Blog posts of '2013' 'May'


 Well, this is what an electron gun looks like: -

It comprises of an indirectly heated cathode, a grid and two anodes.  The inner anode is maintained at a positive potential of up to 7kV on these early electron guns, yes, really!   Let's now look at the bits and pieces individually but first, an exploded diagram: -

The cathode assembly - this comprises of a short nickel tube, one end of which is closed by a disc coated with an emissive admixture of barium and strontium oxide. The heater, which comprises of a spiral tungsten wire coated with an insulating film of alumina was inserted into the open end of the cathode tube.  The cathode was mounted onto the centre of a mica mounting disc which carried connecting lugs for both the cathode and heater.

The grid assembly - this comprises of an inverted metal cup, the solid end of which is pierced.  The mica disc carrying the cathode was inserted into the grid cup and the electrons emitted by the heated cathode emerge through the grid piercing.

The anode assembly - this comprises of two anodes.  The first anode is a nickel cup with a central aperture on its closed end and a wide flange at the open end.  The final anode is a bent nickel tube with a flange at the outer end which is closed with a metal disc having a central piercing through which the electron strem emerges.  The final anode carries three springs which allow a path for EHT and also to allow centering of the gun when fitted to the tube neck.

Each of these individual assemblies were mounted onto two steatite ceramic support rods whilst aligning essential clearances with the aid of a mounting jig and distance pieces.  The electrode system was then mounted onto a glass base 'button' which was fitted with five connecting wires and a vacuum takeoff spigot for final pumping.   A final 100% inspection was made of the assembled electron gun using a projection shadowgraph and remedial tweaking to fine tune alignment was made at this stage.  The electron gun assembly was then passed for incorporation into the cathode ray tube - more on this in a later blog entry.


Today, we are going to look at the Mullard CRT numbering system which gave type numbers as two letters followed by two groups of numbers which translated as below: - 

the first letter

D - electrostatic focussing  and deflection CRT as used in oscilloscopes.

M -  magnetic focussing and deflection CRT  as used in TV and radar.

the second letter

B - screen phosphor gives a blue short persistence trace.

F - screen phosphor gives an orange long persistence trace.

G - screen phosphor gives a green medium persistence trace.

P - screen phosphor is double layer combining a blue short persistence trace with a yellow long persistence afterglow.

R - screen phosphor is single layer giving a long persistance green trace.

W - screen phosphor gives a medium persistence white luminance for television picture tubes. 

first figure group

4 - screen diagonal or diameter of 4 cm or 1.75 ins.

6 - screen diagonal or diameter of 6 cm or 2.5 ins.

31 - screen diameter of 31 cm or 12 ins.

36 - screen diameter of 36 cm or 14 ins.

41 - screen diameter of 43 cm or 17 ins.

second figure group

This figure grouping which is generally separated from the first grouping by a hyphen is a design development reference, for example the MW36-22 and MW36-24 are both direct viewing television picture tubes designed for magnetic deflection and focussing with a 36 cm or 14 ins screen - the only difference is the 22 suffix was superceded by the 24 suffix unit. 



Magnetic cores need to be readily magnetised by a small force and conversely loose their magnetismjust as easily  - just like soft iron or Ferroxcube as we discussed last time.  For the production of permanent magnets a whole different set of properties is required where they need to be magnetised readily but NEVER loose their magnetism and that's where Magnadur, another Mullard invention comes in.

Magnadur is a sintered oxide admixture of Barium and Iron and is practically impossible to be demagnetised once magnetised - unlike conventional steel and steel alloy magnets which can demagnetise with a sharp knock or with temperature change.   A chief use of Magnadur was for CRT focussing magnets where their stability and light physical mass were a real boon.  Here's a picture of a Magnadur focussing ring by Elac: -


The inductance of a wound component can be greatly increased if the coil has a core of magnetic material, not only that, such a core can reduce power sapping eddy currents too, trouble is, magnetic formers such as soft iron are heavy, magnetic - which brings it's own problems and generally a pain in use!    Wouldn't it be great if a non magnetic, light and durable coil former material could be found - WELL, Mullard boffins did just that and christened their prodigal progeny, Ferroxcube!

The chemistry of the Ferroxcube material is interesting in that it comprises of mixed ferrites with manganese, nickel and zinc salts which are ground in a ball mill and then sintered which is a fusing process similar to pottery firing.

Ferroxcube cores have a high specific resistance which imbues the former with negligable eddy currents even at high frequencies.

Ferroxcube cores are unaffected by humidity.

Ferroxcube formers can be smaller and therefore much lighter than a metallic-magnetic material.

