We looked at glass bases earlier in my blog but today, I would like to add a little detail about the lead through wires. To recap, the lead through wires are of composite construction with one end forming the valve pin, the other end, the support for the electrode cage with the in between section being a short length of wire to pass through the glass base.
To make a perfect mechanical and impermeable seal between the metal and glass, the metal and glass require near identical co-efficients of expansion otherwise seal leaks and the destruction of valve vacuum would ensue.
The metal chosen for the middle section of the lead through wire is borated copper clad iron wire of 0.015 mm diameter and 2.5 mm length.
The valve pins are nickel of 1 mm diameter and 9 mm length.
The support wire is nickel of 0.5mm diameter and 12 mm length.
In the Mullard picture below you can see the individual components and the completed lead through joined pins: -
Joining was accomplished using an automated machine. The pins which had been previously cut to length and rounded on a linisher were placed in a rotating hopper and automatically fed, pre aligned, to the centre of the joining machine. The supporting wire and copper clad wire are supplied from two reels, passing first between two rollers to straighten the wire and then to a position inline but 2.5 mm from the pin. Two clamps would then close to hold each part in alignment and the support wire was cut accurately to length. Simultaneously, the end of the copper clad wire was fed along another guideway until opposite the gap between support wire and pin. Here, it was gripped and cut and carried down to position.
The clamps holding the support wire and pin formed an output terminal and the clamp holding the copper clad wire formed another terminal. As soon as all three components were in alignment, a heavy current was passed through them hence welding all three pieces together.
The clamps then opened allowing the finished lead through wire to fall into a collecting chute incorporating a sizing grid that allowed the removal of misshapen or mis-sized items. At the end of the chute was a perforated shaker tray that aligned the lead through pins to fall into cardboard boxes in which they were stored until issue to the valve assembly department.