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Morgan Jones on thoriated tungsten filaments

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Posted on June 23, 2014 at 13:56:04
hvbias
Audiophile

Posts: 396
Location: New Hampshire
Joined: December 18, 2005
 In Morgan Jones book he says "a 1994 study of transmitter valve longevity found that each off/on cycle reduced filament life by 0.2% from its maximum life of 30000 hours. ...it implies that 500 off/on cycles will destroy the filament, so if you switched the valve off and on every day you could expect it to expire in less than 17 months"

Anyone experience this?

 

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RE: Morgan Jones on thoriated tungsten filaments, posted on June 23, 2014 at 18:27:53
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Thoriated tungsten is brittle, but the brittleness varies a lot with the amount of thorium used, as does the expected emission lifetime. So tube manufacturers could, and did, vary the parameters depending on the expected use profile. Pity the "1994 study" reference did not indicate what tube was tested.

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 23, 2014 at 18:46:47
hvbias
Audiophile

Posts: 396
Location: New Hampshire
Joined: December 18, 2005
 Hi Paul

From the references section of the book:

Johnson W. Techniques to Extend the Service Life of High-Power Vacuum Tubes. In: NAB Engineering Conference Proceedings. National Association of Broadcasters; 1994. pp 285-93

Doing a search I wasn't able to turn up more details from that publication.

The thoriated tungsten tube I was interested in was the 801a.

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 23, 2014 at 22:02:29
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 "each off/on cycle reduced filament life by 0.2% from its maximum life of 30000 hours. ...it implies that 500 off/on cycles will destroy the filament"

No, that would be a reduction of 2% for each cycle.

.2% would be 5000 cycles to eat up 30000 hours of life.

I ran it on a spread sheet.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

GM70 anecdote, posted on June 24, 2014 at 09:56:52
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 I've been using the same pair of copper plate GM70s since 2001 with no problems.

20.0 vdc on the filaments, running 685 vpk.

 

RE: GM70 anecdote, posted on June 24, 2014 at 12:07:32
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 and that's with daily cycling?

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

RE: GM70 anecdote, posted on June 24, 2014 at 13:00:00
Jim Dowdy
Manufacturer

Posts: 1518
Location: Atlanta, GA
Joined: July 22, 2000
 I travel a reasonable amount - so I'd guesstimate that it averages out to something like 200 days per year of 4+ hours per day.

 

and ..., posted on June 24, 2014 at 13:43:51
gluca
Audiophile

Posts: 1102
Location: SomeWhere Islands
Joined: February 17, 2005
 ... I have been running the same 211s, 801As and 75TLs on a daily basis with multiple on/off per day for years

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 24, 2014 at 15:36:34
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Thanks!

Google the title and you will find a PDF of the article. The tube in question is the 4CV100,000C which dissipates 100,000 watts (filament is 10 volts at 300 amps). The article is from the VOA tech support organization.

Such a tube would naturally be designed for maximum life, and not expected to be moved during its lifetime, so a very high thorium content would be indicated. Probably not exactly the same performance as an 845 in a home stereo!

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 24, 2014 at 15:45:57
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 In the original article, the actual measurement was a 75 hour reduction of life for each turn-on cycle. To get 500 hours, they used a conservative 60 hours lost per cycle. 60 hours times 500 cycles is 30,000 hours. (I used a caculator :^)

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 24, 2014 at 21:54:22
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
Maybe I should read the article.

Thanks.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

OK, I read the article, posted on June 24, 2014 at 22:46:21
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 "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"

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 25, 2014 at 04:30:43
morricab
Distributor or Rep

Posts: 9178
Location: switzerland
Joined: April 1, 2005
 0.2% of 30000 hours is 60 hours. 30,000*0.002 = 60, 30,000/60 = 500 cycles. However, it is even simpler than that because 500 * 0.2% = 100% of whatever number start with at the beginning.

5000 * 0.2% = 1000%! you got a zero off somewhere in your spreadsheet.

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 25, 2014 at 08:43:00
Tre'
Industry Professional

Posts: 17302
Location: So. Cal.
Joined: February 9, 2002
 That is true only if each on/off cycle kills .2% of 30000 hrs.

If each on/off cycle kills .2% of the remaining filament life then it take 5000 on/off cycles to kill the tube.

I don't understand why the article used the .2% figure if the only on/off cycle that kills only .2% of the remaining life is the first one.

If, on the other hand, 60hrs are lost for each on/off cycle then each on/off cycle kills more and more than .2% of the remaining life and the last on/off cycle kill 100% of the remaining life.

Get it?

Download and look at my spread sheet and you will see what I mean.

The first column shows a loss of .2% of the remaining life per on/off cycle so 60hrs are lost for the first cycle and the number of hrs lost goes down from there because the remaining life is going down.

The second column shows a loss of 60hrs for each on/off cycle and the third column shows what the percentage of remaining life is for each on/off cycle.

For the first 60hrs IS .2% of 30000 but 60hrs is 1% of the remaining life by the 400th cycle and 60hrs is 100% of remaining life for the 500th cycle.

This isn't difficult to understand.

I just think the article should not have used percentages and should have just said that 60hrs would be lost for each on/off cycle.

When percentages are used they are normally used to speak to the percentage lost of the remainder.

In the end it's no big deal. None of us use those types of tubes to start with and the types of tubes we do use do not suffer that kind of life lost for each on/off cycle.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

:^), posted on June 25, 2014 at 10:38:48
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 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...

 

RE: Morgan Jones on thoriated tungsten filaments, posted on June 25, 2014 at 10:44:29
Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001
 Yes again, the article should not have used percentages! There is no cited data to link the on/off transient life effects to the emission lifetime; a tube with an expected 3000 hour life might still lose 60 hours on each cycle.

There probably is a link between cycle damage and emission life, since both are affected by the thorium content, but no data are provided here (or anywhere else that I've found) to quantify that.

 

It would also depend on other factors..., posted on July 18, 2014 at 15:26:24
Naz
Audiophile

Posts: 2184
Location: Sydney
Joined: September 2, 2005
 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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