Blog posts tagged with 'ELECTRODE CAGE'



Today's photograph from the Mullard archives shows Albert Pumphrey, a valve engineer from the Blackburn Works examining Hilda Brooks' efforts at delicately constructing an electrode cage in May 1952.    The archive record does not record whether or not Albert subsequently asked Hilda to accompany him to see the new hit film Singin' In the Rain at the Savoy Picture House in Darwen some time after their Blackburn Valve Assembly tete-a-tete or if she said yes or no!


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..............................


We last left valve assembly where we had formed the electrode cage, today, we will describe what happened next in the valve assembly process.  Before the electrode cage could be mounted on the valve base, four additional components needed to be added and welded into position.  The first two components were the top and bottom screen plates and projecting tags which are welded through onto tags on the outer screen as per the photo below: - 

The third extra component is an L shaped piece of metal which is first welded to the bottom screen plate and then bent over and welded to one of the suppressor grid support rods, finally, the last piece, the getter frame is welded to the top screen plate as shown in the photo below: -

Whew, and now you can see that after all of this dexterous handiwork, the elctrode cage is finally assmbled, ready for the next stage in valve assembly.... and my next blog entry.


Let's look at the PINCH today.  The PINCH is something borrowed from incandescent lamp manufacture and is where lead-in wires pass through a valve envelope in a cleverly wedge shaped piece of glass which not only provides a perfect impermeable seal but also provides a very nice mount upon which an ELECTRODE CAGE can sit.

As can be seen from the photo sequence below,  as we work from A to C, in A you can see the sectioned PINCH at the base of the envelope showing clearly the transit of the wires through the impermeable glass seal.    In B we can clearly see an electrode cage being mounted atop the PINCH.   In C we can see moving from L to R, the support structure and thence the cathode being added as the mounted electrode cage is successively built up.

This next photo shows the semi-molten glass stem being made into a PINCH on a Mullard glass blow-form machine.

So, there we are, we now know all there is to be said about valve PINCHES!


Continuing our journey to look at what's inside a valve, today we are going to look a little further at our EL37 pentode with the envelope broken open.  After a little more surgery on the ELECTRODE CAGE we can see in the centre. the white vertical cylinder of the CATHODE, surrounding that are the three concentric grids, the inner being the CONTROL GRID, then comes the SCREN GRID and finally the SUPRESSOR GRID.


I thought I would write a series of blog articles on what is inside a valve.  Famously, the head of Mullard UK, the ex-Philips SS Eriks once stated that the only British thing inside a Mullard valve was the vacuum and after I recently waffled on to some poor chap about electrode cage construction, pinches and getter flashing, they were bemused so I thought that I had better expand on the great Dutch master's scathing comment, so, without further ado, let's talk about......... what's in a valve..................................

The picture below shows a sectioned Mullard EL37 output pentode, as you can see, this valve is typical of those which have PINCH consttruction.  The EL37 valve has an OCTAL BASE and you can clearly see the glass PINCH upon which the ELECTRODE CAGE is mounted then secured between MICA plates.   We will go on to discuss each of the components mentioned and highlit in upper case text in future articles but for now, just enjoy this scintillating picture of an undressed EL37!