Blog posts of '2013' 'February'


We had an interesting case in the workshop over the weekend, a Philips Monoknob from 1936 with a squeaky Bowdon cable control.  Now these as some of you will be aware are tricky.  We've gone hi-tech here at Mullard Magic none of this 3 in 1 stuff, instead, I reached for my handy dry slide spray et voila, squeak sorted.   You too can get similar results by using the same wonder product I do so without further ado, here are the details of what you need to ask for from your friendly supplier: - 



FUKKOL PTFE DRI-SLIDE SPRAY & MOULD RELEASE AGENT is designed to give fast, efficient and clean release with faithful reproduction of the mould profile however complex the mould or minute the detail. Suitable for use on all plastic and rubber mould tools including injection, compression and blow mouldings.


Penetrates and spreads to all parts of the most intricate moulds to give full, clean release whilst the dry film will not affect profiles or interfere with precision fits. Applied film has excellent adhesion, is non-staining, will not transfer to product and can be safely used without fear of blemish or any adverse effect on subsequent finishing operations.





Well, just look what we have got for you today, these images of a Lancaster having a scrap whilst mounting Operation Chastise are very detailed and very impressive if a little pop art.    I am sure you will be captivated by these as I was especially when you discover that they are painted on a Ford Cortina Mk1 - simply super.


There was a puritanical streak amongst the top echelons of Mullard management you know.  As well as competitor product analysis, Mullard kept a weather eye on the advertising and technical literature of their contemporary competitors.  I have here a company memorandum dated 1953 which detailed their concerns with the following photograph which was said to demonstrate sexual overtones not ideally suited to the wholesome promotion of thermionic products: -

The report does not go on to say whether or not it is the model's Mona Lisa - esqe smile or the implied message that size matters that may have caused a maiden aunt to flush or a stout young blade to swap his well thumbed Gamages foundation wear and corsetry catalogue for the rather more prosaic Cossor valve manual.

But Mullard's would never use such subtle suggestiveness in their literature as you will see in the following example photograph taken from a piece of early 1950s advertising material, note the demure smile and the buttoned collar that served to inform without inflaming the unwary reader: -



I thought that today, we would conclude my review of the Mullard battery D series valves with the arrangement of the filament circuit in which they may be used.  The filament current for all valves in the D series is 50mA for a nominal filament voltage of 1.4V. The output pentodes however, have two  filament sections which may be connected in either series or paralllel configuration, a useful facility where one can either use a single dry cell with parallel heater connection or conversely, a series chain.

To use series operation, precautions need to be taken to ensure that the voltage acrosss any single 1.4V nominal filament device does not exceed the range 1.3 - 1.6 V and this may be done by shunting resistors being fitted across the individual 1.4V filaments as shown in the example below: -

A good rule of thumb is to design the filament circuit such that the voltage across each filament section has a nominal votage of 1.3V and a maintenence range of 1.2 - 1.4V whether or not the supply voltage is the 1.5V battery, the 2V accumulator or a mains PSU.  

The big question then is how to calculate the values of the shunt resistors needed?   When operated at 1.3V the filament current is 47.4mA with no cathode current flow, however, in actual operation, cathode current flows through the filament in a non linear manner. Another consideration is that in starting the filament from the positive end of the chain then each filament has to bear the cathode current of each of the preceeding valves.  Hence the value of the shunt for each individual section must be calculated separately on the basis of the cumulative value of the cathode current at that particular point in the chain.  

Going back to the non linear flow, we need to consider that the current at the positive end of each filament is less than the average filament current by an amount equal to one third of the cathode current.  Similarly the current at the negative end of each filament is greater than the average filament current by an amount equal to two third of the cathode current.

Still with me, not to worry, let's try some practical calculations using the filament chain diagram shown previously:-

If the cathode current at output valve, V4 is 6mA then 2.4mA will flow to the positve end and 3.6mA to the negative end.    

The filament current at 1.3 V is 47.4mA exclusive of cathode current so: -

the total current at the positive  end  of the positive limb is 47.4 - (1/3 x 2.4)  = 46.6mA.

the total current at the negative end  of the positive limb is 47.4 + (2/3 x 2.4)  = 49mA.

the total current at the positive end of the negative limb is 47.4 - (1/3 x 3.6)  = 46.2mA.

