VALVE TYPES & MANUFACTURING TECHNIQUES (xiv) - EVEN MORE ON B7G & B9A VALVE ASSEMBLY - CAGE TO BASE MOUNTING

Now we come to the bit where the valve base and the electrode cage are married together.  It was vitally important that the cage and base were in perfect alignment otherwise the assembly would not fit centrally within the glass envelope and hence a perfect seal betwen base and envelope would be difficult if not impossible to achieve.

Correct alignment was ensured at Mullard by making three key welded connections whilst the base and cage were held in a precision jig as shown in the picture below: - 

As you can see, these jigs were beautiful precision pieces wrought by extremely skillful toolmakers.   Taking the jig, an operator would locate the cage in the perspex cradle which slides on two cylindrical guides.  When the base and cage were mounted, the cradle carrying the cage was moved such that an anode and two screen tags were correctly aligned with the corresponding base support wires.  The two were joined using a spot welding machine which used two pointed copper alloy electrodes and a heavy electrical current to effect the weld.

Once these three key welds were made, sufficient structural rigidity was provided to remove the partially welded assembly from the jig in order to access and make the remaining welded connections to the control grid, screen grid and remaining anode tag.  

I think you are by now getting the message - these valves truly were hand made by a very labour intensive process using specialised custom made equipment expertly wielded by very skilled operators having supreme dexterity.   So, I would exhort you all to revel in the enjoyment that these difficult to make, enigmatic, thermionic devices make to your audiophillic pleasure now you're finding out how much of a kerfuffle making them entailed!

Once all welded junctions were made, the assemblies were placed in metal dust proof caddies to be returned to the parent factory for the next stage in valve assembly...... to be continued..............................