AMATEUR VACUUM TUBE MAKING
WELDING THE TUBE ELEMENT CONNECTIONS
All of the elements of the tube are made of metal and must be joined in some way. Welding is the most practical. Very early tubes used mechanical means to connect elements by bending wire through holes and crimping. This will work for simple situations, but there are many situations where you just can't get to the area to bend and crimp. The pincher welder is the answer.
CD OR TRANSFORMER?
There are two popular types of welder. One is the capacitive discharge, the other is the pulsed transformer. Each has advantages and disadvantages. The capacitive discharge type is very good for fine wires but takes a very large capacitor for larger wires, especially copper. The main disadvantage of the capacitive discharge type is the pulse length is extremely short, typically 1/10 the shortest pulse from a pulsed transformer. This drastically affects the type of weld produced. The welds tend to be surface layer welds with very high residual stresses. This can cause the weld to pop loose unexpectedly, especially when welding to sheet steel.
The answer to this problem is the pulsed transformer type welder. A high current transformer that outputs a low voltage, typically 5 volts, has a relay or solid state control to set the output pulse length. The pulse can be easily controlled from 1/10 second to a second. The current during the pulse can also be easily controlled by a simple rheostat in the primary circuit. The transformer type welder is the type of choice for tube making because of its complete adjustability of operating parameters. It can easily be adjusted to weld fine stainless steel wires (very short pulse, low current) or to weld copper to tungsten. (half second pulse, very high current)
The photo above is a homeade pincher welder hand piece. The pen is shown for size comparison. The width of the clamp sides is 3/4 inch. They are made of black delrin sheet. The hinge is made of white delrin blocks. Cut one to have a U shape and the other to have a T shape so the vertical of the T fits just right into the U. Drill the very back for the hinge screw. The electrode holders are made of aluminum blocks but copper would be better if you have it available. The heat radiating fins are not necessary. Drill the ends for the welder wires which are made of #10 solid copper wire. #10 screws are used to clamp the wires into the block. The #2 welding cable is stripped back and insulated with shrink tubing to get the diameter smaller. Drill appropriate holes in the hinge to get the wires through to the clamp blocks. The ends of the wires are tinned and shoved into drilled holes. Set screws are used to hold the wires in place. The back of the clamp sides has two pieces of delrin to hold the welding cable tightly. A tywrap through two holes in the clamp pieces would work just as well and be easier to make. The cables should be no more than 30 inches long to avoid excess voltage drop.
The power for the welder is supplied by a large transformer. The voltage output should be 5 volts at a current of 100 amps. Since the duty cycle of the transformer is very short, it can be overloaded considerably so a much smaller core can be used than what would be proper design for continuous duty. Pick a core that you can wind at least 4 layers of #10 wire onto. Make each layer 5 volts and paralell up the layers. Connect the winding directly to the welding cables. All connections should be large and soldered. The control box above is only 8 inches on a side, and has plenty of room for the transformer and the control relay. A rough idea on the size of transformer is the core is about 5 inches on a side and it weighs 12 pounds. A good source of the transformer core is an old microwave oven transformer. Carefully cut the high voltage winding off and wind the high current secondary on. Also knock out the magnetic shunt blocks to allow full power to the secondary.
The welder is controlled with a rheostat in the primary. Use a 50 ohm 25 watt unit. The duty cycle is very low so it will not overheat.
The circuit is controlled via a relay and foot switch. The weld time can be made automatic via a delay relay with a time adjust rheostat, or you can just use the foot switch to time the weld. Gets to be a very quick tap on the switch for some small wire welds! Be sure the relay you get to switch the transformer winding has large contacts. The peak primary current can reach 10 amps when the welder is set to full power.
Some welds are very easy to make some are very difficult. (if you don't know the tricks!) Some wires commonly used in tube making are listed here with the weldability and problems.
COPPER: Very difficult to weld due to high conductivity. In most cases must use a buffer metal.
IRON (carbon steel): Welds easily. Welds to other metals successfully.
NICKEL: The universal welding metal. Nickel will weld successfully to almost any other metal. Has high resistance so it generates a lot of heat for use as a buffer metal.
NICHROME: Resistance heater wire cheaply available. Welds easily. Tends to break off during welding if too much heat is used due to low melting point.
STAINLESS STEEL: Very easy to weld due to high resistance. Used a lot in tube making. Small diameters tend to burn through. Requires very low current and short weld pulse.
TUNGSTEN: Difficult to weld to itself due to high melting point. Welds excellently with iron, nickel, and by using a nickel buffer metal can be routinely welded to copper.
