Loose Top Cap - UK Vintage Radio Repair and Restoration Discussion Forum

Adjuster · 12th Mar 2012, 9:47 pm

I have an EBL31 valve where the top cap terminal has come loose from the bulb, although the wire connection is still good. I would like if possible to save the connection from breaking off completely.

What sort of adhesive might be suitable for this - an epoxy like Araldite? Bear in mind that as this is an output valve, the bulb will normally get quite hot.

Alternatively, should I simply get a new valve? I have no access to a valve tester, but the sound seems adequately loud and not particularly distorted. I would prefer to avoid spending money unnecessarily.

Phil G4SPZ · 12th Mar 2012, 10:26 pm

The cement originally used to stick top caps and bases to glass valves was, I believe, a mixture of shellac and wood dust, although my memory may be wrong. It has certainly been discussed on this forum before.

I have successfully used Araldite on a number of occasions, although in my case the top caps had come off completely rather than being just loose; in your case, you may have to devise a method of injecting some Araldite into the tiny gap between the loose cap and the glass envelope, or alternatively grasp the nettle:

1. Unsolder the top cap, remove it and clean away any excess solder
2. Apply Araldite and glue the cap securely to the glass envelope
3. When the Araldite has cured, quickly resolder the wire to the cap

Good luck!

ThePillenwerfer · 12th Mar 2012, 10:48 pm

The original cement was indeed shellac and wood floor. Epoxy will be fine but I've read somewhere that you must NOT use superglue as it will crack the glass when it heats up.

- Joe

12th Mar 2012, 9:47 pm	#1
Adjuster Retired Dormant Member Join Date: Jan 2010 Location: London, UK. Posts: 78	Loose Top Cap I have an EBL31 valve where the top cap terminal has come loose from the bulb, although the wire connection is still good. I would like if possible to save the connection from breaking off completely. What sort of adhesive might be suitable for this - an epoxy like Araldite? Bear in mind that as this is an output valve, the bulb will normally get quite hot. Alternatively, should I simply get a new valve? I have no access to a valve tester, but the sound seems adequately loud and not particularly distorted. I would prefer to avoid spending money unnecessarily.

12th Mar 2012, 10:26 pm	#2
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: Loose Top Cap The cement originally used to stick top caps and bases to glass valves was, I believe, a mixture of shellac and wood dust, although my memory may be wrong. It has certainly been discussed on this forum before. I have successfully used Araldite on a number of occasions, although in my case the top caps had come off completely rather than being just loose; in your case, you may have to devise a method of injecting some Araldite into the tiny gap between the loose cap and the glass envelope, or alternatively grasp the nettle: 1. Unsolder the top cap, remove it and clean away any excess solder 2. Apply Araldite and glue the cap securely to the glass envelope 3. When the Araldite has cured, quickly resolder the wire to the cap Good luck! __________________ Phil Optimist [n]: One who is not in possession of the full facts

12th Mar 2012, 10:48 pm	#3
ThePillenwerfer Octode Join Date: Mar 2011 Location: Sheffield, South Yorkshire, UK. Posts: 1,453	Re: Loose Top Cap The original cement was indeed shellac and wood floor. Epoxy will be fine but I've read somewhere that you must NOT use superglue as it will crack the glass when it heats up. - Joe __________________ "Experts are only good at one thing: explaining why something will not work!"