Ferroxcubes were successfully used throughout television and radio manufacture as IF coils, blocking oscillators, timebase transformers, deflection coils, control inductors, transformers - and of course ferrite aerials!!!

Sintering allows complex shapes to be homogenously manufactured provided you have a tool, just look at the complexity of this Ferroxcube deflection coil: - 


The last stage in the preparation of the CRT bulb was the application of an internal graphite coating which forms in part the connection between the EHT terminal and the final anode.   This was done by mounting the bulb into a lathe and whilst rotating it, a flexible handled directable brush loaded with colloidal graphite paste was pushed into the tube neck and thence into the bulb then manipulated to coat all internal surfaces of the bulb except the faceplate and here is a picture of Lester Creed, graphite coating at Mitcham during the early 1950s.   

The bulb was removed from the lathe and placed on a suspension carousel which transported the bulbs through a tunnel furnace where the grpahite coating was dried and the faceplate luminescent layer was hardened at which point - we have a finished bulb - simples as the meercats say!

Next time we'll look at the other major component of the CRT - the electron gun.


The luminescent screen of a CRT is composed of a mixture of barium and zinc salts in colloidal form which are applied to the internal surface of the face plate.  This was done by placing the tube face down on a work table and then adding a buffer layer of distilled water to an approx depth of two inches.   The luminescent mixture charge was suspended in a potassium silicate solution and quickly added to through the tube neck thouch a special funnel which had a shaped rose fitted to ensure minimal disturbance of the buffer layer yet ensured even distribution of the luminescent mixture.

The tube was then allowed to stand for several hours by which time the luminescent charge settled on the faceplate in an even deposit whilst the silicate formed a layer of gelatinous deposit on top which aided the stability of the settled film.    At the end of this period, the tubes were carried to a decanting table and secured to suction pads.  The table was slowly tilted until the supernatant liquor very slowly poured away leaving the luminescent screen deposit on the faceplate inner surface.  

When all supernate had drained, the bulbs were suspended on a hanging carousel and transferred to a drying table where a sparge tube was introduced through the neck to bleed cool filtered air into the bulb to dry any remaining liquid - just as shown in the photo below: - 


Last time we left this story we had a complete cathode ray tube bulb but we still have a few steps to go before we have the finished tube, so, what's next?   Well, before the phosphor can be added to produce a luminescent screen, the interior of the bulb must be perfectly clean and this was done by pouring a portion of dilute Hydrofluoric Acid (HF) into each tube whilst situated on a shaking table where the bulb contents were continuously swirled for 20 minutes.  HF is a particularly nasty commodity and is commonly used to etch glass and you may remember sachets of this reagent being sold along with a stencil set to allow you to permanently etch your car windows with your registration number - quite in vogue during the early 90's but I suppose 'Elf & Safety' and lefty nanny-ness has scuppered that particular usage these days!!!

After twenty minutes, the acid is tipped out, the bulb rinsed with tapwater and then given a final rinse with distilled water. Once washing was completed, the bulbs were supported on an suspended carrier system and passed through to the next production department and next process where the luminescent screen was laid down.

We'll take a look at the phosphor and luminescence laying process in a future blog entry.


Well, we had a walk around the National Vintage Communications Fair yesterday and there were all sorts of things on offer and the attendees were as diverse as the treasures on offer for sale as these photos show: 



In January 1953, Mullard management got a letter of praise from Messrs. Cuttriss of 49 Frederick Street Birmingham, purveyors of fine electrical apparatus.    This television and radio shop, nestling in the heart of Birmingham's jewellery quarter was well renowned for its wide ranging stock of the latest equipment.   This area was home to over 3000 people within it's approximately 1 square mile area and today, still has the largest concentration of jewellery related businesses in Europe which produce 40% of all the jewellery made in the UK as well as being home to the world's largest Assay Office, which hallmarks around 12 million items a year.

With an affluent customer base and local community the area also attracted it's fair share of scoundrels too and It seems that some jolly naughty fellows hell bent on obtaining a telly for the coronation decided to nick a couple of swish receivers from Cuttriss' shop window in an unsuccessful midnight raid.   It seems that the not inconsiderable weight of each television was too much for the thieves as they dumped them into the Birmingham and Frazeley canal from a canal bridge.   The 30 foot fall caused an impressive splash and the barge running over one of the sets completely smashed the cabinet..... but..... the tubes and valves in the chassis of each set worked perfectly after drying them out hence the reason for the Cuttriss praise for Mullard products.  Historical records unfortunately do not record whether or not the thieves were caught bang to rights, summoned before the beak and banged up in chokey!


What more can I say, here's a period photo of an avid viewers who is very upset that Britain's Got Talent hasn't taped properly:-