[Amps] Outgassing tubes.

[Will Matney]

George,

Your correct. The tubes though that do intermittantly spit or arc I've noticed folks are running them right at the maximum or over the maximum rated plate voltage. Tubes aren't evacuated 100% as that would be about impossible. If the potential is high enough, and the elements are close enough together, with just a very minute amount of gas, they could arc. If they couldn't there wouldn't be a maximum plate voltage rating. Even though a vacuum is pulled, when an element gets hot it can release a small amount of gas, even though they say the material has been de-gassed. Plus with age, a small amount of atmosphere can seep in, especially on glass tubes and where the ceramic to steel meets on a steel tube. Though on any old tube that finally does start to do this more, one should think about replacing them sooner or later. They could end up damaging the power supply.

Best,

Will


*********** REPLY SEPARATOR  ***********

On 10/2/05 at 2:21 PM exray at bellsouth.net wrote:

>Just a quick note
>
>You must stay within the anode dissipation rating.
>The powersupply MUST be able current limit!!
>The idea is to allow the tube to "hold-off" the operating voltage for a bit
>longer. This may extend the useful life a bit.
>All tubes spit(arc)...it's a fact of life. Most are little micro-arcs that
>go unnoticed. The idea is to reduce or eliminate the larger or more
>catastrophic ones.
>George
>----- Original Message ----- 
>From: "Will Matney" <craxd1 at verizon.net>
>To: <amps at contesting.com>
>Sent: Saturday, October 01, 2005 1:23 PM
>Subject: Re: [Amps] Outgassing tubes.
>
>
>> George,
>>
>> Yup, I have heard some call this gettering too which really isn't the
>correct term per say, but understand what they're trying to do. I always
>called it "burning them in", but it's the same thing. What I was getting
>at,
>someone had mentioned that a tube could getter itself by an internal arc or
>flash which is happening inside a gassy tube. Once a vacuum is lost to
>where
>a tube wont withstand it plate voltage rating, it's pretty much shot or the
>way I was always told. Even on receiving tubes, I remember what a gassy
>tube
>would do to a TV set, you'd have one heck of a picture going on. Of couse
>if
>gassy enough they'd flash around inside looking like a bunch of little
>flash
>bulbs going off. Those were the ones that the getter material on the glass
>had already turned white. Most of the transmitting tubes dont even have an
>internal getter though like the smaller tubes have. I guess they just
>evacuated the tube with a higher vacuum pump then let them bake in.
>>
>> Best,
>>
>> Will
>>
>>
>> *********** REPLY SEPARATOR  ***********
>>
>> On 10/1/05 at 9:36 AM exray at bellsouth.net wrote:
>>
>> >Wiil,
>> >
>> >For years it was and is common practice to outgas tubes by applying HV
>at
>> >low current i.e. one or two ma. till the tube arcs. Then reduce the HV a
>> >tad till the tube stops arcing. Leave the HV at this value for 30
>minutes
>> >or so. Then raise the HV slightly.
>> >
>> >Also try to run the tube at reduced HV and a heavy load. Get it as hot
>as
>> >possible. Then reduce the current to 1 or 2 ma. and raise the HV. This
>is
>> >a long process. BE certain to use a current limited supply.
>> >
>> >When you can hold off 20 to 30% more than rated HV your finished!
>> >
>> >George
>> >W4IW
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>>
>>
>>
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