Repair of high current 'flush-severed' leads

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
Kind friends:

This is to inquire as to whether any of you are aware of (or have suggestions to offer) as regards an acceptable (even if less than ideal) method of repairing low impedance (Spec: high current) connections to electron tubes where lead{s} are broken/severed flush with the envelope-to-metal seal?

Granting that repair of high impedance (i.e. Low I, high E) connections may be readly acomplished, I've yet to discover or devise a practical remedy for low impediance connections....

Specifically: I've a quantity of NOS YU181s wherein the former owner 'snipped' the cathode pins/leads flush with the envelope (a long, and, I might add, rather sordid story!:mad:)

Inasmuch as said lead{s} must continuously carry 25A and withstand power-up inrush surges of at least 100A, 'common solutions' (e.g. conductive adhesives, etc ) are of no avail -- Although I've had marginal sucess with 'spring loaded' contact schemes, such have proven less than reliable -- nor does any manner of welding seem an option for the proximity to the (relatively 'delicate') seal...

FWIW inasmuch as each end of the rather low impedance cathode 'filament' is (although similarly 'snipped') accessible -- and, better still, said filament is roughly 'center-taped' to the the metallic 'exhaust nipple' (an 'undocumented' connection in all YU181s and certain other Eimac 'Power Grid'' tubes) I'm envisioning some manner of electrochemical process by which leads may be attached (à la 'electroplating' techniques)? ...

Any ideas, thoughts, wild guesses or 'magic spells' will be greatly appreciated!:)

With very best regards and advance thanks!
HP
 

drc_567

Joined Dec 29, 2008
963
There is a type of metal joining called 'friction welding', whereby the heat used to bond two pieces is produced by the rotation of one segment pressed against the other. As a trial, take a suitable lead extension piece and use normal torch heat to apply a small length of silver solder to one end. Then find a high speed tool such as the Dremel variety ( ... As high as 37,000 rpm ) and insert the silver soldered lead into the chuck end. Activate the lead rotation, applying pressure against one of the clipped lead ends.
It will be necessary to insure that both metal contact surfaces are clean, in the sense that no oxidation or contaminants are present.
... As a possible alternate approach, bypassing the silver solder, which may prove difficult to accomplish, try using a short length of brass rod as the lead extension. Brass rod will have a significantly lower melting point.
 
Last edited:

jpanhalt

Joined Jan 18, 2008
10,933
In 2012, a researcher in Chicago modified the well-known Tollen's reagent (one use of the original was to silver telescope mirrors) to give relatively thick nano-particle deposits of silver. It is claimed they can be soldered. Very low temperatures are required for curing as the other reaction products are volatile. Here's the original:
https://pubs.acs.org/doi/abs/10.1021/ja209267c

Unfortunately JACS quickly removed anything useful from the abstract. If you have an academic account, I would recommend getting the original. If not, I used the information to write a DIY procedure (attached).

That product is sold commercially, but it is hard to find, because there are so many "silver inks" that are just a silver filled carrier like epoxy. It's price has also crept up. Aldrich/Sigma used to have it.

My approach would be to thoroughly clean the exposed metal and surrounding glass. Add a little of the ink, then place a suitable pad (silver) over it and let it cure. Slight heating is required. You might be able omit the pad and just solder to the deposit on the glass directly, but I would not trust that. I would maintain some sort of mechanical fixture for stability.
 

Attachments

shortbus

Joined Sep 30, 2009
8,175
Is there any metal at all above the glass? If so have you considered, "electroless nickle plating"? From my years of tool and die work, I've seen the results of the process. It isn't like standard electroplating where the surface gets covered mostly uniform, though even in that edges build up more than flat planes. With electroless edges build up several times faster and thicker than the plane surfaces. The plating needs no electrical contact or continuity, it joins to the metal surface at such a way that it will withstand machine work, like milling, grinding or lathe work.

After building up the lead surfaces it would be kind of tube like(maybe, not sure about how it works with the end of a wire). Building it up then soldering a new wire to the nub. Almost any big city area will have places that do the plating. https://en.wikipedia.org/wiki/Electroless_nickel_plating
 

jpanhalt

Joined Jan 18, 2008
10,933
I am a chemist and just hobbyist machinist. Thus, I viewed the silver ink as modified a Tollen's reagent. Thanks to @shortbus, I looked up electroless silver plating. Lo and behold: https://patentimages.storage.googleapis.com/34/1e/8b/dfb6a3034a1805/US6387542.pdf

2002 and written broadly to maybe cover the modified Tollen's patent. For what HP proposes to do, I don't think she is likely to be served by Jones Day for patent infringement. :) That is not legal advice.

Back OT. Some electroless platings are displacement, but it appears nickle self catalyzes and is truly additive (as is the Tollen's reagent). I have found rolled nickle strips to be readily soldered with ordinary rosin core solder. My question is whether electroless nickle is as easily soldered? There is also electroless copper, but I didn't research that area as much.

Another consideration: If silver doesn't work, it is easily removed (nitric acid), and you could move to nickle. If nickle doesn't work, it is probably more difficult to remove.
 

shortbus

Joined Sep 30, 2009
8,175
There is also electroless copper, but I didn't research that area as much.
That is how they do plated through holes on PCBs, from what I understand. Never worked with it, so can't vouch for how it addheres to steel, which is probably what the leads are on the tubes. Electroless nickle has saved many high value molds and mold cores in places I've worked.

