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Blog posts of '2012' 'October'

THE MULLARD WORLD SERIES VALVES (vi) - VIDEO OUTPUT PENTODE PL83

Although in 1951, we still had miniscule telly screens and 405 lines, Mullard was looking to the future as they knew the time would come when bigger screens and 'high definition' TV with more lines would be introduced.  But with increases of this type, the pre-exisiting video output valves were decidedly weedy and underrated.

To meet these future requirements, in 1951, the Mullard boffins designed the PL83 which essentially is an EF80 on steroids, whereas the EF980 has sand kicked in it's face because of a maximum anode disippation of 2.5W, the PL82 didn't as it boasted 9W, similarly, size is everything with a comparison of the EF80's mutual conductance of 7.4 mA/V against the big bad boy's 10.5 mA/V.  The steroid analogy actually is quite a good one as the PL83 had a very robust electrode cage to prevent microphony. 

 

MULLARD AT THE 1951 EARLS COURT RADIO SHOW

The national radio show was a big thing for Mullard and indeed any other post war radio manufacturer as they all pulled out the stops to present their wares to a burgeoning post war consumer boom.

The Mullard stand was large and extensive as this photo shows where chassis work from many manufacturers that utilised Mullard components were suspended on show - very outre!

 

The media in the form of Auntie Beeb was in attendance and here we see a BBC TV cameraman using real film ( and later presumably tele-cine) to record the new range of Mullard valves for the waiting nation.

With the BBC also came - was this term coined yet - celebrities - first we have Arthur Ferrier, the cartoonist drawing one of his charcters on the telescribe.   I quite like Arthur as he was an Analytical Chemist like me and then turned to cartoon drawing as a way to supplement his meagre earnings as a scientist.  He made it big in 1930 when he produced a weekly strip glamour cartoon called "Film Fanny"..... The mind boggles............

 

And just look at the photo above, even Sylvia Peters, the Televeision Announcer  was there looking looking at Mullard CRT tubes. 

Or what about some urchins trying to 'Beat the Electron' on a Mullard display.

 

Awefully spiffing and redolent of the time of lashings of ginger beer, or more correctly, lashings of boiled eggs - yum!!!!

 

 

THE MULLARD WORLD SERIES VALVES (v) - OUTPUT PENTODE PL82

With the introduction of wide angle picture tubes in the early 1950s came a commensurate increase in scanning deflection power after all going from 55o to 70o took some power -  but how to get that power?  This called for a new Mullard World Series valve - the PL82 output pentode.

This valve was specifically designed in 1951  as a frame timebase output valve capable of maintaining 70o frame deflection when operated from a 170V HT line.

VALVE TYPES & MANUFACTURING TECHNIQUES (vii) - MICA DISCS

Yesterday, we looked at what mica actually is, today we will chat in detail about the mica discs used in valve manufacture.     As you look at the electrode cage of most valves, you will see that the individual support rods and elements of the cage are supported in their relative positions by two or more discs of mica.  These discs are shaped and pierced by a series of precise holes which act as location points for the components to be supported.    The function of a mica disc is threefold; to insulate the electrodes from each other; to maintain correct electrode spacing; to hold the electrodes rigid within the envelope.  Below you can see examples of a typical mica in this quaint Mullard photograph:-


At Mullard, the mica washers as supplied from India or Canada are stamped to shape and punched with the necessary holes to produce the mica disc.  Very close tolerences of these holes is a must and hence samples of each batch of mica discs were inspected using a Shadowgraph - a magnifying projector whose screen is fitted either with a comparator overlay or with a measuring vernier.  The magnification used was 30 times and the tolerence demanded was +/- 0.05mm.  The acceptance criteria was >95% pass.   If this specification was not met then the whole batch was rejected.   The next stage of inspection was a 100% visual inspection where operators would reject any broken, bent or mis-shaped mica discs.