From this little lot then, the current, I  in R1 must be 49 - 46.2 = 2.8mA for a V of 1.3V therefore R of R1 must be (1.3 x 1000)/2.8 = 464 Ohms.

Whew, in these blog entries we stumble from chemistry to higher maths, well, that's enough for me so if you want to complete the calculations for V3, V2, V1 then please ava go by yourselves in a darkened room with no gibbering and gnashing of teeth!

At least you now know how to design filament circuits to suit the operation of these smashing little valves should you so wish to. 


After the staid learnings on photocells, I thought that perhaps today's blog entry might be historical and factual, yet  frivolous too!  For your delectation then I present the following Edwardian advertisement - not radio or valve related I know - by The North British Rubber Company: -


What a wonderful contraption and following recent events, I can think of someone I would happily demonstrate this fascinating devices usage on and not to keep them warm with either!

The North British Rubber Company was situated in Edinburgh, Scotland and was started in 1855 by an American chap who brought the technology for working and forming rubber to the United Kingdom.  Some of you may be thinking . " Well fancy that, I've never heard of them" but they were a large concern and in their heydey employed nearly 5000 people making not only hot water bottles but car tyres and even rubberised cloth for the production of barrage baloons, yes, some 225,000,000 yards of it during WW2.  

You will however have heard of the company after it changed it's name  in 1965 to Uniroyal.  I am sure you will also have heard of or indeed even posess a pair of Hunter Wellington Boots, the Prince of wellies with their traditional logo proudly embazoned on the front upper of each boot, as yes, these too were manuafctured by North British Rubber/Uniroyal

LIke most things in life, all good things come to pass and North British Rubber/Uniroyal, closed it's Edinburgh factory doors for the last time in 1973, however, their legacy lives on and today, you can still buy a pair of Hunter brand Wellington boots but alas, they will have been made in China.


Photo-electric cells may be distingusished as belonging to one of three different types, these are: -

The barrier layer photo-electric cell; in which a voltage is produced when light falls upon the junction of two different materials. - a good example of this type is the Selenium cell fitted to the famed Weston Master (Megatron) light meters.

The photo-conductive cell; in which the resistance of the cell varies in proportion to the amount of light falling on them - a good example of this type is the Cadmium Sulphide (CdS)  cell which was the mainstay of most SLR cameras having through the lens (TTL) light metering from the early 1960s to the mid 1980s.

The photo-emissive cell; in which electrons (and hence a current) are liberated from the cathode when irradiated by light.

The first two types of photo-electric cell are mentioned for completeness and interest only and all further discussion will centre around the latter type - the photo-emissive cell which just like a typical radio valve, has an anode and a cathode.  The cathode of a photocell consists of a flat metal plate coated with a photo-emissive material.  The anode typically takes the form of a metal rod or loop situated in front of the emissive cathode.




The operation of any valve type depends on the current flow caused by drawing free electrons from an emissive cathode to the anode by a positive potential being applied to the anode.  By applying a potential to grids interposed between the anode and cathode of a valve, the flow of electrons may be controlled - vary the grid voltage and a corresponding variation in anode current results which may be greater than the voltage applied to the grid and hence this is how a valve ampifies a signal.

However, rather than using grid potential to produce an electrical variation, we sometimes need to produce electrical variations which correspond to variations in light and this is where the photocell comes in.    To understand the photocell we have to do a little chemistry, as we know, some elements are very stable, however others are less so.  In Caesium, the energy from light can upset a Caesium atom's stability and force it to give up electrons.   However, 'dope' Caesium with Silver and Antimony to form an amalgam and an element of stability is introduced whereby only specific quantities of electrons are produced in reaction to a specific level of light and it's colour and it is on this basic premise that a photocell works  Right, that's enough chemistry for today so for anyone still awake, I shall just let you know that in my next blog entry we will look at types of photo-electric devices in a little more detail


An acquaintance came to see me, all self important and gloating - he had a Baird Televisor lamp and it was picked up for a fraction of it's true value, obtained for a song and Stukey Bill had sat in front of it for ages, yeah, right.    Well done said I with a brittle smile and a demeanor tinged with a soupcon of envy, however, this soon evaporated just like his dreams of a healthy profit when he produced his prize with a flourish and I lampooned his hopes by showing him a photo from contemporary Mullard documentation showing a similar specimen of his 'Televisor lamp' : -  

Pretty good huh, but alas not Televisor material.   What my hapless pal had stumbled upon was a photocell designed for sound-on-film projectors, although there were no markings I pronounced it a Mullard emission photocell type 20CV - unfortunately, slightly less desirable than a Televisor lamp!!!

