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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 6th Sep 2009, 10:24 am   #1
YT2095UK
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Question "Black" PL81s

I was just going through one of my boxes of valves to try and sort/organise them a little better and came across 4 PL81`s, but they all seem to have some sort of Black/Brown deposit inside them, something akin to creosote to look at.
the getters all seem fine and since they are from different makers I can practically rule out some sort of contamination during manufacture.
one of them is in a Really sorry looking state with the 2 fins at the top that are spot welded to the copper posts at what Looks like G1 having a crackle effect like the plating is flaking off almost peeling off in places.
now I`v seen times where there has been a hole in a plate and you get a rainbow effect on the glass probably from some sort of vaporised metal carried as Ions and then reformed on the glass as it loses an electron, but this black stuff is nothing like that.

anyone Know what it is? and is it indicative of a dud valve?
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Old 6th Sep 2009, 12:18 pm   #2
barretter
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Default Re: "Black" PL81s

I think it just means they have been well-used and are somewhat knackered. I have lots of EL84s which have gone the same way.
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Old 6th Sep 2009, 12:21 pm   #3
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Default Re: "Black" PL81s

The PL81 is a line output valve used in many TVs. It works VERY hard for a living and all the used ones I've seen have some kind of brown deposits on the glass. Presumably metal that has evaporated from the anode.

The only way to test them is to try them in a suitable TV. Despite the deposits they may be OK. Or not.
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Old 6th Sep 2009, 7:51 pm   #4
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Default Re: "Black" PL81s

In my experience, probably O.K. J.
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Old 6th Sep 2009, 8:03 pm   #5
Boom
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Default Re: "Black" PL81s

I've spent the afternoon sorting my 'U' valves out and trying them out in a test set. I have several UL41s all which look terrible with black and brown thingies all over the inside of the glass but they all work fine with no hum either.

You can't tell anything at all by looking at them.

Dave
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