Post inspection, the discs were coated with an aqueous solution of Magnesium Oxide - once dried this forms a white powder layer on the mica disc which has very high insulating properties and served to break up the smooth mica surface increasing potential leakage path lengths and repel getter deposition as a valve ages.

Finally, the mica discs are placed in glass flasks which are evacuated and sealed until they were required for use by the valve assembly department.   Here we see Dilys Dyke studiously sealing mica discs into evacuated bulbs for storage at Mullard Blackburn: -

 

 

VALVE TYPES & MANUFACTURING TECHNIQUES (vi) - MICA

Valve lore contains a lot of references to mica but what is a mica?   Well, let's start with the raw material.  Mica is a silicate material having the chemical formula  Na2Al4–6Si8O20(OH,F)4.   

The only thing we need to remember about this chemical formual is that it provides a material which splits into very thin sheets of a material that is impervious to gas, rigid but brittle and a useful insulator.  Interestingly, the material is named from the Greek word mica which means crumb and also the Greek word micare for glitter.      

The mica supports used by Mullard were obtained as cut 'washers' from sources in India and Canada.

In the next blog entry, I'll tell you how these washers were received and treated at Mullard before being incorporated into valves.

THE MULLARD WORLD SERIES VALVES (iv) - BOOST DIODE PY80

In a previous blog entry I discussed the early 50s vogue for transformerless techniques in television design, the manufacturers said it reduced set footprint, I said it was a cost saving measure, whatever, it caused the brainiacs at Mullard’s development lab to think long and hard about the next ‘World series’ valve to be brought out.

Transformerless is a good way to go but it means that realistically HT voltages much over 170V cannot be achieved, however, it would be better if more juice could be squeezed, after all, the anode of the line output valve as well as the first anode of the tube could do with a much higher belt.


This was provided by a handy little dodge in which the energy in the line output transformer which would be dissipated during flyback is used to boost the HT power supply after it had first been rectified – probably by another Mullard valve, the EY51.

The little gem doing the boosting is of course that redoubtable Booster Diode, the PY80.


 

 

 

VALVE TYPES & MANUFACTURING TECHNIQUES (v) - THE MANUFACTURE OF GRIDS

In the preceding two blog entries, I have discussed how tungsten wire was produced by Mullard's - incidentally, exactly the same process was used to make molybdenum wire too.   Both of these wire types were used to manufacture the grids we see in multi electrode cages.   As you can no doubt imagine, the performance of a valve is dependant upon extreme accuracy in the winding and positioning of these grids so let's have a look at what's involved in doing so.

Let's consider the Mullard EF86, a B9A pentode used for many applications from TV IF strips to a floating paraphrase phase splitter in say a Quad II amplifier.  The control grid in an EF86 comprises of two copper wires 20mm in length and 0.75mm diameter spaced 5mm apart which serve to form a rigid support for the grid spiral.  Onto this support, molybdenum wire of 0.05mm diameter is wound over the copper wires with each turn spaced 0.125mm apart.   That's what I call precision, so, how was it done.....

First, here's a nice picture of a Mullard grid winding machine: -

As you can see, the grid winder is very similar in appearance to a typical engineering lathe.  This machine produced grids in continuous lengths of 120cm which were then cut into individual pieces.    At the outer end of the machine was a bracket carrying two reels of support wire which were parallel drawn into the machine via a travelling tailstock.  The rotating machine head was fitted with a reel of grid wire and two wheels - sharp edged and flat rimmed -  that rotate with the head.

In operation, the sharp edged wheel cuts a series of notches in each grid support wire, the grid wire is wound into these notches and finally, the flat edged wheel presses over the edge of the cut support wire thus cinching the grid wires in place. When each continuous length of grid had been wound, it was chemically cleaned then cut into individual grids and 100% visually inspected.  In the picture below, you can see Agnes Shufflebottom diligently inspecting grids for winding defects: -

If any grids are found to be mis-spaced, they can be delicately adjusted with a special gauge which can correct overall dimensional defects, here you see the hand of Agnes' cousin Euphemia, correcting a wayward grid: -