How do your re-glue a plate cap?

[W5COA]

Howdy,

I did a search but couldn't find anything.....

I have managed to pull the metal plate cap off some old 811's and a couple of 6146's. I tried some hi-temp rtv which seemed to hold for a while, but it lifted today after I tried to remove the ceramic plate cap.

What was originally used? It is brownish and brittle. Too bad it only lasted 50 years.

I suppose that JB Weld would do the trick forever. I worry about differential expansion between the solder connection, glass seal, plate cap, etc.

Any successful recommendations will be greatly appreciated. Hate to have to scrap a good tube.

Jim

[W2PFY]

have you tried super Glue? I have used it on 811 caps and also used it on loose bases and it seems to work fine.

[WV Hoopie]

JB Weld!

[K5WLF]

Devcon 5-Minute Epoxy

ldb
K5WLF

[KA2DZT]

Devcon 5-Minute Epoxy

ldb
K5WLF

You need to use high temp epoxy.  J-B Weld makes a high temp epoxy.

I've had some regular epoxy break down at high temperatures.

Fred

[KA2DZT]

What was originally used? It is brownish and brittle. Too bad it only lasted 50 years.

Jim

I often wondered what type of glue or epoxy was used to hold tube caps and bases.

Not sure what it was but you think they would have used something better than the cheap 50yr-brown stuff.

Fred

[KA8WTK]

Like is stated above, regular epoxy will not work at high temperatures. The last one I did with a high temperature gasket sealer, then soldered the cap.
One other thing I do is try to expand the metal "grip" in the ceramic cap. I don't want it loose on the tube, but I find that some of the ones I have here fit so tight you really worry about breaking the tube when trying to remove them. These radios aren't mobile, in airplanes or going to vibrate a cap off.

Bill KA8WTK

[WBear2GCR]

The original cement is a phenolic based cement.
The formula can be found, but it is not a diy thing, imo.
There are a number of similar cements that are in use today.
One of them you can see mounting the glass to ceramic & metal pins on things like halogen lamps. This cement can be had, but likely only in small industrial quantities.

Epoxies get softer with temp... dont "grab" glass well.. ymmv...

cyanoacrylates also have temp issues, and don't grab glass that well... ymmv.

             _-_-

[W7TFO]

I've had very good luck with Dow 3140 flowable silicone RTV.  Grabs glass, doesn't crap out from heat.  Forms a strong but resilient bond.

This works best if the cap is not completely off, rather just rotating about the lead axis.

I invert the tube, make a dam out of tape, and flow it in until it won't hold any more.  

Let it set up, trim with an Exacto knife, done.

73DG

[WV Hoopie]

The plain metal spring plate caps are easier to work with. When pushing them on the toobs, leave some space between the top of the plate cap and the top of the toob plate connection.

Then use a small screw driver, insert between the two and give a little twist. The plate cap will come off without putting pressure on the toob plate connection and the glass! Can't do this with the porcelin types.

I've got an 811A with years in the Junkston "500" and J-B Weld has not lost its grip yet.

Craig,

[k4kyv]

I have used JB Weld as well as Crystal Clear 2-Ton epoxy.  The crystal clear stuff turned brown and then black, but the cap never came loose.  I used it on an 810 I was running in the modulator.

I never use those crappy ceramic or bakelite push-on plate/grid cap connectors, and promptly replace them whenever I pick up a piece of equipment that uses them. They appear to be expressly designed to pull the cap loose from the glass envelope of the tube when you remove them.  I prefer National "Grid-Grips", since they are made in the form of a clip that relieves the pressure from the cap in order to remove it.  They made them in numerous sizes from small receiving tube grid caps all the way to large transmitting tube plate caps. Another alternative would be the all-metal heat radiating caps held in place with a set screw, but you need to use those with caution too, since they can freeze to the cap just like a knob with a set screw may freeze to the shaft. I also have some 833A caps heat radiating caps that are split into two sections and held on the cap with springs.  Those are safe to use if you are careful to pry them loose from the cap before trying to slide them off.

With the Grid Grips, if they freeze to the cap, they can be safely loosened by inserting a small, thin screw driver and prying them off the cap without pulling on the cap.

[KA1ZGC]

Furnace cement. Don't bother with anything else. Bonds nicely to glass and metal, and has a very low thermal coefficient so expansion and contraction won't be a problem. Just scrape the old brown crud out, clean the solder from the top of the plate cap, pack the cap about 3/4 full, and stick back on the tube.

