8000's with loose plate caps

[KE6DF]

I just received two 8000 tubes which look to be unused, or close to it -- getters silvery, glass nice and clear, base and pins not scratched.

The problem is, the plate caps are loose.

As you all know, on an 8000, the metal plate cap is separated by a ceramic piece that touches the envelope.

I realize this question has been asked before in various contexts, but what should I do?

One possibly is to just be careful with them and do nothing.

Another is to glue the caps in place so there won't be a risk of breaking the plate seal by the cap moving around in handling or connecting the tube.

Normally when gluing a plate cap, you run into the issue that the metal cap has different thermal expansion than the glass and so when glued at room temp you end up with stress on the glass at high operating temps. 

But in this case, I would be gluing the ceramic washer piece and it probably has closer expansion characteristics to glass (perhaps that's why it was used in the first place).

What do people suggest? 

If gluing it is the answer, then do I just use super glue? Or is there some kind of high temp glue that is findable for this purpose?

8000's are kind of rare so I want to treat them right. 

[KM1H]

Furnace cement is a close cousin to what was used and Ive used it on many RX and TX tubes over the years. That includes a few that got rather orange with no ill effects.

Carl
KM1H

[WBear2GCR]

Furnace cement, IF it hardens may work ok.
There are some that do not harden.

You really want "lamp cement" - finding a ready source could be daunting.
A web search might turn some up.
If I recall it is a phenolic compound in part. It grabs glass.

One other solution is to use "flowable" silicone rubber.
It is sold in auto parts stores for use on windshields.
This stuff works well on tube bases.
I think it would work ok on the tube cap as well.

          _-_-bear

[K9ACT]

8000's are kind of rare so I want to treat them right. 

The most important thing is that you make RF with them.  That would make you a member in a very exclusive club.

As far as I know, there is only one other ham (K9WEK) aside from me, actively using 8000's in a transmitter.

I built mine from scratch (see my Radio Page for details) and he converted his Globe King to run a pair.

js

[k4kyv]

I used to use a pair in my rf final, but I finally wore them all down to the point that I couldn't find a pair that would come anywhere close to matching in the push-pull output stage.  Right now I use Amperex HF-300's that seem to be even more rare than the 8000's.

I have never seen the metal part of the plate cap come  loose from the ceramic ring, but the whole assembly does easily come loose from the glass.  If you are very careful with it, you might get away with doing nothing at all.

I have had good luck using epoxy.  I once glued a cap back on a 810 using Two-Ton Crystal Clear epoxy.  It worked well, but after a while, turned black.  I have also used JB Weld.  I have never had one break the glass due to thermal expansion, but I am  not saying that couldn't happen.

I'm not exactly sure what "lamp glue" or "furnace glue" is, but I assume they are adhesives designed to withstand high temperatures.  There must be someone out there who knows what the stuff is that tube manufacturers originally used, and if it might still be available from somewhere.

[KE6DF]

Don, Interesting you mention JB Weld. I went to Lowes looking for furmace glue
but ended up getting a tube (actually it comes in two tubes you have to mix)
of JB Weld. The package says it holds up to 600 degrees F.

I'll put it on the plate caps tomorrow.

Also, it is the ceramic part that has come loose from the envelope. The metal part
 is still attached to the ceramic.

If I had a working rig that needed new tubes so I could just drop these in and go
I'd be tempted to leave them alone. But I have to home brew a rig so I'll no
doubt have to take tubes in and out and mess around with changes so I  think
I'd better glue them.


Dave

[KX5JT]

This somehow seems pertinent.... if nothing else certainly interesting:

http://home.earthlink.net/~lenyr/vt-vac.htm

http://www.youtube.com/watch?v=gl-QMuUQhVM

http://www.youtube.com/watch?v=9S5OwqOXen8

[KE6DF]

The post about vacuum pumps and tubes is interesting.

I recently scored a 805 tube off ebay. I often put what I call low "stink bids" on interesting stuff and usually get outbid.

I put a $10 on a couple 805s, and for some reason none of the Audio hobbyists saw them and I won.

In taking one of them out of the wrapping, my hand slipped and I "tonked" it on my bench. Didn't break the envelope, but I must have broken one of the base seals or maybe the plate seal because I watched the getter turn white over a 10 minute period.

I was so angry I almost smashed it, but I put it away and still have it.

Maybe at some point in the future people will find it worthwhile to recondition gassy tubes like that. Who knows.

But at this point, it will make a nice tree ornament next Christmas.

[W1RKW]

I've epoxied (JB welded) plate cap on smaller transmitting tubes without any issue. Granted they are peanut whistle tubes in comparison to the 8000 but the glue held up. Maybe the key is just to use enough to hold the cap in place and not glop it on the glass. Keep the glue underneath the plate cap ie. minimal surface area.

