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Old 17th Jan 2021, 9:31 am   #1
high_vacuum_house
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Question Cutting glass on B9A valves.

Good morning,
I am trying to restore a Mullard valve component display box where the glass bulb has got loose and smashed. I managed to cut off and make smooth the pump off tip on a dead EF80 but spent 2 hours trying very carefully to separate the bulb from the base. This was done with the corner of a diamond file, which is now rather worn!!. I very nearly managed it but the bulb cracked as the file broke through the cut I had made.

Is there a better way of separating the bulb from the base from an all glass valve ?

Many thanks,
Christopher Capener
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Old 17th Jan 2021, 10:03 am   #2
vinrads
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Default Re: Cutting glass on B9A valves.

Good morning Christopher, I remember some time ago wrapping some resistance wire around the valve just one coil connecting to some volts until it got hot , then quickly immersing the valve in cold water ,it worked for me , Mick.
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Old 17th Jan 2021, 10:23 am   #3
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Default Re: Cutting glass on B9A valves.

As above. CRT reconditioners used that trick to remove the tube neck, it left a neat crack which they then left for a while to release the vacuum.

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Old 17th Jan 2021, 5:02 pm   #4
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Default Re: Cutting glass on B9A valves.

A piece of cotton string soaked in meths and ignited (water immersion not required) works for cleanly removing the bottoms of glass jars and bottles, but I haven't tried it with anything small like a valve.

Last edited by emeritus; 17th Jan 2021 at 5:04 pm. Reason: Typos
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Old 17th Jan 2021, 5:47 pm   #5
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Default Re: Cutting glass on B9A valves.

Yes, a length of Nichrome wire wound round the tube-neck a few times, then stagged across a car-battery until it glowed bright orange was what the backstreet tube-reconditioners used to get the gun-assembly out of a tube.
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Old 17th Jan 2021, 9:28 pm   #6
high_vacuum_house
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Default Re: Cutting glass on B9A valves.

Good evening,
Many thanks for your replies, I have got some thin resistance wire on order (I smashed open a wire wound resistor only to find the resistance element wasn’t wound with wire!!) and I have found another dirty unlabeled secondhand EF80 as a candidate for dissecting. Will have to go hunting fore more dead B9a valves!
Will find out what happens next!

Christopher Capener
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Old 17th Jan 2021, 9:36 pm   #7
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Default Re: Cutting glass on B9A valves.

The sort of 'murder-wire' you'll need is what was used in convector-heaters or night-storage-heaters - the sort of things that drew 12A from 250V mains.

Cull one, uncoil the element, stretch it out so it's kinda straight, then wind 3 or 4 turns round the valve at the point you want it to separate.

Then hook it up to a car-battery and watch the glow! If you're lucky you will hear a ~~ping!~~ when the job's done.
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Old 18th Jan 2021, 5:49 am   #8
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Default Re: Cutting glass on B9A valves.

Resistance wire scavenged from old heating elements often goes very brittle and cant be re-used as it breaks when un-coiled, or when otherwise manipulated.

New resistance wire should work fine as described.
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Old 18th Jan 2021, 6:36 am   #9
its ur aerial
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Default Re: Cutting glass on B9A valves.

I assume this is the modus operandi used by these people that make the Bottle Lamps ?
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Old 26th Jan 2021, 8:27 pm   #10
David G4EBT
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Default Re: Cutting glass on B9A valves.

No one has mentioned using a 25mm diamond cutting disc in a Dremal.

Out of curiosity I gave it a go. I cut the base off a TV valve, but as it had a top cap, I had to cut the top off as well as the base to be able to extract the innards. I wrapped a piece of masking tape around the valve as a guide to keeping the cut to a straight line. I kept the speed down on the Dremel it only took a couple of minutes as the glass was only .0025 (25 thou) thick - 0.063mm.

Lots of videos on youtube on how to cut a glass bottle, but cutting glass this thin usinghthe same technique thin is quite another matter.

Obviously in an all glass valve with no top cap, only the base would need to be cut off to separate it from the 'bottle'.

I'm not exactly sure if cutting the base off in this way is what's required, but anyway, two pics are attached.
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