I reattached the plate cap to one of my 811 mod tubes with it, and have never seen a plate cap with a better bond. Will easily outlive you.

I even re-soldered the plate cap while the cement was still wet. Wear eye protection if you try that, because the water in the cement will boil and vent steam through the solder while the iron is applied.

If you wait for the cement to cure before soldering, make sure the plate lead is clean. Once the cement cures, you won't have much luck getting it off the lead.

[k4kyv]

Where do you buy furnace cement? I tried the local hardware store and they said they had never heard of it.

[KA1ZGC]

They may know it as "fireplace mortar", or they may just not have ever had any demand for it.

A Google search for "furnace cement" yielded no shortage of hits and information.

Given the small amount you need for plate caps, I would recommend a can instead of a caulk-gun cartridge. Cans reseal better, and once this stuff cures, you're never ever breaking it free again. Unlike RTV, you can't just jam a nail into the tip to clear out the bit that cured in the end of it.

The stuff I used was made by Hercules Chemical. They used to make it with asbestos (well-incorporated into the cement, so no risk of it becoming airborne), but looking at their latest literature, they no longer do:

http://www.herchem.com/specs/furnacecem.pdf

That's a very slight caveat, as the asbestos itself wasn't an active ingredient in the cement. I haven't tried asbestos-free furnace cement, but I'd be really surprised if it worked any worse for plate caps.

[W5COA]

Try a fireplace shop. The one here in Plano TX carries fireplace cement. It may be different from the caulk.

[KA1ZGC]

The only real difference is the color. So-called "light body" furnace cement is tan in color, whereas "heavy body" is gray or black. The stuff I used was gray.

Your typical home-improvement chain store probably carries it too.

[W1ATR]

Man I go through cases of the stuff every year. There's 6 5lb buckets to a case for about 8 bucks a bucket.

Maybe there's a market selling small film canisters of it to the audio phools for say, $52/ea.

The gray stuff for normal toobs and the tan colored "Oxygen free tube cap adhesive" for situations where melodious sonic transparency is required.

[W2PFY]

They may know it as "fireplace mortar", or they may just not have ever had any demand for it.

When you buy it, buy the smallest amount possible because it will dry out in no time at all. It comes in tubes and tins. The type I buy is used to glue on those fiberglass seals on your wood stove doors. Good Idea Tom. I never gave it a thought.

[k4kyv]

Does this look like the same stuff?

The smallest size is 8 oz (1/2 pint).  That would be enough to glue a lot of plate caps.

For purists, the tan would probably most closely resemble the original goo used by the tube manufacturers.

http://www.herchem.com/specs/furnacecem.pdf

[WBear2GCR]

Tom.

Good lead on that one... I suspect, but don't know that all furnace cements do not grab the glass. The old lamp cement depended on some chem combo with the phenolic to etch and grab the glass...

              _-_-bear

[W2PFY]

Does this look like the same stuff?

That's the stuff I use Don. Never tried it on tubes  but will soon as I can find a loose cap.

[KA1ZGC]

Don: Yes, that's not only the same stuff, it's the exact same link I posted. 

Bear: I also already wrote that it bonds very well to glass.

Let me just reiterate that I have an older asbestos-based version of their product. They no longer use asbestos. Asbestos wasn't an active ingredient, so the bonding properties shouldn't have changed.

Still, if anyone has a batch of their asbestos-free cement and a tube with a loose plate cap, I'd be curious to know how it works out, since I know I can't get any more of what's in the can I have (which should last me three lifetimes if I keep it sealed properly, but 10-1000 happens).

[w1vtp]

Finally a good answer to the loose cap (actually several good ones)  I'm getting me some of that magic stuff and fix at least a couple 6146's a 811a and a 866a

thanks, Al

[Todd, KA1KAQ]

Somewhere I have the recipe for the stuff RCA used, Larry Ware or someone provided it back in the 90s. I recall a big discussion of the same topic back then, and as Thom mentioned, there was an issue of differing expansion rates of some epoxies vs glass which caused problems for folks. Can't recall if it was super glue type stuff or something else, but a number of guys had failures around the top seal after applying it in a cap repair.

I wonder if the furnace cement is basically the same stuff as they used in muffler repair kits years ago? Black and somewhat gritty as I recall.

[k4kyv]

Somewhere I have the recipe for the stuff RCA used, Larry Ware or someone provided it back in the 90s.

Do you recall if it could be made from present-day easily obtainable materials?  If so, why not post it, if the black hole in your shack hasn't already sucked it up.