Assembling the tube

A typical glass audio tube is made on an assembly line by people wielding tweezers and small electric spot-welders. They assemble the plate, cathode, grids and other parts inside a set of mica or ceramic spacers, then crimp the whole assembly together. The electrical connections are then spot-welded to the tube's base wiring. This work has to be done in fairly clean conditions, although not as extreme as the "clean rooms" used to make semiconductors. Smocks and caps are worn, and each workstation is equipped with a constant source of filtered airflow to keep dust away from the tube parts.
Once the finished assembly is attached to the base, the glass envelope can be slid over the assembly and flame-sealed to the base disc. A small glass exhaust tube is still attached, and enters the envelope. The tube assembly is attached to a processing machine (sometimes called a "sealex" machine, an old American brandname for this kind of device). The exhaust tubing goes to a multistage high-vacuum pump. The sealex has a rotating turntable with several tubes, all undergoing a different step in the process. (See more pictures of glass tube assembly and production)
First comes vacuum pumping; while the pump runs, an RF induction coil is placed over the tube assembly and all the metal parts are heated. This helps remove stray gases trapped in the parts, and also activates the cathode coating.
After 30 minutes or more (depending on the tube type and the vacuum desired), the tube is automatically lifted up and a small flame seals its exhaust tubing.
The turntable rotates, and there may follow an electrical "break-in" period where the tube is put through a series of operational stresses, such as higher-than-rated heater voltages.
Then the tube is rotated to the getter-flash station, where a combination of RF induction heating and/or high-voltage discharge flashes the barium getter.
Finally the tube is removed, the base wiring is attached to the external base (if it is an octal base type) with a special heat-resistant cement, and the finished tube is ready for aging in a burn-in rack. If the tube meets a set of operational specs in a special tester, it is marked and shipped.

By Eric Barbour

Information from www.vacuumtubes.net