These devices crop up from time to time and the technology behind the is interesting and not often described so, in  my next blog entry, I shall describe the theory, operation and usage of these valves and indeed other photo-electric phenomena - I'll bet you can hardly wait!!!!








Mullard used a few dodges to ensure that blemished face plates were not used in tube manufacture such as when fitting the EHT connection in the cone of the 12 inch tube, this would always be located immediately above the gather or shear marks so that when the tube was fitted in the set, the tube mask would hide these marks - pretty clever.

Otherwise, it was a matter of 100% inspection and selection and this is how it was done.   Firstly, only blemishes which could be seen from a distance of  3 feet 6 inches distance either under external lighting or with a raster on the tube were considered.

Any blemish which would be hidden by the tube mask could be disregarded.    The planar surface of the faceplate was divided into a number of zones with the specification for the central area being more stringent than for the edges.  Consideration was given to size location and distribution of blemishes such that they would have no obvious effect on picture viewing.  In this way, Mullard enjoyed a les than 1.5% failure rate on screen blemishes.   We'll close today's blog entry with this photograph of a Mullard man going 'bog eyed' looking at screen blemishes and phosphor coating faults: -


Today, let's look at the process for tube screen (or faceplate) manufacture that was employed by Mullard in the early 1950s.   If we had visited Mullard's tube manufacturing area we would have seen a tank holding 250 tons of molten glass and nearby, a work bench holding moulds which were contoured to match the required screen shape.   Glass blowers working in this area armed with iron rods, topped with a ball of fire clay would dip their implement into the molten glass tank and withdraw a bolus of molten glass known as a 'gather'.    The 'gather' would be held over a mould and with a quick jiggle, gravity would cause the treacly molten glass to flow into the mould.  The molten glass would spread slowly within the confines of the mould with final shaping being helped along by a power operated plunger that would descend to press the glass to fill the mould hence completing the faceplate.

I am sure that having read the account above, two things may be appreciated, first, how manual and potentially dangerous the manufacturing process was and secondly, the potential for marking of faceplates was pretty great.     As can be appreciated, when the 'gather' first strikes the relatively cold mould, it starts to solidify, causing an arc shaped mark in the centre of the faceplate.  These marks were termed gather marks and were usually seen on the outside surface of a faceplate and looked a little like this: -

As the 'gather' broke away from the transfer rod and plopped into the mould, again, a surface blemish could occur, again in the form of an arc but this time pointing toward the faceplate centre.  These marks termed shear marks were usually found on the inner surface of the faceplate and looked a little like this: -

Where localised cooling occurred when the initial flow of glass over the mould resulted, sometimes the central portion of the faceplate could have a polished appearance whilst the remainder was noticeably matt.  In order to reduce this defect, every mould would be reground daily or even chromium plated to reduce the occurence of this blemish known as matt & polish which looked a little like this: -

The above described examples comprised the most commonly seen and hence troubling faceplate or screen blemishes, however, there were more......    If the face of a 1950s moulded tube were examined closely, four small glass pips could be seen around the peripheral endge of the faceplate surface  - these artefacts correspond to vent holes which were provided in the plunger to permit the escape of air trapped betwixt plunger and mould.  The pips were carefully placed to be in the extreme corners of the faceplate when the tube was installed in a TV receiver.

When you consider that faceplates were produced by a manual process in an industrial environment, occasionally tiny ripple marks, very small bubbles and foreign matter such as sand or scale particles could end up embedded in the faceplate as well.

Next time, we'll disuss the quality control checks that Mullard applied to minimise detection of these blemishes by a discerning consumer populace.