Nail polish or acrylic glue to repair old tubes

I have some questions regarding the repair or the prevention of damage in old tubes.

1) If the grid cap or anode cap is turning around the glass the vacuum is lost?

2) If the bakelite on the base has a slight movement the vacuum is lost?

Can I repair these movements (and prevent further damage if the vaccum is still present) with nail polish or even that kind of super glue (acrylic)?

I will attach some pictures.

Thanks

Alvaro Ernani Georg

Rio de Janeiro - Brazil

YouTube Channel: Rádios Antigos & Cia Ltda

• Grill cap (217 KB)
• Bakelite base (158 KB)

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Greetings Alvaro.

My answer to your two questions would be to say that, if you're lucky, the vacuum will not be lost unless the glass envelope, or the top cap, has been turned through several degrees or mishandled. So, what to do to repair? First, I would not use nail polish or super glue as neither of them stands up to high temperatures. I believe this tube is an RF/IF pentode, so it may not become really hot in use.

1. The top cap problem. You will likely be able to desolder the top cap if you can see solder at the very top. Use an iron with a relatively large bit so you can do the job quickly in a few seconds. Discard the old solder in the cap and you should have a top cap with a small hole in it. My recommendation for adhesive or sealant would be an RTV (Room Temperature Vulcanizing) silicone designed for high temperatures such as is used to seal valve cover gaskets in automobiles. Although it's designed as a sealant, it should provide enough strength to hold the top can firmly once it has cured. You could also use a two part epoxy adhesive, preferably one that sets over 24 hours as it will likely stand up to the heat better than a five-minute cure type. Just remember that unless you can find a superlative epoxy, such adhesives are not quite as good as silicones for heat tolerance. Once the top cap is secure, resolder the cap to the wire coming out of the top of the tube with a quick dab of solder.
2. For the base, if you can inject the silicone, then it should do a good job and any excess is easy to clean off. Epoxy adhesive flows more easily and could likely do a better job if the base of the AF7 doesn't become too hot.

In both cases, use a Q-tip or small paint brush to clean all parts with iso-propanol before applying the sealant or adhesive to improve the bond.

These are just suggestions. Perhaps other members can provide simpler or better techniques.

Kind regards,

Stan

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Many thanks Mr. Roberts,

I do have two silicone sealants, I guess that you are referring to the black one for automotive use and not that one for electronics use (see the pitures attached).

In fact, I have two tubes with problems, the first one is a rectifier (AZ1). On that one, I did try to repair the base with the nail polish. When I did put it in place, there were some sparks and the smell of something burning (problably the nail polish burning). I am not using it anymore, because I am worried about some problem of short circuits on the connections. It will be on reserve for the new one that I did put in place now.

The second one is the AF7 (which is a pentode) and has the problem on the cap.

You are saying that I should desolder the cap, pull it out and put some silicone or epoxy inside of the cap (I mean on the inside of the cap but not on the top of it), put it on the glass again and make a new solder on the top. That is correct?

Thanks again,

Alvaro.

• Clear silicone (251 KB)
• Solder on the grill cap (197 KB)
• Black silicone (262 KB)

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Hi Alvaro.

Yes, I would use the sealant designed for automotive applications because it will have a higher temperature capability than the 200 deg. C limit of the white one.

Concerning the top cap, yes, desolder it carefully and quickly and underneath you will typically see the connecting wire protruding through the glass envelope via a glass nipple. Don't try bending the wire as it may chip off some of the glass, which will have become brittle over the years. The cement that was used originally will likely remain in the top cap. I would just clean that with alcohol so that the silicone sealant can key itself to that cement as the sealant oozes up into the top cap. Don't overdo it with too much sealant to avoid getting it on to the protruding wire. Then replace the cap carefully and let the sealant set. Again, make sure that no sealant covers the wire. Once set, put a little flux on the wire and solder it back to the top cap.

Builders' construction adhesive may come to mind as a substitute as it will stick to brick, drywall, wood and even ceramic, but I've found that it has little tolerance for high temperatures.

Kind regards,

Stan

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Thanks again Stan.

I will do that.

Alvaro.

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If I remember correctly, the solution to attach the caps to the glass envelope was waterglass, or sodium silicate, mixed with additives (fillers). Make sure that whatever glue you use does not shink while hardening, or if it does, to not get into a situation where e.g. a ring of glue excerts pressure on the envelope.

You can also search our forums for "wasserglas" which will yield a number of (German) forum entries. Some interesting information e.g. in this thread.

Regards
Mark

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Vielen dank Herr Hippenstiel.

Alvaro.

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Hi Alvaro,

a lose top cap or bakelite base does not affect the vacuum of a tube, as long as the glass bulb is undamaged.

If you want to fix a lose bakelite base of a tube that has a conductive coating, such as the AF7,
make sure that the grounding wire protruding from the base will be connected to the conductive coating again. First clean the grounding wire and then use conductive paint to make the connection.

Best Regards,
Jacob

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Vielen dank Herr Roschy.

Alvaro.

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