Advice wanted on possible tube repair

[sortedaudio]

I have a minature triode with a broken grid connection at the top pip of the glass envelope. My thoughts are to possibly grind some glass away from the tip and solder to the exposed wire. It sounds easy as I type this, but I know the odds are stacked against me.
Does anyone have any experience, advice or suggestions on making a successful repair?
The valve in question is a Telefunken AC701K, and a quick search will show why I am keen to make a repair!
Thank you.

 

[Tubetec]

My advice would be grind the top surface where the grid pins comes out ,only take it back so you can see fresh metal , apply a blob of solder to what remains of the grid , then tack on a short lenght of flexible wire .
Theres no guarantee the seal wont be overheated and fail if you try and solder that close to the glass ,
low melting point solder might be worth using , but you also want to get the soldering done very quickly with minimal dwell time .
Id avoid damaging the glass in any way if possible .

 

[mrclunk]

Break into your local cancer unit and use their proton beam to laser weld it.
https://www.researchgate.net/figure/Power-loss-of-the-laser-beam-through-the-glass_fig2_275309855

 

[Script]

I'd say, forget it. Pics are a bit out of focus. So it's not clear to see how much wire is left.

Yet, if rusty (and it looks like it?), I might try to very very very carefully scratch first under jewellery magnifying glass or like just so that the copper(?) shows again , and then definitely use some high-quality flux when trying to apply solder - or maybe solder paste and a hot gun?

I checked the tube on the nets. Best of luck and please report back if successful.

 

[metalockpick]

Conductive glue would be my first suggestion. Sorry, don't have a brand to suggest.
If you destroy the tube, try 5703 (think m49V)

 

[gyraf]

Agree on the glue. Top is grid, a bit of series resistance more or less dosent matter

 

[lassoharp]

Might want to buy some cheapo subminiatures to practice on first.

Maybe consult a glass engineer and see if there are any alternatives like using acids. The idea would be to find one that dissolves/cuts glass but won't dissolve copper. Or maybe there is some type of specialized instrument or hand tool for removing small areas of glass.

Even with a clean exiting wire to work with, I would be concerned over strength. It looks like a small collar like a grommet or some other suitable material could be fitted to the nipple area and act as a strain relief. I know space inside a C-60 is pretty tight so it might be a tedious fit.

Maybe its time for the nouveau repro industry to put this on the list alongside the VF-14.

Good Luck!

 

[k brown]

https://www.amazon.com/Resist-Elect...86793&sprefix=conductive+epoxy,aps,226&sr=8-3

 

[sortedaudio]

Thank you for all the suggestions, it's very helpful to have this input.
I have other valves I can practice on. I had been considering using a Dremel, but I think the diamond coated file is a better approach. I will see what a little experintation reveals regarding attaching a new wire. I think I have both conductive adhesive and paint, flux and various solder types. The valve is to be fitted in a customers Neumann KM56 mic, which is not being sold. The customer is aware of the risks, but the valve is useless as it is!

 

[TromNek]

I think I would try the diamond files as the safest way to to begin.

But first I would try and find an artist who works in glass to get their advice.
They may have some great ideas from experience. And some warnings to be careful about during the process.

 

[Markku01]

Join a Facebook antique radio repair group, many members have had this problem with old style radio tubes. Here's three examples of groups to join.