Conserving our good CRT's

[jr_tech]

Originally Posted by old_tv_nut

My understanding is that the getter does nothing after its first firing.

No, the barium surface deposited by the getter on the neck or funnel is a long term active pump, that will maintain the tube at a low pressure until it is depleted. All tubes leak, but the barium surface can insure many years of decent vacuum.

jr

[Tubejunke]

Originally Posted by jr_tech

All tubes leak, but the barium surface can insure many years of decent vacuum.

I have never heard this before. Now it is common knowledge that some of the early metal/glass tubes (19AP4) developed leaks over the course of many years, but I am compelled to ask the basis behind all vacuum tubes leaking when there are so many around from the 40s-60s that are as strong and void of air or gas as they were when new.

If they do leak, then it must be immeasurably slow. Too slow for our lifetime. My 1920s radio tubes (O1A and the like) are holding up well too. This might be an interesting topic.

[jr_tech]

Originally Posted by Tubejunke

I have never heard this before. Now it is common knowledge that some of the early metal/glass tubes (19AP4) developed leaks over the course of many years, but I am compelled to ask the basis behind all vacuum tubes leaking when there are so many around from the 40s-60s that are as strong and void of air or gas as they were when new.

If they do leak, then it must be immeasurably slow. Too slow for our lifetime. My 1920s radio tubes (O1A and the like) are holding up well too. This might be an interesting topic.

My major point was that the barium deposited by the getter flash continues to function to remove gas in a sealed off CRT for many years after it is flashed. There are several sources of this gas.
1. Incomplete processing... some surfaces may still be outgassing.
2. Virtual leak... gas trapped in very fine porosity of the CRT materials that may come out.
3. Real leaks... such as around base pins, tip off seal, frit joints.

Indeed, with decent techniques these are very small sources of gas...but get back to me in 100 years or so about the condition of your tubes.

jr

[old_tv_nut]

Originally Posted by jr_tech

No, the barium surface deposited by the getter on the neck or funnel is a long term active pump, that will maintain the tube at a low pressure until it is depleted...

jr

Thanks - I learned something!

[dtvmcdonald]

Originally Posted by jr_tech

No, the barium surface deposited by the getter on the neck or funnel is a long term active pump, that will maintain the tube at a low pressure until it is depleted. All tubes leak, but the barium surface can insure many years of decent vacuum.

jr

Barium is not exactly a great getter. Its great for oxygen and water,
but that's about all. Its totally useless for nitrogen, hydrogen argon, helium,
and hydrocarbons. I'm not sure about carbon monoxide.

The gasses that ooze out of hot nickel, iron, molybdenum or tungsten are mostly hydrogen and carbon monoxide. I've seen this over and over
again in vacuum systems with mass specs in them. I had one that
stayed sealed for over ten years and after initial bakeout and
pumpdown that's what I saw.

For vacuum tubes with soft glass envelopes you will always see
Argon developing due to radioactive decay of potassium in the glass.
For tubes with thoriated tungsten filaments of course you will see
Helium developing.

The great getter is Titanium. It will getter anything except rare
gases and methane. That sealed system used a titanium based
ion pump. I believe that large transmitter klystrons still use such pumps.