21AXP22 tragedy

[ohohyodafarted]

About 2 weeks ago I made another trip to Hawkey with 2 under vacuum
21AXP22's that were under vacuum. The purpose of my trip to Hawkey was to rebuild both tubes.

Unfortunately, something went wrong in Scotty's evacuation oven. BOTH, yes BOTH of my good under vacuum duds incurred a catastrophic failure of the glass face plate of the tubes. Both of my tubes were a total loss.

I am now short 2, 21AXP22's for my sets.

IF there is anyone out there who has any under vacuum 21AXP22 duds that they would be willing to sell me, please PM me. I need to find 2 more rebuildable tubes before Scotty closes shop.

Surely someone out there must have some under vacuum duds that they are willing to part with. I have a CTC4 Director, and a CTC4 Hallicrafter Clone that now both need a crt due to this tragedy.

Thanks in advance to anyone who can sell me an under vacuum dud or give me a referal to someone who has one that is willing to part with it.

Bob Galanter

[zenithfan1]

Sorry to hear that Bob, do you wanna take a close look at mine from the Wingate? Although I'm pretty sure it's down to air, but you can have it if you want.

[leadlike]

Sorry to hear about that, Bob-do 21ax's have a higher mortality rate during rebuilding, or did you just get a run of horrible luck?

[ohohyodafarted]

Mark, Thanks for the offer. I will take you up on your offer of the tube full of air. It is better than what I have. At least It will serve to fill the empty hole in a display set. And John and I are working on trying to find and seal leaks. We have purchased a Helium leak detector/ mass spectrometer to locate the leaks. Sealing them will be another whole issue.

Leadlike, Not sure what went wrong. Dont know if it was just bad luck, equipment malfunction or a human screw up. I have a pretty good hunch but can't say publically. None of these tubes have ever had a catastrophic failure using the slow 6hour ramp up/ramp down oven cycle at Hawkeye. And for 2 tubes to fail, both during the same oven cycle points a common problem. The tubes failed right when the oven reached 600F on the ramp up portion of the cycle, within 30 seconds of each other. Massive multiple cracking of the face plates of each tube indicating that the glass was under tension and not heating at the same rate as the metal bell of the crt.

So I am in need of rebuildable tubes to replace these that were destroyed. Any help will be greatly appreciated.

[kx250rider]

Very sorry to hear... I know how touchy those are, as I lost one that way when Dunbar rebuilt one for me. It made it all the way through the process, then imploded on the cooling rack (back in the 80s).

I haven't any duds (or any good ones for that matter)... But I'm sure you'll get some with a little patience.

Charles

[ohohyodafarted]

Charles,

Rate of heating and cooling is the key to successful rebuild of all crt's with a metal shell. You have to ramp up and ramp down very slowly. At Hawkeye, the oven cycle for a metal tube is 6 hours long, (as opposed to 3 hours for an all glass tube) using a very long and slow ramp up and ramp down. Then the tubes sit in the oven overnight to cool to room temperature. I suspect that Dunbar would have been succesful with your tube if he had just left it in the oven overnight to cool down instead of taking it out of the oven and puting it on the cooling rack.

I read somewhere that there was a seperate cooling down oven that was used on metal tubes after they exited the old pump down production lines, in order to bring the tubes down to room temperature at a very slow rate.

Bob

[jeyurkon]

Could be something as simple as a thermocouple gone bad in his oven too.

John

[ohohyodafarted]

Nope, not a thermocouple. Scotty has two independent temperature sensors in the oven. One drives the controller and there is a second that just monitors the oven temp. Both were in agreement within about 5 degrees. So 600F was pretty much right on the mark.

[WA3WLJ]

RE : I'M THINKING 30 HOUR BAKE - OUT

[Steve McVoy]

Bob, I'll check to see if I have any duds tomorrow.

[WA3WLJ]

[SIZE="4"]I NEED TO STEAL (BORROW AN OMEGA[/SIZE
Timer from WWORK !!]

[WA3WLJ]

Does Scotty drink or eat steak ?
TAKE Him out for dinner and let the tubes cool to ambient.....Slowly.

[freakaftr8]

I have worked on plenty of Omron temperature controllers. Yes, it is always possible for the controller to go faulty resulting temperature variations. Im not saying that this is what happened, but could alwayus be a possibility. If these two theromouples "most likely watlow type J thermo's" are wired to the same controller, of course.

Im sorry to hear about the CRT's Bob. Hopefully Mark's CRT will be a success!

[eberts]

I suggest you get out of your dream world and use a 21FBP22 Rare earth in those relics. 315C is not a critical temperture where tube faceplates fracture during processing.
Usually tubes that fail during the up "ramp" cycle have been exposed to rough handling in the past.
6 hours for a bakeout is too long, unless he is using a 350C peak temp for the metal tubes.
4 hour bakeouts with a peak of 385c for 15 minutes is good.
Tubes don't like it when they are in the oven, and you have a garage door open, even a small gust of wind will make them pop.

[drh4683]

Thats unfortuante. I would offer you a 21AX if I had an extra of any kind but my CTC-5 Westcott is the only set i have that uses this tube. That CTC-5 that I picked up at the estate sale last year (not trying to start anything here, just as a reference, it was the one that Mark and I got upset over) is in the hands of a TV guy in Milwaukee who probably won't do much with it. The cabinet needs alot of help but it has a good tube. You might be able to buy the whole set off him, either restore the set or use the crt. send me an email if interested at drh4683@gmail.com and I can get you his contact info.