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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 1st Apr 2019, 10:45 am   #1
David G4EBT
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Default Refixing loose valve bases

From time to time, the topic of how to re-attach loose valve bases crops up, often with comments on what not to use, rather than what works best. The issue is that as the glass heats up, it expands and with some adhesives, notably super-glue, it's claimed that the glass can crack due to the rigidity of the bond.

I've converted quite a lot of NOS EBL1s to EBL31s (electrically identical apart from the bases and top caps) for use in Ekco A22s, by removing the side contact base and 3/8" top cap and fixing an octal base and 1/4" top cap. To do that I've used two-part epoxy known as 'Super Steel' which sets rock had and it's never posed a problem. After all, the original adhesive used by valve manufacturers also set rock solid. That cement consisted of shellac, wood flour, stone dust and other ingredients. (It can be dissolved with meths, known outside the UK as 'de-natured alcohol').

A professional woodturner (Emma Cook, known as 'The Tiny Turner' as she's just 4' 10" tall), recently demonstrated at our woodturning club. The item she made consisted of glass bauble with wooden finials top and bottom. The glass globe was about 75mm diameter, illuminated internally by an LED 'string'.

The relevance to this thread is that she'd tried a range of adhesives to fix the wooden finials to the glass globe, the most obvious of which was 2-part epoxy glues such as Araldite. She discovered that due to the expansion and contraction of the wood, which occurs naturally, the rigidity of the adhesive resulted in the glass cracking.

She did a lot of research and discovered that the best glue for this purpose - which has completely eliminated cracking - is 'Serious Glue' by Evo-Stik, which dries transparent and as it cures, forms an elastic rubber bond. It's available in 33 gram tubes and can be bought at Halfords, than it costs half as much again as on Amazon.

https://www.amazon.co.uk/EvoStik-663...96492859&psc=1

Here's a quote from the maker's description:

"High performance, single component, moisture curing, Modified Silane (MS) Polymer based adhesive. Will bond most surfaces even if both are impervious. Will bond damp surfaces and may even be used underwater. It has a low odour and cures to form an elastic rubber, with almost no shrinkage and low hazard bonds to most surfaces quickly".

End quote.

Below is a picture of the glass/wood baubles and the glue.

Hope that might be of interest, though I guess it risks sparking off a 'beauty parade' of the most efficacious glues!
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Old 1st Apr 2019, 10:55 am   #2
Reelman
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Default Re: Refixing loose valve bases

Thanks for the information David since this is a problem we are all likely to come across at some time or another. I shall have to get some “serious” glue and try it. The last thing you want is to glue your PX4 and have it crack!

Peter
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Old 1st Apr 2019, 12:11 pm   #3
GrimJosef
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Default Re: Refixing loose valve bases

Just a couple of comments - Henkel, who make it, claim Super Steel is metal-loaded. So whatever you do don't let it run down inside the valve base and touch the valve wires or pins. Serious Glue is rated, again according to the manufacturer https://diy.evo-stik.co.uk/sites/diy...rious-glue.pdf, as good for temperatures up to 130C. Hard-working power valves can get hotter than that though.

Cheers,

GJ
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Old 1st Apr 2019, 2:40 pm   #4
David G4EBT
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Default Re: Refixing loose valve bases

Quote:
Originally Posted by GrimJosef View Post
Just a couple of comments - Henkel, who make it, claim Super Steel is metal-loaded. So whatever you do don't let it run down inside the valve base and touch the valve wires or pins. Serious Glue is rated, again according to the manufacturer https://diy.evo-stik.co.uk/sites/diy...rious-glue.pdf, as good for temperatures up to 130C. Hard-working power valves can get hotter than that though.

Cheers,

GJ
Thanks for reading and for your comments GJ.

Yes, they say it's 'metal loaded' but it depends on one's definition of 'loaded'.

Before ever I used if for such purposes as re-fixing valve bases and re-stuffing capacitors, I applied 'Super Steel' to a strip of glass 50mm long x 15mm wide, then tested it on my Victor V60B insulation tester at 1,000V. The tester will show leakage up to 200M Ohms. As will be seen from the pic below, (with the strip in the foreground connected to the test leads), far from being conductive, 'Super Steel' makes an excellent conductor!

