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Posted by STEVE M on

In the preceding two blog entries, I have discussed how tungsten wire was produced by Mullard's - incidentally, exactly the same process was used to make molybdenum wire too.   Both of these wire types were used to manufacture the grids we see in multi electrode cages.   As you can no doubt imagine, the performance of a valve is dependant upon extreme accuracy in the winding and positioning of these grids so let's have a look at what's involved in doing so.

Let's consider the Mullard EF86, a B9A pentode used for many applications from TV IF strips to a floating paraphrase phase splitter in say a Quad II amplifier.  The control grid in an EF86 comprises of two copper wires 20mm in length and 0.75mm diameter spaced 5mm apart which serve to form a rigid support for the grid spiral.  Onto this support, molybdenum wire of 0.05mm diameter is wound over the copper wires with each turn spaced 0.125mm apart.   That's what I call precision, so, how was it done.....

First, here's a nice picture of a Mullard grid winding machine: -

As you can see, the grid winder is very similar in appearance to a typical engineering lathe.  This machine produced grids in continuous lengths of 120cm which were then cut into individual pieces.    At the outer end of the machine was a bracket carrying two reels of support wire which were parallel drawn into the machine via a travelling tailstock.  The rotating machine head was fitted with a reel of grid wire and two wheels - sharp edged and flat rimmed -  that rotate with the head.

In operation, the sharp edged wheel cuts a series of notches in each grid support wire, the grid wire is wound into these notches and finally, the flat edged wheel presses over the edge of the cut support wire thus cinching the grid wires in place. When each continuous length of grid had been wound, it was chemically cleaned then cut into individual grids and 100% visually inspected.  In the picture below, you can see Agnes Shufflebottom diligently inspecting grids for winding defects: -

If any grids are found to be mis-spaced, they can be delicately adjusted with a special gauge which can correct overall dimensional defects, here you see the hand of Agnes' cousin Euphemia, correcting a wayward grid: -

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