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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 14th Jul 2021, 2:32 pm   #1
Nanozeugma
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Default Valve bases - a question.

Regarding the bases of valves where not of all glass design, like octal for example, does anyone know the material used for the base holding the pins, and the type of adhesive used to cement the base to the bulb?

It came up in conversation today and I have no idea...

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Old 14th Jul 2021, 2:43 pm   #2
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Default Re: Valve bases - a question.

Here's RCA's basing cement ingredients and measures....It'll do a lot of valves...

Coarse marble flour 170 lb.
Orange flake shellac 19-1/2 lb.
Durite phenolic resin LR275-2 7-1/2 lb.
Medium-color (grade G) rosin 3-1/4 lb.
Denatured alcohol 9 liters
Malachite Green aniline dye 10 g.

A brief article here for RCA:

https://antiquewireless.org/wp-conte...acuum_tube.pdf

Lawrence.
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Old 14th Jul 2021, 2:54 pm   #3
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Default Re: Valve bases - a question.

To summarise, the bases are one or other form of bakelite. The adhesive is normally methyl cellulose or shellac resin.
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Old 15th Jul 2021, 11:12 am   #4
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Default Re: Valve bases - a question.

Lawrence, Paul - thanks for responding.

Lawrence, that article is fascinating - many thanks.

I wonder whether the same adhesive was used to cement the top cap to valves which sported one?

Kind Regards,

Colin
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Last edited by Nanozeugma; 15th Jul 2021 at 11:14 am. Reason: Additional Text.
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Old 15th Jul 2021, 11:28 am   #5
ms660
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Default Re: Valve bases - a question.

Quote:
Originally Posted by Nanozeugma View Post
I wonder whether the same adhesive was used to cement the top cap to valves which sported one?
I would have thought so, having said that it's something I've never studied as such.

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Old 15th Jul 2021, 6:16 pm   #6
David G4EBT
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Default Re: Valve bases - a question.

In my experience, it's invariably shellac based as outlined by Lawrence in post #2 above.

Where the recipe refers to 'de-natured alcohol' that's what in the UK we call methylated spirits.

I've converted quite a few valves by removing bases and top-caps - mainly converting side-contact EBL1 valves to the much rarer EBL31s as used in Ekco A22s. Apart from the bases and the top caps, the two valves are identical. All NOS 'EBL31' valves sold by UK dealers are converted EBL1s that have simply had an Octal Base placed over the side contact base with it still in place, and have left the EBL1 3/8" top cap in place too, whereas a genuine EBL31 has a 1/4" top cap. That results in a valve which is too tall to use in an Ekco A22 where the valve is mounted horizontally, so the radio back will not fit on.

To remove top caps, I've simply unsoldered the grid wire on the cap, then carefully sliced open the top with a Dremel and diamond disc to dribble in some meths. That quickly dissolves the cement and the cap can be lifted off. On EBL1s, I don't remove the whole of the side contact base to fit a short octal base, but I've covered how I do that in other threads over the years. The aim is to create an EBL31 valve no taller than a genuine one and has the correct top cap.

Mostly, when I've removed octal bases on other valves it's been when they've been cracked and I've wanted to fit new ones. First thing I do is to cut a ring about 1cm wide around the old base and remove that ring to expose the wires emerging from the glass 'pinch' at the bottom of the valve. (The wires emerge from the 'pinch' in a straight line - not a circle). I've then tied a small tag around each wire to identify the pin and it's role. Then I unsolder the pins and remove the lower part of the valve base. Next I tip the valve upside down and pour a little meths into the portion of the base still attached to the valve. The cement is quickly dissolved and the old base can be completely removed.

If you ever unsolder the pins and remove a base from a valve, you will discover that the wires don't stay in a neat circle and apart from the heater wires, you'll have little idea which wire belongs to which pin unless you've done as I said above and labelled each wire with the bottom of the base still attached.

To re-fix replacement or loose bases and top caps, I long ago abandoned Araldite, and the only product I use is two-part epoxy resin 'Super Steel'. It's very runny when mixed, can be dribbled around the base, quickly sets, and is heat-proof. (It's black when set). Despite its name, it isn't electrically conductive.

Hope these ramblings might help.
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Old 16th Jul 2021, 12:49 pm   #7
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Default Re: Valve bases - a question.

David, as ever you are a goldmine of interesting insights.
My thanks, as usual
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Old 16th Jul 2021, 6:24 pm   #8
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Default Re: Valve bases - a question.

The 'stuff' used to fit bases to general-purpose low-power valves is as described - but for valves dissipating significant power [think 807s, 813s and 'sweep tubes' like the PL500-series] there seems to have been something different used - it looks more like a sort of grey ceramic 'frit' paste - that's only to be expected because such valves are specced to operate with an envelope-temperature of 250C or more!
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