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Old 3rd Mar 2018, 10:30 am   #1
mark2collection
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Default Valve has gone to air, 'my vacuum has escaped!'

Hi folks,

I'd been cheerfully using a set over the Christmas period & up until recent days with no issues at all, she's been great. Yesterday, I fired her & she lit up as usual but not a peep to be heard, not even the usual gentle hum from the speaker.

Removing the back revealed the original Mullard UL84 has a white top with a 'V' crack emerging from one of the heater pins. Gadzooks the vacuum has escaped! (As a colleague once amusingly exclaimed).

Has anyone else had this? This is my first true valve failure, normally I see low emission or poor valve to valve base contact, but this took me by surprise.

I can only imagine the indifferent materials (valve pin vs glass envelope) change temperature at different rates, add some fatigue into the mix & she's a gonna! The room is a stable/sensible temperature, around 68 degrees.

Would welcome anyone's thoughts on this.

Many thanks,

Mark
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Old 3rd Mar 2018, 10:37 am   #2
HamishBoxer
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

I cannot spell the word correctly but, conduedual flaw comes to mind. It does happen to glass and the defect has been there a while or some stress in the past.
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Old 3rd Mar 2018, 10:37 am   #3
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

I've had various glass-base valves develop those sorts of cracks: one thing to check is that the contacts in the valve-base are clean and tight. If there's corrosion or poor contact causing a significant resistance on the heater-pins in particular, you can get a significant amount of the 'heat' being delivered to the heater-pin rather than the heater, which over time doesn't do the metal-to-glass seal much good.
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Old 3rd Mar 2018, 10:50 am   #4
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

Somewhere in the distant past, at Technical Collage we were told that the valve pins were made of a type of metal which had the same heat coefficient as the glass. Mick.
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Old 3rd Mar 2018, 10:52 am   #5
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

If the vacuum had escaped from the valve it would be in the cabinet.
It has simply ceased to be, it is an ex vacuum.

I have had a few B9A valves do this but mostly rimlocks I suspect because the glass is thicker and hence the pins create more stress.
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Old 3rd Mar 2018, 10:55 am   #6
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

There was a thread here some time ago about valves doing this while in storage. I don't remember whether any firm conclusion was reached about the cause(s).
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Old 3rd Mar 2018, 11:31 am   #7
M0FYA Andy
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

Make sure you don't breathe in the escaped vacuum, it'll do you no good at all.
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Old 3rd Mar 2018, 11:34 am   #8
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

It happens, but surprisingly not very often, at least it was a UL84, easily obtainable for now.
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Old 3rd Mar 2018, 12:00 pm   #9
mark2collection
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

Many thanks for the replys thus far chaps, seems a combination of conditions could cause this, I can also see how a poor valve base contact would increase the amount of heat on the pin, particularly for the heater as it'll be consuming a bit of current.

We still humour our fellow colleague about 'vacuum escaping'. Could the same be said of some transistors, they only fail when the compressed smoke escapes!

I shall hunt around for a replacement valve, in the meantime, & having re-strung my daughters guitar, the old guitar strings prove very useful passing through valve sockets & removing the tarnish, they come up lovely.

Just make sure any residual HT decays first!

Mark
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Old 3rd Mar 2018, 12:03 pm   #10
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Smile Re: Valve has gone to air, 'my vacuum has escaped!'

Hi,
Back in my distant youth, I had a Bush TV43 14" B&W telly. Of all the valve holders within, only one was ceramic. It held the ECL80 frame osc/output, and I must have gone through at least four as they used to crack across the base at odd times. I'd switch it on from cold, and be greeted with a white line!
Cheers, Pete.
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Old 3rd Mar 2018, 12:17 pm   #11
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

Langrex have 72 in stock.
https://www.langrex.co.uk
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Old 3rd Mar 2018, 12:41 pm   #12
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

They do very occasionally fail like this. The cold temperatures and low humidity over the last week may have been a factor.
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Old 3rd Mar 2018, 1:20 pm   #13
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

Quote:
Originally Posted by mark2collection View Post
.

We still humour our fellow colleague about 'vacuum escaping'. Could the same be said of some transistors, they only fail when the compressed smoke escapes!


Mark
Shouldn't that be magic smoke? Though I guess that given the amount that comes out it's probably well compressed to get it in during manufacture.....
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Old 3rd Mar 2018, 1:54 pm   #14
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

You can get replacement "magic smoke" although this is car smoke which may be different to transistor smoke.

http://www3.telus.net/bc_triumph_registry/smoke.htm

Keith
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Old 3rd Mar 2018, 1:57 pm   #15
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

Your vacuum has escaped?

Vacuum hunters will be converging on Royal Berkshire once they know there is some on the loose. They are easily recognised by their weaponry, a blunderbuss end attached to a solder-sucker barrel.

They'll catch it before the public is endangered.

David
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Old 3rd Mar 2018, 4:22 pm   #16
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

During my servicing days it was quite common to find a value had gone “soft”.
We never looked for a reason, just replaced valve, tested set (Radio or TV) and away on next call.
Cheers
John
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Old 3rd Mar 2018, 4:33 pm   #17
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

It gives pause for thought that a once-common and relatively inexpensive technology is usually able to retain a hard vacuum so reliably.
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Old 3rd Mar 2018, 4:33 pm   #18
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

I've had a couple of valves spontaneously go to air, with a crack starting at a pin. Really upsetting when this happens. It's always been button bases, I have never had this problem with a pinch seal.
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Old 3rd Mar 2018, 6:08 pm   #19
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

I've always suspected stress cracks started when someone wiggles them trying to get them out. If so it can take years to show up.

David
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Old 3rd Mar 2018, 6:40 pm   #20
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Default Re: Valve has gone to air, 'my vacuum has escaped!'

Quote:
Originally Posted by Radio Wrangler View Post
Your vacuum has escaped?

Vacuum hunters will be converging on royal Berkshire once they know there is some on the loose. They are easily recognised by their weaponry, a blunderbuss end attached to a solder-sucker barrel.

They'll catch it before the public is endangered.

David
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