‘Burn’ marks on rear of B-7971 tubes

[Robert Norman]

I have 9 tubes, 5 show no sign of any marks, 2 show marks on a couple of the connections and 2 show more extensive marks, all work perfectly.

Are the ones with marks more likely to have a shorter life?

Rob

[Nicholas Stock]

I have a variety of B7971's with different micas, 'burn' marks, pins in the top of the envelope etc etc. I have yet to have a tube fail on me in day to day use after many years. I don't think it's an indication of any life expectancy for these tubes.... they appear to be built like tanks.

Now that I have said that, I await the inundation of messages talking about B7971 failures...... LOL

Cheers,

Nick

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[gregebert]

I've only seen 2 types of functional failures in 7971's

• One completely-dead segment (A gift to me; that's how I got started with 7971's)
• Adjacent segments shorted together (it just happened one-day on my 8-tube clock w/o warning; very peculiar)

I have one exhibit-only device with a crack in the base, but that's from mishandling. The same tube has two black spots on the mica right next to rivets, and I dont know what to conclude from that.

They are pretty rugged, especially considering their massive size  /mechanical complexity, and versatile because you can display numerals, text, and a few special characters. That said, they are butt-ugly and getting expensive.

With 9 tubes that all work, you're in great shape to build a 6-tube clock and have spares.

[Jeff Walton]

I have a few of the mica-backed variety of 7971 tubes that do not show burns around the mica by connections.  They don't seem to test any differently than the tubes that do show the markings.  Most have the burn marks but none have ever failed in years of use.  The earlier versions with wire backplates do not have these markings.  They all seem to be super reliable and can even withstand the hard shock to dislodge the shorted adjacent corner segments, which sometimes happens in shipping.  I've seen some cathode poisoning on the underscore carets, but no other issues.  I'm not sure that the "burn" marks are anything other than a manufacturing caused discoloration.  The 7971 seems to be super reliable.

[Jeff Walton]

Construction examples:

[Dekatron42]

The only "burnt out" ones that I've seen was from a few that I bought from a German seller on eBay where he had over driven them with too many segments lit at the same time so that the thin anode strip from the pin to the anode had burned through completely making the tubes utterly unusable. I am not sure how you could put that much current through the anode under any normal condition but that was what he told me he had done - don't know if that was by accident or from poor design.

/Martin

[Robert]

Already using 6 in the Mod 6 Lumina 

Rob

On 25 Apr 2023, at 08:50, Dekatron42 <martin....@gmail.com> wrote:



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[H. Carl Ott]

I've seen one tube where the segments disintegrated to the point where pieces broke loose and shorted other segments. 

I have no idea how much current it took to do that,  or even what caused it to happen. 

But I don't remember any burn marks on the mica. Will have to dig it out of storage and look again. 

 

carl
--------------------------------------------------------
Henry Carl Ott   N2RVQ    hcar...@gmail.com

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[Robert]

Third type of construction, this one is from 1964

Rob

On 25 Apr 2023, at 12:52, H. Carl Ott <hcar...@gmail.com> wrote:



I've seen one tube where the segments disintegrated to the point where pieces broke loose and shorted other segments. 

I have no idea how much current it took to do that,  or even what caused it to happen. 

But I don't remember any burn marks on the mica. Will have to dig it out of storage and look again. 

 

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