Blog posts of '2012' 'November'


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


With the electrode cage sorted, the next stage was to prepare the valve base.  In the valve build described - the DF91 - this has a B7G glass base, the manufacture of which was the subject of an earlier blog entry where we left the valve base as a glass button having seven composite wires, each comprising of a valve pin, a seal and an electrode support wire, all very nice, but an electrode cage cannot be mounted on the valve base as it stands.

The valve base prep team had to first prepare the base using a special machine that both cuts the electrode support wires to correct length and then bends them to the exact position to enable mating with the corresponding connection from the electrode cage.

In the photo below, on the left, you can see a valve base as supplied and then to the right after electrode trimming and bending prior to mounting the electrode cage onto the base: - 




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.


Here's a photo to warm the cockles of your heart.  A deputation of eminent Pye representatives visited the Mullard Mitcham valve manufacturing facility on 29th November 1951 - nearly 61 years ago and the visit was photographed for posterity.  Can you see Hilda Trumper peeking coyly around her MEK task lamp? - '' Eeee ducks, as ee tekken the picture yet?"  

Some of you may be asking why is that man in the light grey suit behind her wearing a pained expression?  The official classified Mullard management report on this visit records that this photo was taken at 13:30 after everyone had dined in the canteen (including Hilda) on Brown Windsor soup, followed by Roast Pork, red cabbage, sprouts and boiled potatoes then rounded off with lashings of spotted dick and custard........ hmmm, Trumper by name and by nature perhaps.



In previous blog entries we have seen how many individual valve components were made, now we will look at how they were assembled.   For component manufacture, we also saw how many automated production machines were utilised, however, when we come to valve assembly, things were quite different in that manual dexterity utilising just hand tools and a few light jigs are essential - yes - your valve was truly 'hand made'.

Another difference is that component manufacture and the final stages of valve making - pumping, sealing and testing - were done at the two main Mullard factories at Blackburn and Mitcham, a large proportion of the assembly work was undertaken at a number of smaller 'feeder factories.' These feeder factories used small work groups to produce certain valve types with the record being held by the Gillingham feeder factory where 86 people produced 12500 miniature battery valves (D series) in a day with a rejection rate of just 60 - or 0.5%.

Let's now look at how this performance was achieved.    We start with the individual components required being delivered to the feeder factories twice daily by a van which on it's return journey carried the assemblies prepared since it's prior visit. Each type of component was transported in a sealed metal container to prevent dust contamination.  

From the transport containers all metallic components were fed into a muffle furnace where they were stoved under a hydrogen stream at 950oC for 10 minutes before being cooled for 20 minutes prior to issue to the valve assembly stations where runners filled the bench hoppers at the valve assembly stations: - 


At these work stations, teams of girls worked to assemble the electrode cages of a particular valve type.  A typical team comprised of one grid cutter, two assemblers and two welders.  As you can see in the photo below, there were quite a few bits to put together for this DF91 RF pentode and unlike a typical chap assembler, when they had finished they didn't have any bits left over!!!!! : -




Today, I have had an e-mail from Veaceslav in Moldova, asking what tests the MHSVT can do.   This valve tester can do seven main tests sequentially and these are: - 

Test 1: check heater filament continuity.

Test 2: check insulation between electrodes - cold.

Test 3: check heater - cathode insulation.

Test 4: check insulation between electrodes - hot.

Test 5: check reverse grid current is acceptable.

Test 6: measures emission indicating 'goodness' on a CRT screen.

Test 7: checks correct connection of each electrode to each valve pin.

Parameters for each test are set using the test card for a particular valve which is slotted into the rear of the tester.

A useful tester but for me, I prefer the AVO VCM series and this is what we use at Mullard Magic HQ.



Hey, just look at this nifty invention from Mullard in 1951.  Using flying spot scanning, the time bases of two cathode ray tubes, one a transmitter and the other a receiver are synchronised.  In the transmitter, a raster is projected onto a glass overscreen.  In the absence of any writing on the overscreen, the light beam passes through the glass.  If however, a mark is made on the overscreen then some light is reflected then detected by a photocell.  The electrical currents produced in this way are amplified and used to modulate the beam in the receiving cathode ray tube which results in a facsimile reproduction of the original work.

This was hailed as a very clever piece of kit in 1951 - no PDA, tablets or iPAD then you know and these pieces of kit were so big that no-one could steal them easily....... unlike a PDA, tablet or iPAD!!!!!!


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.


As we have previously seen in this blog series, grids were produced in continuous  4 feet lengths and then cut to size for valve assembly - just like in the photo below: -

As Mullard used millions of grids annually - all of which were cut by hand - in order to increase consitency and throughput, something had to be done.   Accordingly, the Mullard Engineering Department developed an automated grid cutting machine in which grid lengths are loaded into glass hopper tubes which allow whole strips to be cut to length sequentially.  All an operator needed to do was keep the glass tubes filled and use the shadowgraph screen to monitor the quality of the cut lengths.  As needed, a limited range of adjustment was allowed for the operator to optimise the correct length of cut grid.

Here we see Trudy Firbush operating the automated grid cutting machine at the Blackburn works: - 



Well, we're going to talk about the ECC81 twin triode today, originally designed for TV applications, yes, that's right - TV!!!!     All the other valves we have discussed in this World Series of blog entries had been designed for use at the then used British TV frequency bands of 45 - 70 MHz.  However, other countries used carrier frequencies at a much higher range within the 100 - 300MHz region at which a low noise level high grade triode would be needed so enter stage left.... the ECC81.

The ECC81 was great because it was inherently a 'low noise' device, for TV, noise manifests itself as random variable emission giving a 'shot silk' screen effect or alternatively due to electron distribution variance which gives a 'partition veil' type of noise but this is normally seen with pentodes only.    The ECC81 as a triode would hence only be susceptible to  'shot silk' noise and even then not much of it at all.

It is strange to think of the ECC81 as a TV RF amplifier or frequency changer due to it's useage in many audio iconic amplifier designs but we cannot change Mullard design historical documents!!

Unfortunately, a photo of a Mullard ECC81 is fairly boring so instead, please enjoy this photo of what is regarded as the best ECC81 variant of all time, GEC's A2900 or in military guise, the CV6091: -