It does solder pretty well. The one place I worked used the process to make paint masks for painting car instrument panels and the electroless was soft soldered to a wire holding frame, the frame was made of mild steel wire. But they did use acid flux to solder it.
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
Greetings all! Many thanks for your time and effort taken on my behalf - as well as your many helpful replies!

As regards my (unplanned) 'disappearance' - NO! I promise I've not become a 'hit and run' poster!:oops: -- 'tis merely that my 'vacation' has become a bit more 'complected' than anticipated...:rolleyes: Moreover, owing to my location, connectivity (whether via local services or by LEO [Spec. 'Iridium'] relay to my W. WI 'hub') is 'shaky' at best...

Well, anyway... Enough with the 'alibis':oops: -- Please know that I intend to properly respond to/continue this thread at such time as same is practically feasible - Again, please know that your kind, knowledgeable, suggestions and information is greatly appreciated:)

Most sincerely
HP
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
Addendum -- By way of avoidance of any misunderstanding owed my (admittedly) irksome habit of failure to expand brevity, please be advised that 'LEO' (as used in the above post) is in reference to a mobile terminal ('phone' if you must) network employing Low Earth Orbit repeater satellites --- That's right! A 'marooned bird' I may be -- but no 'yard bird'! - Apologies to any whom this may disappoint!;):cool:

TTFN
HP
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
To the mod/admin having spontaneously moved this thread to 'Technical Repair'-- Sincere thanks!:cool: - Sometimes 'general chat' is... well... just that - To wit: a tad too general for 'full exposure' to interested parties, as it were...

Much appreciated!
HP:)
 

Thread Starter

Hypatia's Protege

Joined Mar 1, 2015
3,226
Kind friends

So... A (likely fleeting) 'lull' in 'offline obligations' has granted me a few 'ticks' that I might explore your kind suggestions.:)

I've only just begun work on the 'eletrochemical approach' -- I find that (as expected) electrolyte baths compounded of aqueous solutions of alkali-cyanides (e.g. KCN) and, to a lesser extent, NH3 (aka ammonia, monoamine, etc ) and N2H4 (aka hydrazine, diamine, etc) provide good results to the extent that the metals are deposited as a contiguous 'plate' (as opposed to fine granules) -- that said, I've yet to achieve sufficient 'bond strength' (Still it's early days). The principal encumberment to rapid investigation of this technique owes to the rather high order of toxicity exhibited by the cited nitrogen compounds (hence requiring highly time-consuming --albeit altogether necessary-- safety measures...).

I'll keep y'all posted!:)

In response to some of your inquiries/comments:

There is a type of metal joining called 'friction welding', whereby the heat used to bond two pieces is produced by the rotation of one segment pressed against the other. As a trial, take a suitable lead extension piece and use normal torch heat to apply a small length of silver solder to one end. Then find a high speed tool such as the Dremel variety ( ... As high as 37,000 rpm ) and insert the silver soldered lead into the chuck end. Activate the lead rotation, applying pressure against one of the clipped lead ends.
It will be necessary to insure that both metal contact surfaces are clean, in the sense that no oxidation or contaminants are present.
... As a possible alternate approach, bypassing the silver solder, which may prove difficult to accomplish, try using a short length of brass rod as the lead extension. Brass rod will have a significantly lower melting point.
Thanks! Such indeed represents a promising approach to my problem inasmuch as the seals in question are ceramic (as opposed to glass) to metal! -- As I understand it, the (thermal) coefficient of expansion of the ceramic 'tracks' the metal (monel?) over a significantly broader range --and to much higher temperature-- than does glass...

In 2012, a researcher in Chicago modified the well-known Tollen's reagent (one use of the original was to silver telescope mirrors) to give relatively thick nano-particle deposits of silver. It is claimed they can be soldered. Very low temperatures are required for curing as the other reaction products are volatile. Here's the original:
https://pubs.acs.org/doi/abs/10.1021/ja209267c

Unfortunately JACS quickly removed anything useful from the abstract. If you have an academic account, I would recommend getting the original. If not, I used the information to write a DIY procedure (attached).

That product is sold commercially, but it is hard to find, because there are so many "silver inks" that are just a silver filled carrier like epoxy. It's price has also crept up. Aldrich/Sigma used to have it.

My approach would be to thoroughly clean the exposed metal and surrounding glass. Add a little of the ink, then place a suitable pad (silver) over it and let it cure. Slight heating is required. You might be able omit the pad and just solder to the deposit on the glass directly, but I would not trust that. I would maintain some sort of mechanical fixture for stability.
I'll definitely check it out! -- Again, the ceramic (as opposed to glass) envelope construction greatly reduces liability to absolute-temperature and Δ-temperature 'issues'... -- Many thanks!

Is there any metal at all above the glass?
Sadly no -- The leads are shorn flush with the ceramic envelope -- Sincere thanks for your interest and consideration of my difficulties!


The "steel" may be "Kovar" Fe-Ni-Co alloy (Fe: 54%, Ni: 28%, Co: 18%), which is what we used in the 60's with borosilicate glass (Kimax or Pyrex) for vacuum tight glass-metal seals..
---Emphasis added---

Hmmm... Sounds similar to Monel -- Perhaps the (thermal) coefficients of expansion are such that kovar is used with glass whereas monel is used with BeO ceramics?

Very best regards
HP

PS -- I leave you all with a sentiment I believe we can all relate to -- To wit: Wake me when 2020 ends!
 
Top