If a picture tube loses its vacuum...

[John Marinello]

Yea... How do you prevent a metal CRT from imploding while in the oven?

[julianburke]

21AXP22's have to be put into the oven even when new which we did, and had a 80% success rate with no problems. They went through the same process as a 21FBP22. We never did any 15GP's (too old then) and they were not a good item in the '60's.

[John Marinello]

Monochrome = black & white. Those mentioned are tri-color tubes

[julianburke]

B & W tubes are rebuilt the same way as color. All you are doing in the process is installing a new gun mount and evacuating the glass bulb.

[John Marinello]

Any experience in rebuilding metal B&W tubes? I was referring to the high failure rates with these while in the oven.

[Tom_Ryan]

Originally Posted by John Marinello

Yea... How do you prevent a metal CRT from imploding while in the oven?

Ans. You prevent metal CRT from imploding through a very careful control of heat during the annealing process when it is in the oven.

There are different glasses and metals used in the manufacture of all CRTs. Each will have a different coefficient of expansion (rate at which physical dimensions change as heat is applied or removed). The trick is to match the coefficients between glass to metal or glass to glass so the different materials that are joined together have a similar coefficients of expansion. Otherwise, the materials joined expand at different rates and break apart from each other. The problem is compounded a bit with older CRTs. An old CRT has had time to "age" in a way that can affect chemical changes to glass to metal seal or glass to glass frit seals (15GP22 is a case in point) that may make it impossible to re-apply heat to those joints years down the road without causing them to fail. The secret to success is to find the minimum acceptable temperature and cycle time, or repetition of annealing cylces thereof, to safely anneal the join without inducing failure to other joints. A lot of experience and knowledge is necessary to determine the best cycling time and annealing temperature to achieve acceptable results so the tube won't fail. For the rebuilder it often means selecting a low temperature gradient and lengthened cycle time to minimize thermal shock.

Annealing used during the rebuilding of a CRT is not the same as the annealing process used during the original manufacture. That's why most CRT rebuilders either discourage this business or avoid it entirely. They simply may not know the correct annealing parameters. This is not really the fault of the rebuilder - even the manufacturer may not know without extensive resaearch. What's really needed is an exceptional understand of the material science and chemistry of the problem. Two very difficult skill sets to find in any practitioner with a sound understanding of vacuum technique.

The second reason to apply heat is to help the evacuation process. But this is a double edged sword. High heat destroys seals and may cause electrical feed throughs to fail. However, unlike the manufacturing process, the CRT rebuilder needs to take more time, often because the residual gas content of any old tube is hard to determine, unless RGA (residual gas analysis using a mass spectrometer is employed). This kind of anyalsis is often way beyond the ability of any rebuilder. Avoiding high "outgass" temperature is crucial to preventing implosion due to the mechanical failure of the envelope or failure of the seals. Typically in the world of restoration time is on our side. So, use the lowest heat possible and simply pump under vacumm for the longest possible time, and then seal the tube and flash the getter is often the best approach. Rebuilders who try to hurry the pumping process only short change the life of a rebuilt tube.

Rebuilding any tube is really a process of situational awareness involving knowledge of structurual and chemical engineering. Hidden pitfalls lurk everywhere. Breathing life back into any old tube can be a unique challenge every time.

[julianburke]

It's built the same way as a glass tube. We actually had less trouble with the metal tubes than the glass ones.