probably the same or better today, thanks to better pumping and computer controlled manufacturing techniques. This agrees nicely with Shoog's observations.

Another interesting point of fact about the exponential failure rate is that if you take two identical valves, one which has been running for, say, 5000 hours, and one which has been running for 50000 hours, the chances them lasting another given period of time are actually equal!

Last edited by Merlinb; 17th March 2010 at 12:41 PM.
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Old 17th March 2010, 12:46 PM   #8
ksporry is offline ksporry  China
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JRN: Not true, the analogy to "a piece of string" does not apply to mean time between failures of industrial parts.
I admit I may have made my statement incorrectly. I should not have said "To be honest, I don't see why this info appears to be difficult to obtain.", because maybe it IS difficult to obtain. However, I am convinced the data does exist. Simply because it must.
Manufacturers of any industrial equipment always specify longevity under NOMINAL conditions, because it is an industrial requirement to do so (after all, which sane company would ever buy equipment for industrial applications that cannot be planned for maintenance?).
Please note "under NOMINAL conditions", which means under specified testing conditions used by the manufacturer, which means, specific loading, specific input signals, specific voltages etc (usually what is stated on the spec sheet). How do you think the industry can maintain their equipment if manufacturers just decided to not provide mean time between failure data?
Vacuum tubes have been industrially used for a long time. Therefore, this equipment needed to be maintained. Therefore maintenance schedules have been generated. To do so, information on Mean time between failures MUST be provided by the manufacturer. Therefore the data exists.

I'm not saying the data is easy to obtain, I am saying the data exists.
(Again, with possible exception of chinese made valves.)
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Old 17th March 2010, 12:49 PM   #9
ksporry is offline ksporry  China
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Merlin is correct in his rationale. What's more, in the 50's process control was not as developed as it is now. Process control nowadays have a huge effect on controling failure rates. Automation is part of process improvement (as you take out the variable called "human"), and therefore caters for a significant change in process improvement.

Back to the main topic: SDB, have you got a specific tube type in mind? this may make it easier for us to help find the data for you...
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Old 17th March 2010, 01:48 PM   #10
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Quote:
Originally Posted by ksporry View Post
JRN: Not true, the analogy to "a piece of string" does not apply to mean time between failures of industrial parts.
I admit I may have made my statement incorrectly. I should not have said "To be honest, I don't see why this info appears to be difficult to obtain.", because maybe it IS difficult to obtain. However, I am convinced the data does exist. Simply because it must.
Manufacturers of any industrial equipment always specify longevity under NOMINAL conditions, because it is an industrial requirement to do so (after all, which sane company would ever buy equipment for industrial applications that cannot be planned for maintenance?).
Please note "under NOMINAL conditions", which means under specified testing conditions used by the manufacturer, which means, specific loading, specific input signals, specific voltages etc (usually what is stated on the spec sheet). How do you think the industry can maintain their equipment if manufacturers just decided to not provide mean time between failure data?
Vacuum tubes have been industrially used for a long time. Therefore, this equipment needed to be maintained. Therefore maintenance schedules have been generated. To do so, information on Mean time between failures MUST be provided by the manufacturer. Therefore the data exists.

I'm not saying the data is easy to obtain, I am saying the data exists.
(Again, with possible exception of chinese made valves.)
I don't dispute your explanation of MTBF being a more-or-less knowable value in controlled conditions. You are quite right, but not in the context of the OP, wherein the term "nominal conditions" is not defined. George (Tubelab) regularly demonstrates (much to our entertainment) that any such figures are hopelessly inaccurate when a tube is subjected to real world (or in George's case, unreal) conditions. This being a forum for tinkerers of similar, if less extreme, "ingenuity", the "piece of string" question is most appropriate.
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