[Amps] Outgassing tubes.

[Will Matney]

Ian,

Thanks as I didn't understand what you meant. I take it when this coating is heated, it traps the gas molocules by the reaction. Receiving tube getters absorbs the gas during the flash from a chemical reaction. That's why the silvery coating turns white when more air is introduced inside the envelope. I was servicing an old hybrid TV when I first started my shop years ago, around 1984. I had replaced the fuse and noticed what I thought an arc emitting from around the Horizontal output tube. When I fused it again, I seen it was inside the tube. It actually looked like small dull flash bulbs going off all over the inside of the tube. It didn't make any bang, or pop, just a slight tinkleing sound. After that first experience, I knew what to look for. Anytime though you see a receiving tube which the silvering is white, you might as well can it then as it's full of air. Generally, you'll find a crack in the glass around one of the pins or under some anode caps. Some slightly gassy tubes, this silvering will discolor around the edges where it's faint around the outside and silver in the middle. These generally will make the gassy light go off on a tester but I have seen some that didn't. I've replaced them anyhow and cured the problem.

Best,

Will


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

On 10/3/05 at 12:02 AM Ian White G/GM3SEK wrote:

>Will Matney wrote:
>>
>>Your correct about the differences! I've seen the innards of several 
>>types of transmitting tubes though and never seen a true getter like 
>>the ones used in receiving tubes. Where are they in them as I've not 
>>seen them. Generally you have a pin which connects directly to the 
>>getter where they use at the factory to flash it. They used to call 
>>these pins out on tube specs as NC or do not use. I imagine you could 
>>attach one to one of the elements but I've never seen that.
>>
>The basic principle of a getter is to form a chemical bond with stray 
>gas molecules. This is a two-way process because the chemical bonds are 
>often quite weak, so trapped molecules can also be released. Even so, at 
>equilibrium the getter holds the pressure down to an acceptably low 
>level.
>
>What the getter cannot do is instantly absorb a sudden release of gas. 
>Such releases can happen if impurities trapped within the structural 
>materials eventually diffuse to the surface. Even a small release can 
>cause a pressure spike if it happens quickly enough, and it only takes a 
>few microseconds to strike an arc. However, once the arc is over, the 
>chemical reaction will generally mop up the gas over a timescale of 
>several seconds, which is why these small releases can cause an arc but 
>still be completely recoverable.
>
>The "silvery" getter used in receiving tubes and small transmitting 
>tubes (807, 6146) is usually barium metal, which is very effective but 
>has too low a melting point for use in high-power transmitting tubes.
>
>In many glass transmitting tubes, the getter is the grey zirconium 
>finish on the outside of the anode. That material works best at high 
>temperatures, so the anode is a good location.
>
>Graphite anodes probably act as their own getter.
>
>In ceramic/metal tubes the anode runs quite cold, so the getter is 
>usually in the warmest available place, deposited somewhere on the 
>cathode structure.
>
>
>
>-- 
>73 from Ian G/GM3SEK
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