Blog posts tagged with 'CRT'


A number of tests were undertaken on the finished CRT under actual operating conditions, below, we shall discuss each of them in turn: -

Cathode emission -  insulation resistance between all electrodes was checked, anode current, Ia was checked as was "cut off" which is the negative grid potential required to nullify emission

Vacuum integrity - the degree of vacuum was determined using an ionisation gauge.

Final visual check - the focus was checked, the raster was checked, the faceplate was checked for any blemishes, unevenness of colour or stray emission, the whole tube structure was checked for mechanical defects.  Only then was the tube given a final polish clean, marked with a type number and then packed in it's sale carton for storage in the stockroom.

Above, we have described the Quality Control (QC) tests that were carried out on each and every CRT that Mullard manufactured but in addition, Mullard were early exponents of Quality Assurance (QA) and from each production batch of CRT a sample was taken (√n+1 sampling plan) then independently checked for the same range of tests as the on sale devices. Each test was logged using Shewart charts such that trends in performance could be seen and corrective action taken to ensure product consistency.  Pretty impressive and all implemented long before our American colleagues reinvented this form of quality management and renamed it Six Sigma!

Of the QA samples taken, a portion were stored as batch retained samples whilst the remainder were subject to Accelerated Life Testing where operating conditons were chosen such that a 1000 hour continuous run would mimic the typical domestic useage experienced in approximately 6 years of operation.   This data was used again to assess product consistency but also to give valuable data on expected product Life Cycle and End of Life product performance.







Well, it's been a long journey, having inserted the electron gun, withdrawn the air from the bulb and sealed it off, the only remaining thing to do is to perform that final process of "ageing" and a Mullard Cathode Ray Tube was available for sale.   More deatil on this final process in my next blog entry but for now, I just wanted to share with you this evocative photo of tubes being aged - don't they look cute in their Moygashel linen bags.   As we all know, linen from Maigh gCaisil (the place where the stone fort is)  is much prized for it's softness and hard wearing characteristics and was just the thing for 1950s Mullard  CRT manufacture for whilst the valve assembly girls wore nylon, the CRTs wore only linen sourced from Ireland: - 


Now here’s something you don’t see every day – it’s a Monoscope. These were invented in the immediate post war period in the GEC laboratories at Wembley following on from the need for broadcasters to derive a signal with which to test their equipment.

Within the cathode ray tube (CRT) of the monoscope, a solid silver plate, photo etched, curiously using photo sensitive fish glue (yes, really!) and carbon coated depicted the information that had to be produced – in the case of this device, the redoubtable BBC Test Card C.

A signal was generated as a focused beam of electrons scanned the target and a sharp facsimile of the target was produced due to differential secondary emission between the silver plate and the carbon screening where electron impingement onto the silver produced an output and conversely, the carbon screening, no current. In place of the fluorescent screen seen on a typical CRT the faceplate connects to a metal plate inside the tube. This plate has an insulated photographic pattern engraved upon it. The scanning beam produces an output where it strikes the plate and no current where it strikes the insulation.

The monoscope was electrostatically focused and magnetically deflected and had an anode voltage of 5kV to ensure a tight focused beam which was essential to achieve good definition of the picture plate, the maximal value being approx.100 lines per inch with a beam current of 5uA and a signal output of 2.5uA peak to peak.



This arrangement was an elegant way of producing a stable image and was much more cost effective than having a camera pointed at a printed test card. It was, of course, limited to a single design. Additionally it was a monochrome only technology and its use ceased with the advent of colour television.

Apocryphally, R.A Dixon & D.M. Mackay proudly presented their invention to an excited GEC board of directors – one less enlightened chap stated, “Very nice, but what does this thing actually do that a magic lantern and candle cannot?….”

I am indebted to my pal Donovan C from Chinnor for allowing me to be the custodian of this exquisite piece of thermionic engineering.


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. 



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.


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.


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!




Today's evocative photo shows Mulard Long Life picture tubes undergoing life tests at the Mitcham plant sometime in mid 1952.    

On the test rig, applied voltages were regulated and the beam current was fixed.  The test card used closely simulated a screen raster.  Here we see Trevor Wibbley about to take voltage characteristc measurements, light output measurements and assess subjective picture quality.

What is special about the tubes pictured is that they were the first of those produced by Mullard with tinted glass faces.   The tinting was introduced as an attempt to boost contrast when viewing in daylight conditions.due to the tinting agent cutting down reflected light hence making the black  picture content appear much darker even though the tint did cut down the bright emitted light!