When I've converted EBL1s, it's necessary to extend the wires inside the octal valve base so I sleeve them to obviate the risk of them shorting to each other.
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Old 1st Apr 2019, 4:39 pm   #5
ColinTheAmpMan1
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Default Re: Refixing loose valve bases

Quote:
Originally Posted by David G4EBT View Post

far from being conductive, 'Super Steel' makes an excellent conductor!
Insulator, surely?

Colin.
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Old 1st Apr 2019, 4:48 pm   #6
kalee20
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Default Re: Refixing loose valve bases

I've used epoxy (Araldite, not Araldite Rapid) to glue valve bases in the past, without problems. However, I'd prefer something with a little bit of compliance. My thoughts would be silicone, RTV with a neutral cure (not the acetic acid curing, bbath sealant stuff), thixotropic. RS and Farnell sell a decent selection.

Silicones stand high temperatures well. And they have a bit of 'give'. Even if used on a low-power valve, where temperature excursions, differing thermal expansion rates, don't 'bite' there is still the matter that phenolic bases do sometimes warp over time, especially in sunlight. It would be a shame if they tried to distort the glass at the same time... and won!
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Old 1st Apr 2019, 5:47 pm   #7
GrimJosef
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Default Re: Refixing loose valve bases

Quote:
Originally Posted by David G4EBT View Post
... Yes, they say it's 'metal loaded' but it depends on one's definition of 'loaded'.

Before ever I used if for such purposes as re-fixing valve bases and re-stuffing capacitors, I applied 'Super Steel' to a strip of glass 50mm long x 15mm wide, then tested it on my Victor V60B insulation tester at 1,000V ...
Thanks for that. It's interesting (and also good to see the trouble being taken to check before getting started ). Clearly there isn't enough metal in that stuff to make a continuous conductive path through it. So that just leaves the question of breakdown strength. Since the croc clips aren't at the ends of the glass perhaps the 1000V was applied over 33mm (makes the sums easy ) ? That would put the test field at 30V/mm, which the glue withstands.

I rooted out an old IO base and sacrificed a bit of mm graph paper and I reckon that the peened-over metal on adjacent pins is about 3mm apart. In a cathode-biased EL34 the adjacent anode and heater pins, or screen grid and control grid pins might (worst case, I admit) be 450V apart. The field would then be 150V/mm. If you still have the bit of epoxy-coated glass it would be interesting to know whether it's fine with a 1000V test and the croc clips 7mm apart. It would also be interesting to know whether temperature changes anything, but now this is becoming a student project !

EDIT: What made me think of this was a post on here a few years back from someone who had tried a metal-loaded epoxy as a base glue and found that he was getting a breakdown problem. I can remember who it was (at least his real name, if not his current username) but I can't track down the post right now.

Cheers,

GJ
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Old 1st Apr 2019, 5:58 pm   #8
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Default Re: Refixing loose valve bases

https://www.ebay.co.uk/itm/704-Silic...8AAOSwMVdYFxz3

Cheap as chips, good for 250 degrees C. Not a recommendation, just a happy user.

There is also silicone flue sealant that is good for 1200 degrees i think!

Last edited by Boater Sam; 1st Apr 2019 at 5:59 pm. Reason: corrected
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Old 1st Apr 2019, 8:19 pm   #9
David G4EBT
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Default Re: Refixing loose valve bases

Quote:
Originally Posted by ColinTheAmpMan1 View Post
Quote:
Originally Posted by David G4EBT View Post

far from being conductive, 'Super Steel' makes an excellent conductor!
Insulator, surely?

Colin.
Ooops - an 'intellectual interlude' AKA 'senior moment'.

How embarrassing!
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Old 1st Apr 2019, 8:49 pm   #10
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Default Re: Refixing loose valve bases

Fantastic post, something I have coming up in my latest valve-based (pun intended) projects. The Pye sig-gen, Bush SAC35 and the Sobell all have valves which have become loose on their bases to some extent so it's good to know that there's flexible options out there.

I had considered silicone RTV but felt that it may not be tough enough, then again handling the valves sensibly means no real stress put on it anyway. I wonder how much of the problem of loose valve bases is down to natural degradation and how much is actually down to less sympathetic handling of the valves in the past.
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Old 1st Apr 2019, 8:52 pm   #11
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Default Re: Refixing loose valve bases

Please do not use 'Super Glue' type adhesives to bond valve bases!! It is a fantastic bonding agent but not for delicate glass valves.