There are other less common metals that are weldable especially with nickel, but these metals are not used much in tube maiking due to their high cost.
TUNGSTEN TO NICKEL OR IRON
This is a very easy weld to make. Use a pliers to make a slight flat area on the end of the nickel or iron wire. (the tungsten cannot be flattened unless it is annealed tungsten, which is much more expensive than raw as-drawn tungsten) The flat makes it easy to hold the wires together when the pincher is applied. Set the current and time pulse to make a smooth weld. (try a few scrap wires to get the setup before doing your good wires)
The welding of nickel, nickel alloys, steel, iron alloys, and stainless steel is straight forward and will not be discussed in detail. Simply press the two wires together in the jaws of the welder and apply the current. Use test pieces to get your time pulse and current set right.
Welding tungsten to tungsten wire is more difficult. It is not common to weld tungsten to tungsten so that will not be covered. Use full power on the welder and you can do it, but the joint will be brittle. Do not bend it!
Welding tungsten to nickel or steel wires is easy. Tungsten forms a good weld to these metals and is used routinely in tube making.
To make the weld, first prepare the nickel wire by slightly flattening the end of the wire where it is to be welded. This makes it easy to hold the tungsten wire in contact without it slipping aside. Pinch the joint with the welder and apply current. The weld will not tend to stick to the copper jaws, but sometimes it does. In this case, carefully wiggle the nickel wire side to side. The sticking area will easily break free leaving the weld ok.
Welding tungsten to copper wires requires a different proceedure. A weld interface metal is used to generate the bond. The interface metal can be steel or nickel. Nickel is common so it is used more often. The copper wire has a short (2 mm, 1/16 inch) sleeve of thin (.002 in, .05mm) nickel tubing slipped over the joint area. This sleeve is then lightly crimped onto the copper by slightly flattening the wire, as with the nickel wire joint. The joint is then welded as normal. Note that it takes considerably more current to make this weld, possibly the maximum the welder will put out. The time pulse is set to get good copper flow without the joint overheating.
The source of nickel tubing for making the joint is old TV tube cathodes. Small tubes have cathodes that are small in diameter, some have cathodes the exact right size to fit smoothly over common wire sizes used in tube making for external lead wires.
In the case that the tubing is too large, or if you are using nickel foil, the sleeve is made by crimping the edge of the tube with a pliers. See photo below.
The oversize nickel tube is slipped onto the wire and flattened. The tube can be made slightly longer than needed to make handling it easier. The pliers is then used to grab the edge and crimp it down tightly around the wire. This is surprisingly easy to do. The flattened extra tab is then cut off with a sharp tin snips. Then trim the end to the length needed and slightly flatten it to hold the tube on This method is useful with nickel foil as well. Cut a strip of foil 2mm wide and bend a hook in the end for the wire. Crimp the hook tightly around the wire as with doing a piece of tube. Note that you may need to grind a pair of pliers to get a sharp angled end on the tips for doing this job.
WELDING TO STEEL SHEET
Most elements in homeade tubes are made of nickel or steel. While old tin cans can be used for a source of plate metal, the best metal to use is parts of old TV tubes. These are available for give-away prices since you don't care what type of tube it is. The best tubes for sheet metal are large tubes with large internal elements. Some of these tubes will yield two square inches of plate material and several inches of nickel wire each.
Nickel wire welds perfectly to steel plate. The wire is held in contact with the plate and pinched with the welder jaws. The power level and time pulse is set such that the wire heats red hot for long enough to make the weld but not to burn off. This setting is somewhat touchy for various types of wire, nichrome being the most critical. Nichrome has the tendency to burn in half since it loses all of its strength when it turns red hot. Use a very short pulse time for nichrome wire and you will have success.
Steel and stainless steel weld fine to steel and nickel plate. Steel wire and stainless steel, have a tendency to burn off if the current is too high since they have much higher resistance than nickel, and thus generate more heat at a given current.
To weld copper wire to steel sheet, prepare the end with a sleeve of nickel just as making a joint to tungsten. Use a relatively high current pulse since copper has very high heat conductivity and tends to cool the joint before it welds if it is not hot enough.
Tungsten to steel is easy but it must be considered that the joint will be brittle due to the brittle nature of tungsten wire after it has been heated red hot and then quickly cooled like happens in a weld. Successful joints are easy to make though. Just don't try to bend the tungsten wire around after the weld has been made or it may break at the weld.
Note the excellent metal flow on each weld. The welds are deep and thorough, not just a surface stick. Note the nickel sleeve on the copper tungsten weld. Note the slight flat spot on the end of the wire on the nickel tungsten. Do not flatten the wire excessively.