Valve cement?

[greg_simons]

Ok, so this may have been covered before but does anyone know what the cement was that glued valves into their bases and top caps etc, is there a modern substitute?, I would like to render a rare valve a little less fragile by fixing the top cap to the glass.

[ms660]

Various stuff from all accounts, this was RCA's formula for their "tubes":

"This recipe for RCA's basing cement, yielding about 200 pounds of material, was "standard for all bases.":

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.

(Just for reference: the above recipe yielded enough cement to put bases on 23,000 Type 50 tubes"

Lawrence.

[vidjoman]

I wondered about this last week and though about the possibility of using 'fire cement' that we used for fixing holes in firebacks many years ago. I guess it's still available as many people have coal fires. It came ready mixed in tins.

[Herald1360]

If you can source the materials, converting lbs to gms might make the quantity more manageable.......

It'd still be enough for 50 Type 50s though.

[Leon Crampin]

I use PVA glue thinned with water to repair loose phenolic bases, and slow setting Araldite for top caps.

The thinned PVA will wick right into the space between the cement and the glass, and is not visible when set. It remains soft enough to accommodate expansion, so the glass is not stressed as it would be with epoxy or superglue.

Some strength is needed for a top cap - but the area is small. Araldite fits the bill here because it's strong and the limited joint area means that thermal stresses are minimal. Keep the epoxy away from the soldered joint. High dissipation valves often suffer from broken joints at this point due to solder fatigue and may require re-soldering.

Leon.

[ms660]

High temperature silicone rubber might work (the red stuff) I used it to seal a flue pipe, that got damned hot.

Lawrence.

[mark pirate]

I have successfully used car windscreen sealant, it is a type of runny silicone and works a treat on loose valve bases and CRT's.

Mark

[joebog1]

I agree with red silicone.
I also agree with SOME of the Sikaflex range.
There is a high temp version.
As far as PVA goes, it melts in the australian sun WITHOUT the valve being turned on.
There is a new version of PVA that isnt PVA at all, called external polyurethane.
I have not tried it as yet.
I cannot believe PVA Unless of course its in cold cathode valves, or voltage references.
I definitely would NOT try it with an EL34 in a 100 watt Marshall amp!!
The smoke arising might even put you to sleep.

Joe

[Ti Pwun]

Search for 'Permatex Flowable Silicone Windshield and Glass Sealer' - perfect for the job.

[G6Tanuki]

I use short lengths of large diameter heatshrink sleeve - as did MoD radio workshops on some of the valves in receivers in the late 1960s.

It's fine on small-signal valves [EF39s, KTW61s, EBC33s and the like] but I'd not use it on 'power' valves (rectifiers or output-stages).

[FIXITNOW]

I read a note in practical wireless that said plaster of Paris can be used

[emeritus]

I believe that Plaster of Paris was used in the early days of electric lighting to secure bulbs to the early "Brass Collar" (dimensionally the same as the current BC) bases, so should be fine for valves.

[kalee20]

Plaster of Paris, shellac, phenolic, etc no doubt will do the job but mark_pirate, Ti Pwun, joebog1 and pwdrive suggest modern alternatives which should be better.

Don't forget the original stuff has let go - if it's replaced with the same type of stuff then that's going to let go again!

[David G4EBT]

Basically, apart from the filler, it was shellac and methylated spirits, or as the Americans call it 'denatured alcohol. If ever you need to remove a valve base or top cap, a few minutes soaked in meths will soon have it free. Over the years I've removed and replaced quite a lot of valve bases and top caps and have tried various mixtures, including shellac and wood-flour mixed into a paste. However, for some time now I've used two part epoxy made by Plastic Padding known as 'Super Steel' which is a bit of a misnomer in that the inference is that it might be conductive, and hence unsuitable for some applications. However, I've tested it at 1,000 Volts and it shows no conductivity at all.

It sets in a minute or two and adheres really well to glass. I've repaired several valves with it just this last week. I needed a set of fairly uncommon Mazda valves, (TH41, VP41, Pen45DD, UU6), and when they do come up, they're not cheap, but I saw a scrap Bakelite radio on e-bay with that line-up that had been dropped and smashed, so I had a punt on it, taking a gamble that the valves might be intact. The top cap had ripped off the top of one of the valves, leaving just a whisker of wire protruding from the glass pip. Using a magnifying lamp and a steady hand, I was able to carefully grind away enough of the glass pip with a diamond burr to be able to solder a thin wire onto the wire remnant protruding from the pip. I then applied a blob of super steel to strengthen the joint, then when that had set, I applied more 'Super Steel' to secure the top cap, then soldered the new wire to the cap.

Another valve had a cracked and loose base, so I cut round the perimeter of the base to expose the wires, and labelled them before unsoldering the pins, because in my experience, the wires are springy and if you simply remove the valve base without first identifying the wires, there's a good chance that the wires spring out of shape and apart from the heaters, you'll not have a clue which the other wires are as they don't emerge from the valve in a circle, but it a straight pinch. I got a good M.O. base off a donor valve, (used meths to free it), slipped it over the wires and secured it in place with super steel around the rim of the base, then soldered the wires. Another valve just had a loose base so I was able to poke 'Super Steel' around the perimeter with one of those coffee stirrer sticks, which I find a lot of uses for in the workshop.

All the valves tested like new, so I was pleased with my efforts.

I've attached a couple of pics about 'Super Steel'. The first one is of the two tubes, along with some capacitors that I re-stuffed, which had originally been sealed with pitch at the ends. Super Steel is a good match for pitch. The other picture shows a 5cm length of Super Steel applied to a strip of glass, and subjected to a 1,000V insulation test on my Victor VC60B insulation tester to check that 'Super Steel' is non-conductive. (The display shows '1' and to the RH side MegOhms, but the '1' doesn't mean 1 Meg - it means no reading, or more correctly, in excess of 2,000 MegOhms).

Super Steel is widely available - I get mine from an e-bay supplier for £5.95 post free.

Hope that's of interest.