Tube Jumpers

What Tube Jumpers Actually Are

Tube Jumpers are a method of making electrical connections on PCBs by using small-diameter copper wire (or sometimes the lead from a component) that fits inside or through plated-through holes. You literally insert a thin wire into the via or pad hole, apply solder, and it forms a mechanical and electrical bridge. It is also sometimes called "wire-in-hole" soldering or through-hole jumping. The technique is most common in retro electronics repair, vacuum tube equipment work, and any situation where modern surface-mount alternatives don't fit or aren't available. Here is the actual process. Take a piece of solid-core or flexible wire, preferably tinned already, with a diameter slightly smaller than the plated-through hole you are targeting. Insert one end through the hole from the component side. Apply flux to the joint. Heat the hole and the wire simultaneously with your iron tip, then feed solder into the joint from below. The solder should wick up into the barrel of the via and grab the wire. Clip the excess. That is basically it. The reason people reach for Tube Jumpers is that they solve a very specific problem: you need to bridge two points on a board where the original trace is broken or was never there, but you cannot easily route a surface-mount jumper because the clearance is tight, the board is densely populated, or you are working on vintage equipment where aesthetics and reversibility matter. A tube jumper is mechanically stronger than a flywire and much less visible than a run of enamel-coated magnet wire stapled across the board.

I ran into this on a piece of 1970s test equipment where the original copper trace under a resistor network had lifted due to thermal cycling. The vias were still intact. I stripped about four millimeters of insulation from a 30 AWG silver-plated copper wire, tinned it, and inserted it through the via from the top. Applied a small amount of no-clean flux, touched the iron to the via barrel for two seconds, fed a tiny solder blob from underneath, and let it cool. The joint held at roughly eight ohms of contact resistance, which was fine for the signal path. I did not pot it. The board saw vibration in transport and I wanted to be able to inspect the joint later if something failed.

When Tube Jumpers Are the Right Call

They work well when you need a low-profile, mechanically stable connection in a space where a standard jumper wire would rattle loose or interfere with a chassis. They are also useful when you are working with boards that have unusually thick plating in their vias—some industrial and military boards from the 80s and 90s have barrel plating that is almost too thick for fine wire to make good contact. In those cases a tube jumper actually helps by displacing old solder and creating a fresh metal-to-metal interface. There is a downside that most guides do not mention: the solder joint can look clean but actually be cold if the via barrel is heavily oxidized. I have seen this on boards that sat in humid storage for a decade. The wire goes in, you apply heat, the solder beads up on the surface of the barrel rather than wetting it, and you think you have a good joint. It will fail under thermal cycling within months. The workaround is to scrape the inside of the via barrel with a reamer or a small drill bit before inserting the jumper wire. It takes about thirty seconds per via and it changes the outcome completely.

Common Mistakes to Avoid

The biggest mistake is using wire that is too thick. If the wire fills more than sixty percent of the via barrel, you will block solder from reaching the inner surfaces of the barrel and you will get a hollow joint. The second is skipping flux. Solder does not flow into a via barrel without it, especially on aged boards. The third is applying heat only to the wire and not the barrel. The barrel needs to be hot enough to melt the solder that contacts it. If you only heat the wire, the solder will stick to the wire and slide right out of the hole. Another thing to keep in mind is that Tube Jumpers are not ideal for high-frequency signals. The added length of wire inside the via creates a small inductance that can distort fast edges. If you are working on a digital board with rise times under five nanoseconds, consider a surface-mount jumper or a trace repair with copper tape instead. For audio, power, and low-speed digital, the inductance is negligible and the joint is reliable.

Tools and Materials

You do not need anything exotic. A 30 to 36 AWG enameled or insulated copper wire spool works fine. Solid-core bus wire from a hardware store is cheaper but harder to handle at small diameters. A fine-tip iron with a chisel or conical tip around 0.5 mm is adequate. Rosin-core solder in 0.5 mm diameter is standard. No-clean flux in a pen or syringe is worth the extra cost because it makes every joint more repeatable. A pair of fine wire cutters and a pair of needle-nose pliers complete the basic kit. If you are doing this regularly, a micro-reamer set for cleaning via barrels saves a lot of failed joints. There is no single download or installer for Tube Jumpers because this is a manual technique, not a software tool. What you can look for are reference schematics and PCB layouts from vintage equipment service manuals. Those documents often show where factory jumpers were placed and what gauge wire was used. I keep a folder of scanned service manual pages for common brands like Tektronix, Hewlett-Packard, and Marantz. When a board comes in with a broken trace, I check the manual first to see if there was an original jumper location I can replicate rather than improvising.

Reversibility and Inspection

One advantage of Tube Jumpers over other repair methods is that they are inspectable. You can see the wire coming out of the top of the via and the solder bead on the bottom. If a joint fails, you can desolder it, ream the barrel, and try again. This matters on equipment that needs to pass inspection or that you plan to resell. A poorly done flywire repair looks like amateur work and raises doubts about the rest of the job. A clean tube jumper looks intentional. The tradeoff is time. A properly done tube jumper takes two to three minutes per joint including preparation. A flywire takes thirty seconds. If you are doing ten jumpers on a board, that is fifteen minutes versus three. For a single repair it does not matter. For a production run of repaired boards, it adds up. I usually reserve Tube Jumpers for customer equipment or personal projects where the finish quality matters. For internal prototype fixes I use whatever is fastest. If your board has a lot of broken traces in a dense area, you might be better off with a full trace reconstruction using conductive epoxy or a flat conductor ribbon. Tube Jumpers are not a substitute for proper trace repair when the original copper is gone over a long run. They are a bridge, not a foundation. Use them where the gap is small and the mechanical stress is low. Anything larger and you should consider a different approach entirely.