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In Reply to: RE: Morgan Jones on thoriated tungsten filaments posted by Tre' on June 24, 2014 at 21:54:22
"The next two curves make the further
assumptions that an Off/On cycle will remove 60
hours of life from the filament at the rated filament
voltage. It amounts to a 0.2 percent reduction in the
life of the tube, another way of saying that the filament
is good for about 500 Off/On cycles at the rated
filament voltage before burnout"But if each cycle reduces the life of the tube by .2% then even though the first cycle will remove 60hrs the 200th would only remove 40.44hrs because by then the hours left would only be 20222.64 as each cycle leaves the filament with less hours of life left.
So .2% of 20222.64 is not 60hrs, it's 40.44hrs
I guess my point is the article should not have stated a percentage of filament life lost, it should have stated just the number of hours lost with each cycle if that's what they meant.
If each cycle removes 60hrs then the 499th cycle is removing much more than .2% of the remaining life of the filament and the 500th cycle will remove 100% of the remaining life of the filament.
This is all made clear in the linked spread sheet.
Check it out, you might at least get a laugh that I would take the time to create the spread sheet over an article that has very little to do with the types of tubes we use for audio.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 06/24/14 06/24/14Follow Ups:
Yes, it's not the most carefully written article!
Subtle difference - 0.2% of initial life vs. 0.2% of remaining life. I puzzled over that too, until I saw the earlier data set showing the actual number of cycles before tube death as experienced by the Egyptian transmitters. All the other numbers were derived from this data - there is no actual data to show that you would still have 15,00 hours if you left it on all the time after the first 250 cycles...
and I would think that peak current is one of them, possibly the largest. If the turn on peak was limited then surely it must lead to extended life. I have always designed assuming this principle, though I have nothing to support it other than logic:)
Naz
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