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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 couple of period photos of avid viewers, one of which is very upset that Britain's Got Talent hasn't taped properly:-

Today we'll look at the CRT neck production and their joining to the cone to make a complete CRT bulb.  The neck was the only glass part of the CRT to be made at Mullard Blackburn as the site glass plant was well versed in dealing with tubing which was also used to manufacture valve envelopes as well as tube necks.  In the glass making machine, molten glass flows on the outer surface of a hollow refractory mandrel so that a tube of molten glass is forced vertically through an air blast venturi.  At the top of the mandrel was a series of air jets which cool the produced tube such that it solidified sufficiently to be grabbed and drawn upwards by pairs of friction wheels which lifted the tube two floors to a work station where it was cut to length.

Each length was gauged for external diameter and weighed to check both internal and outside diameter.  Any variation from mean specification was controlled by adjusting the speed at which the tubing was drawn.  The tubing required for CRT necks was cut to the required length and then fed into a glass lathe where it was heated with gas jets to soften the glass which was then formed into a conical bell which corresponded to the end of the CRT bulb.

The shaped neck was then transferred to another machine and held in a jig which held it approximately 40mm from the bulb aperture.  The jig moved around a carousel where gas jets were applied radially whilst turning the jig until the glass on both parts softened at which point the jig moved downwards until the plastic glass touched.  Continued heat and downward pressure ensured a perfect join.   The completed bulb continued to move around the machine with it's temperature being reduced at successive stations.  At one point of the rotation, an operator fused a hole in the side of the bulb into which an EHT lead-in connection was fused.   The EHT connection was a number of lengths of wire sealed into a glass bead ready for fusing into the bulb.  

Bulbs were then removed to an annealing furnace where they were slowly cooled to relax heat induced stresses, the most critical of these being at the bulb and tube joint area.  To ensure a no stress product with minimal risk of implosion, the join area was scrutinised under polarised light to show up any inherant weakness which had not been normalised by the annealing process.   The completed bulbs were at this time in the early 1950s transported by road to the Mitcham factory where the final processes to turn them into finished CRT were undertaken. 

We shall leave this story today with a nice photograph of the individual glass components of a Mullard Rectangular Tube: - 

In the very early days of television, the bulbs for cathode ray tubes (CRT) were blown by hand, however, due to volume requirements, most CRT bulbs were moulded, being made in two parts with the flattened end termed the face-plate and the pear shaped body termed the cone.  As the separate parts were delivered to the Mullard factory, at this stage in the early 1950s that means the Blackburn works, the first stage after inspection for blemishes was to join the parts.  Any components marred by blemishing were returned to the glass moulders for re-melting - waste not want not!

Joining was carried out using a multi stage joining machine with the face-plate and cone being held on a loading jig whilst touching each other, the fixture then commences to revolve and was moved to successive stations where gas blowpipe flames of increasing intensity played on all areas of the join until the individual parts were welded together, successive stations then reduced the glass temperature gradually with the bulbs finally being placed into an annealing furnace where they were left to cool to ambient temperature slowly hence  relieving any heat induced stresses.

So,  that's the cone and faceplate sorted, next time we'll look at the manufacture and attachment of the CRT necks.

Well, a CRT is a thermionic electron tube so I thought we would follow our popular blog series on how valves were made with a similar one detailing the manufacturing process for CRT employed by Mullard.

Let's start with the basics, we can list the CRT main component parts as the envelope, the electron gun and the screen.

The envelope, consists of a funnel shaped glass bulb which was either round or rectangular in section and closed at it's widest end with a flat glass faceplate and terminated at the other end by a narrow tubular neck.

The electron gun, produced a narrow beam of high velocity electrons that move toward the screen, the intensity of this beam being proportional to that part of the picture being transmitted.  It was made up of an indirectly heated cathode, a grid which modulated the beam with picture information and two high potential anodes which accelerated the electrons towards the screen.  Final focussing of the electron beam and it's deflection such that it sweeps the entire screen area was influenced by a focussing magnet and deflection coil combination located coaxially with the tube neck.

The screen, was coated with a fluorescent material which had the property of emitting light when bombarded by high velocity electrons.  The amount of light produced depended upon electron velocity (a function of EHT potential)  and the rate of electron flow ( a function of grid modulation and cathode emission).

More detail on the sequence of operations of production will be given in successive blog entries but for now, feast your eyes on the following photograph of 'modern' Mullard produced black and white cathode ray tubes - simply spiffing!

Well, we are just back from seeing 'Junior' and his beautiful bride duly spliced.  Don't they make a lovely couple? Well done Simon & Tory!  And what a time we had in Northumberland, no radios or thermionic delights in sight though I did try unsuccessfully to get a rural garage owner to part with a rather nice Land Rover Series I!  More Mullard Magic meanderings next week as we have to catch up with our April episodes.