Rebuilding Metal Cone CRT's - Videokarma.org TV - Video

Duke Nukem · #1 10-20-2009, 04:42 PM

Kinda prompted by another thread, and not being that familiar with early colour CRT's, I get the impression that CRT's with large areas of glass-metal seals whose seals have leaked tend to be suspect candidates when it comes to rebuilding ?

I ask because the UK had a brief flirtation with B&W metal cone CRT's and replacements were pretty much unobtainable even by the end of the 50's; so far I only know of one person that has a working metal cone CRT. So, what do you think the probability of rebuilding metal cone tube rebuilt *and* the result last more than 6 months ? I must admit I'm amazed they ever worked at all.

I know the previously mentioned colour tube isn't metal cone, but it does have a large glass-metal seal doesn't it ?.

TTFN,
Jon

ohohyodafarted · #2 10-20-2009, 07:02 PM

Actually if a metal cone crt is still under vacuum it is every bit as rebuildable as an all glass crt.

The trick is that the glass on the face of a metal crt is much thinner than the face of an all glass crt. As a result it is not as strong, and is prone to cracking during the rebuilding process due to heating and cooling to rapidly in the evacuation oven.

It has been determined that if you raise the temperature of the evacuation oven very slowly and cool it down very slowly, so that the stresses of expansion and contraction can equalize, the tube can be rebuilt successfully. The whole oven process will take about 6.5 hours if done slowly. The expansion coefficients of the metal and the glass are different and rapid heating and cooling will cause the face of the crt to crack.

Metal tube rebuilding was a very common occurance in the US back in the 50's. But back then the rebuilders would treat a metal tube just like an all glass tube. Some made it through the oven ok and some cracked. There was such an abundance of the metal tubes at the time it did not mater. They just threw the ones away that cracked and tried to rebuild the next one. Today, the tubes are too scarce to chance a rapid heat up and cool down, so Scotty uses a very long 6.5 hour cycle and has very good success. He also charges more for a metal tube because of that extra long cycle.

old_tv_nut · #3 10-20-2009, 08:42 PM

Just to be complete - IF the tube is under vacuum, the prospects are good. However, if gone to air, it is iffy. The experiment I paid for with an aired 21AXP22A did not come out successfully. (It did not break, but went back to air slowly over several days.)

Duke Nukem · #4 10-21-2009, 12:25 PM

Yep, what I was trying to get to was confirmation that if a metal cone tube had gone to air a rebuild was less likely to be successful.

In the UK, the metal cone was a short lived solution for larger screens (generally 16"), very soon replaced by all glass 17" designs, so the metal cones are quite rare. Despite being rare, the ended up in lots of console sets that aren't particularly valuable today so the tubes aren't really worth rebuilding. However HMV did an oddball 21" metal cone model and that would definately be worthwhile rebuilding, but if success is unlikely as tube had already gone to air then again not worth trying.

TTFN,
Jon

***Eric H*** · #5 10-21-2009, 01:04 PM

Metal cones were popular here too in the late 40's early 50's.
There's at least one 8" type, several 16" the 21AXP4 color tube and of course the 30BP4 used in the Dumont Royal Sovereign.

It would seem that size matters in how well they hold vacuum, all the 16" tubes I've had including a 50 year old, NOS in the box 16AP4 were just fine, many of the 21AXP4's seem to have gone to air as well as a lot of the 30" tubes.

There were some rectangular tubes with metal cones too, I don't know their numbers off hand.

The plus side of the metal tubes is they are very light, also the face plates are precisely made so there's no distortion.

Someone here had a 21AXP4 successfully rebuilt, it was under vacuum to start with though.

jr_tech · #6 10-21-2009, 01:16 PM

Did the 21AXP22 use the same method of attaching the front section of the welded flange to the panel as the 15GP22 did?

jr

David Roper · #7 10-21-2009, 03:17 PM

Nobody mentioned the good ol' 19AP4? That's the big tube used in Zenith porthole sets of 1951 and many other sets. A couple of rectangular metal tubes I've encountered are 17CP4 and 21MP4. 21AP4 is also a metal tube.

ohohyodafarted · #8 10-21-2009, 04:05 PM

21AXP22 and 15GP22 are completely different animals.

The 21AXP22 has the phosphors deposited directly onto the inside of the faceplate and the shadow mask is mounted right behind the faceplate and attached to the rim of the front assembly. The metal funel of the rear section is heliarc'd to the metal rim of front section. Unlike the 15G, the metal components on a 21AXP22 are a single layer of metal that has no metal to metal laminations that could leak. When the front and rear sections of the 21 are brought together the flange that sticks out, is clamped together and then heliarc'd. The weld produces a very good vacuum seal.

Leaks on the 21Axp22 likely occur where the glass neck assembly joins the funnel, and where the faceplate is fused to the front flange assembly.

jr_tech · #9 10-21-2009, 05:21 PM

"Unlike the 15G, the metal components on a 21AXP22 are a single layer of metal that has no metal to metal laminations that could leak."

Thanks! I had forgotten that the 21AXP22 had a metal "skirt" around the faceplate... I was thinking that it was an all glass panel back to the flange. I remember reading or hearing somewhere that RCA shipped carloads of these front and rear assemblies across the country (to Varian?) to be welded together... can anybody confirm that?

jr

Duke Nukem · #10 10-21-2009, 06:19 PM

Quote:

There were some rectangular tubes with metal cones too, I don't know their numbers off hand.

I'd totally forgotten that there had ever been rectangular metal cone CRT's, but I now remember that RCA introduced one (the first?) towards the end of 1951 with a 14.5x11 inch screen area; but the UK were already starting to use all-glass CRT's not long after round metal cones appeared over here so never got to see a rectangular B&W metal cone. If anyone has a picture of one (particularly that first RCA one) I could shamelessly steal for my website it'd be much appreciated.

TTFN,
Jon

nasadowsk · #11 10-21-2009, 07:46 PM

I don't have a picture of one but they are truly freaky things to look at, just because they look so...weird...

old_tv_nut · #12 10-21-2009, 09:51 PM

Here's a1952 ad for the rectangular one, and some earlier ads for the round ones.

mikeh · #13 10-21-2009, 10:01 PM

This is a metal cone crt from a Hallicrafters tv.

***Eric H*** · #14 10-21-2009, 10:06 PM

My Avatar picture is from a 16AP4 metal tube in an RCA set.
It's probably the sharpest picture I've ever seen on a vintage set.

bgadow · #15 10-21-2009, 10:31 PM

Maybe the sharpest picture I have is on a rectangular Admiral with metal tube. Could there be something to that, having better focus with a metal funnel?

Should a good 21AXP22 be vacusealed? Or doesn't it really matter?