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Old 26th Nov 2017, 4:34 am   #1
Otari5050's Avatar
Join Date: Jun 2015
Location: Auckland, New Zealand
Posts: 89
Default Unusual deposit in tube (valve)

Hi folks, got a nice KT88 tube (NOS) which has an unusual white deposit on the inside of the glass.

At first I thought it had gone to air, but the getter is perfect and a check on an AVO MK4 shows it's in perfect health, no gas at all.

Sorry the photo isn't the best but it's hard to get close.
You can see where it's white from the base up, but then goes clear before the bend in the glass.

I've never seen this before in a tube.
I'd like to get others' thoughts on this.
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Old 26th Nov 2017, 9:28 am   #2
Join Date: Jul 2008
Location: Charmouth, Dorset, UK.
Posts: 3,165
Default Re: Unusual deposit in tube (valve)

I have occasionally seen white deposits on the inside of a valve and thought the same as you but if the heater lights up and it works OK I wouldn't worry about it.

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Old 26th Nov 2017, 7:54 pm   #3
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Smile Re: Unusual deposit in tube (valve)

Probably just a peculiarity caused during manufacture. I think that the white is where the gas jets would have been melting the glass and a tool being used to form the joint between the pinch electrode assembly and the glass bulb itself.

I have seen valves with white patches around the base and funny coloured getters but are all fine. Some clean B9a valves have quite powdery white looking bases inside.

Could be the glass to base cement but this is usually much darker and rougher.

Christopher Capener
A proper radio is one that needs to be moved with a wheelbarrow !!
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Old 28th Nov 2017, 9:49 am   #4
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Location: Lincoln, Lincolnshire, UK.
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Default Re: Unusual deposit in tube (valve)

I noticed with a NOS valve the other day that there seemed to be a white milkiness in the glass itself around the base which was not present in other examples of the same valve. It was otherwise identical to others of the same type.
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Old 28th Nov 2017, 10:27 am   #5
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Location: Croydon, Surrey, UK.
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Default Re: Unusual deposit in tube (valve)

More than likely a colouration of the glass itself. If it checks out perfectly just use it and enjoy!
'Can we have everything turned up louder than everything else'? Ian Gillan...Deep Purple
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Old 28th Nov 2017, 1:26 pm   #6
Boater Sam
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Default Re: Unusual deposit in tube (valve)

Overheating of soda glass in the blowing/moulding process can be the cause. Normally re-annealing would remove the whiteness.
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