Repairing Valve Top Caps

[David Simpson]

Advice sought on repairing a couple of vintage valves. Both with missing Top Caps.
Brimar 50CD6G Pentode with wire broken off flush with glass nipple, Mazda ?C2/PEN/DD with 2mm of wire left above nipple. At the moment I havent any old scrap valves to remove TC's from.
The Madza's lettering is a bit worn & the first letter/no is missing, so advice also sought on this ?

Regards, David

[Tim]

I am sure there is a thread on this somewhere.
You can file a SMALL amount of glass away and solder on a "tail" of thin wire, and then solder that to the top cap and araldite onto top of valve. Alternatively try stuffing the top cap with tin foil and then glue on, but that's more of "an old indian trick".
If you have a valve tester test the valve first with a crocodile clip, just so you don't waste time repairing a dud valve-unless you want the practice of course!

[Leon Crampin]

The Mazda valve is an AC2/PEN/DD. Neither of your valves is particularly rare, but fixing them is instructive and preserves a diminishing stock.

You need caps from other discarded valves (ask around). Note that the two valves you have probably use caps of differing sizes - the older British based valves used caps of a larger diameter.

The valve with the protruding wire needs to have some thin tinned copper wire soldered carefully to the stub. Scrape the protruding wire clean and tin with a small but hot iron using good cored tin/lead solder. Araldite your new cap onto the valve with the new wire protruding through the top hole. Leave some slack wire inside the cap to allow for thermal expansion and to minimise heat conduction to the inner joint. Solder up carefully when set. Keep the Araldite away from the site of the final joint or you will have trouble soldering it.

The valve which is broken off flush is more challenging. For a relatively common type such as you have, you can risk grinding or chipping away the glass pip around the lead wire, just sufficiently to enable a wire to be soldered as above. Examine the seal carefully and see how much glass you can safely remove.

For a rare valve, a pressure type of contact to the stub is a safer bet. I have used a pellet of Al foil preloaded with a phosphor bronze spring inside the cap quite successfully.

Best of luck - have a go at it.

Leon.

[David Simpson]

Leon & Tim, Many thanks for advice. Most appreciated. Have now managed to tin both wee stubs of protruding wire with a hot s.iron.
Scrounged the use of my wife's stained glass grinding wheel to remove a mm or so of nipple glass off the Brimar. Shall now test on my CT160 before putting out a post for a couple of surplus caps.
The valves were amongst nearly 100 30's & 40's IO & B7 etc valves aquired last weekend( Including 3- 807's, 2- VT136's & a 6CD6).
I like the idea of a ph.bronze contact spring.Might have one somewhere from an old AVO cut-out.Will try that if I'm unable to sucessfully solder on an extra lenth of wee wire.

Regards, David

[Jonster]

One thing to remember is never to use superglue. The results can be quite shattering!

[Phil G4SPZ]

I have several valves with o/c heaters that could act as donors of various sizes of top cap. If possible, let me know what size you need. There are three common sizes, small (commonly found on GT types), medium, and large (typically seen on Mazda octals).

[David Simpson]

Phil, Many thanks for offer. Both valves tested fine, although the big old Brimar Pentode gave approx 6mA/V at 65mA Ia. Mind you, this was the one with the shortest stub of wire remaining on top, so didn't run the max Ia for too long through the minimal crock clip contact.

Regards, David

[Phil G4SPZ]

David, PM sent with more details.

[FrankB]

I have used furnace cement to re-attach loose and replacement plate & grid caps on the old tubes.
For me it has always worked great.

Frank WB7ELC