I have no knowledge of the chemical reaction but it appears to dissolve an area of the glass resulting in fracture.I have good proof of this.

My own method is to purchase a cheap two tube type adhesive at the pound shop. After mixing, force it into the space between the base and the bottom of the glass bulb. Damp a cloth and while turning the valve, wipe off any excess producing a clean smooth finish. To complete, for a period of a few hours, stretch an elastic band around the bulb and under the base to hold it in mild compression.

Cheap epoxy is ideal as it does not set so absolutely solid as the real stuff does. I have never experienced any problems what so ever.

NEVER USE 'SUPER GLUE TYPE bonding agents. They will crack with time. John.
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Old 2nd Apr 2019, 6:49 pm   #12
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Default Re: Refixing loose valve bases

My preferred approach for small-signal valves is to use a short length of adhesive-lined heatshrink tubing to bridge the glass/valve-base gap.

This was Military SOP in the 1960s/1970s - I've got some VR100 valves that came from a R1155 that have had their bases re-bonded this way.

i wouldn't suggest this as an approach for 'power' valves or rectifiers though!
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Old 3rd Apr 2019, 8:40 am   #13
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Default Re: Refixing loose valve bases

I also use a 2-component Aradite - like glue.
This has the advantage that it softens a little bit when hot, so mechanical tensions stay low enough.

Never had any problems with it, I use it also for top connections of output tubes.

I guess that any rubbery type glue will work fine as well.

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Old 3rd Apr 2019, 9:00 am   #14
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Default Re: Refixing loose valve bases

I've never had problems with clear EVO STIK. It dries slightly 'rubbery' but is more than sufficient for valve bases and top caps. I've just repaired an MM4V where the top cap had come adrift and it's fine.
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Old 3rd Apr 2019, 9:58 am   #15
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Default Re: Refixing loose valve bases

I've used a Unibond branded product called 'Repair Extreme' which sounds to be very similar to David's 'Serious Glue'. It has excellent bonding properties, is very strong and retains a degree of flexibility when cured. The only drawback is that once opened it continues to set and the tube becomes hard within a few months. This almost makes it an expensive single use product and not something you can keep in the cupboard for use when the need arises. I wonder if 'Serious Glue' is the same?

Alan
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Old 3rd Apr 2019, 10:05 am   #16
peter_scott
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Default Re: Refixing loose valve bases

40 years ago I glued the base of my 6/6 CRT to the glass using car filler and after many hours of use it still holds tight and hasn't cracked the glass. Having said that, the glass is Pirex.

Peter

p.s. Thanks David for the "Serious Glue" tip. I've just placed my order with Amazon.
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Old 3rd Apr 2019, 10:08 am   #17
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Default Re: Refixing loose valve bases

I'm wondering if Serious Glue, being a moisture curing glue is similar to Gorilla glue? I have used the latter on many occasions for anything from brick and concrete to wood and it REALLY sticks! The one thing is that it does do is swell - which is good for void filling but it also seeps out of the side of the joint. In practise not really a problem as it can be removed before it is fully cured, or afterwards - I use a very sharp chisel. It has a kind of hybrid, 'hard' surface when cured but not glass hard. I've never given it a thought before but I bet it would be good for refixing valve bases.
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Old 3rd Apr 2019, 4:29 pm   #18
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Default Re: Refixing loose valve bases

There's a small caveat here - Gorilla also make a cyanoacrylate "super glue". I would suggest not confusing them for re-fixing loose valve bases.

Colin.
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Old 4th Apr 2019, 7:55 am   #19
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Default Re: Refixing loose valve bases

Indeed Colin. Like I said, "Gorilla Glue", not Gorilla brand superglue.
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Old 4th Apr 2019, 8:30 am   #20
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Default Re: Refixing loose valve bases

I've used Arctic Alumina White with good success on several loose bases & top caps (even on an output valve). It's a two part thermally conductive compound, we use it where I work for securing heatsinks & the like. Good for about 160C & even when set still has a small amount of compliance. A little goes a long way & the 5g syringes seem to last forever.
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