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Old 24th Feb 2018, 9:41 am   #1
igt
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Default Valve/vacuum tube base repair/reglue

Hi

I have a CRT that its base ( ebonite ? ) came apart from the glass (envelope)
How to reglue it ? What type of glue is suitable - heat, sealant etc.
Many thanks

igt
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Old 24th Feb 2018, 10:04 am   #2
GrimJosef
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Default Re: valve/vacuum tube base repair/reglue

There are several threads on the forum about this.

1. You need a glue which is very adhesive (the glass surface changes over the years and it is harder to stick now to old glass than it was when the glass was new).

2. You need a glue which will not cause electrical conduction or arcing if it comes into contact with the tube's leadout wires.

3. You need a glue which will survive repeated thermal cycling, potentially up to rather high temperatures (perhaps this is a more severe problem with power output valves than with CRTs).

4. You need a glue which will not develop stresses in itself which might lead to the glass cracking (some people have reported cracks with cyanoacrylate glues - so-called 'superglue').

Personally my favourite glue is a mix of one third clear nail varnish (which is very sticky and just a little bit flexible when set) and two thirds acetone (which makes the mixture very fluid, so it will run deep into the joint thereby giving a large surface area for the bond). I tape the joint together then run in as much of the glue as I can using the little nail varnish brush to drop it onto the joint. Then I leave it for at least 24 hours to harden. I have tried 12 hours and quite often the joint seems to crack. Sometimes the joint even cracks after 24 hours, but I repeat the glueing and then it is usually fine.

Cheers,

GJ
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Old 24th Feb 2018, 10:17 am   #3
igt
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Default Re: valve/vacuum tube base repair/reglue

Hi GJ
Very helpful info.
Thank

igt
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Old 24th Feb 2018, 10:18 am   #4
G6Tanuki
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Default Re: valve/vacuum tube base repair/reglue

As well as gluing, you could consider using adhesive-lined heatshrink sleeving: this technique was used by the MoD on some valves when the base became loose [I've got a bunch of WWII-vintage VR100 that have been so treated].

Only possible issue with this is if the tubing made the 'neck' of the tube too fat to get the deflection/focus coils over!
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