[W7TFO]

I use the flowable silicone for many tubes that get loose caps.  Always holds, easily trimmed excess.  Epoxy is tough to apply, whereas the silicone will flow underneath and set up fine.   If I do nothing and use them, I tend to forget and yank the thing apart when pulling a cap off.

BTW, HF300's are great tubes.  A 204A with a regular base!

[KA1ZGC]

I just re-attached an 811 plate cap last night with good ol'-fashioned asbestos furnace cement. No mommy-coddled goof-offs in this outfit. 

It takes quite some time for the cement to cure. I got impatient and started slobbering before all the moisture had escaped. Had to hit it four or five times over the course of a few hours to get it to stop venting through the slobber cup every time I hit it with the iron. I strongly recommend wearing glasses when you first put the iron to the cap if you go that route. It will spit molten slobber right in your face quite readily.

Honestly, I've never seen a bond so strong as this one. This tube will never give me trouble again (which is good, because this is the second time I've had to re-cap this tube, and it probably wouldn't survive a third). Whatever they've been using for the last 40 years or so doesn't hold worth a damn after a few decades.

Ran the 811 all last night after the repair, and it works like a champ.

"Be careful and do nothing" is not recommended. Eventually, you'll break off the piece of wire the cap is soldered to, and a perfectly good tube will be junked for no reason. Better to get it cemented on there somehow. Like you said, 8000s aren't something you find in every trash can these days.

[k4kyv]

I once repaired an 810 in which the wire had broken off even with the little glass tit on the top of the tube.  I used a dremmel tool, with a miniature grinding tip something like what a dentist uses.  I carefully ground away just enough glass to get a  little bit of wire sticking out, without destroying the seal.  I tinned this wire and placed the plate cap back over it and soldered.  I tested the tube to see if it worked and it did.  Then I unsoldered the  cap, put a small amount of epoxy under the loose cap, and after the epoxy had hardened, re-soldered.  I am still using that tube.

Where would I go to buy some asbestos furnace cement?  Wouldn't anything containing asbestos be like trying to buy carbon tet, lead paint or something containing mercury?

[KA1ZGC]

Where would I go to buy some asbestos furnace cement?  Wouldn't anything containing asbestos be like trying to buy carbon tet, lead paint or something containing mercury?

I would expect so. This can's about the same age as me, probably.

Hercules Asbestos Furnace Cement, made by the Hercules Chemical Company of New York. It's a 1 pound can. The price was hand-written on the can, $.59. Couldn't find any other indication of its age (aside from the ample rust on the can), but that gives you some idea.

Mind you, the biggest danger from asbestos comes from breathing its dust. This stuff's pretty well saturated in a grayish paste medium, so I doubt it was ever much of a hazard to anyone.

We had a five-pound can of more run-of-the-mill furnace cement, but that wasn't properly sealed, so the cement cured in the can. The asbestos cement was still perfectly pliable and usable, so I plied and used it.

I suspect any furnace cement will work equally well. It's not like the asbestos is helping any more than it's hurting. It's got one hell of a bond, though. Timtron highly recommended it (the cement, not the asbestos), and he was right: the stuff really has a death grip on both the cap and the tube.

[W3SLK]

Tom said:

I suspect any furnace cement will work equally well. It's not like the asbestos is helping any more than it's hurting. It's got one hell of a bond, though. Timtron highly recommended it (the cement, not the asbestos), and he was right: the stuff really has a death grip on both the cap and the tube.

Google is your friend. Hercules still makes furnace cement. I was looking at their MSDS and it looks as though they substituted silicate for the asbestos. I would suspect any plumbing/heating supply shop would have it in caulking tube.

[KM1H]

I told y'all that the furnace cement was good stuff. These days Im using Harveys Part 045020 and its also got sodium silicate which is closer to what tubes use anyway. Did a couple of 572B's not too long ago but have used one brand or another for many decades. Hasnt failed me yet

The stuff requires heat to cure. Use your favorite method to keep pressure on the cap and place in rig, toaster oven or the big oven.

Carl
KM1H

[KA1ZGC]

Google is your friend. Hercules still makes furnace cement. I was looking at their MSDS and it looks as though they substituted silicate for the asbestos. I would suspect any plumbing/heating supply shop would have it in caulking tube.

I figured they probably still did. I'll never use up this can just doing tubes, so I haven't bothered to look for another source.

Silicate makes sense as an asbestos substitute, though I'd think it would want to act as a dessicant as well.

For tube repairs, I'd still want to buy it in a can. You can reseal a can better than a caulking tube. With exposure to enough air over enough time, the stuff will eventually cure, even if not vulcanized. Once it cures, it's like concrete. You only need a half-teaspoon or so to reattach a plate cap.