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Blog posts of '2013' 'January'

DEAD PIXELS RUINING MY SCREEN - THIS NEVER HAPPENED IN THE OLD DAYS! (i)

I had to laugh as I heard some old duffer chuffing on about dead pixels and the lamentable quality of modern OLED screens "Eeeeh, this never 'appened in t'olden days and the picture is no better either" he bemoaned.  I suppose in some ways he was right, after all, CRT screens didn't have pixels but we did have phosophor burns which were even more unsightly and we certainly had face blemishes.    Moving on to picture quality though, if our Samsung LED TV is anything to go by,  I have never ever seen a picture with more startling clarity and acuity but as we know, opinions can differ.

As to the subject of blemishes ruining screens, as is often the case, the rosy tinted spectacles we use to recollect the past along with our memories often isn't what it should be in terms of accuracy...

In actuality, face blemishes were a fact of life due to the way cathode ray tubes were made and indeed many a panicking consumer thought they had a crack in their TV screen but the marks were just surface blemishes.  Were this not the case, any defect would have been rewarded with implosion as the screen would have catastrophically failed under the pumping process in which the tube envelope was raised to a very high temperature and placed under a high torr vacuum which resulted in the screen surface being subjected to an external pressure of some several tons!  The glassware of a typical cathode ray tube was produced by moulding and comprised of a screen, a cone and a flanged neck - all produced sepreately and then fused together to form a single unit.

To understand the face blemish phenomenon a little further, we shall look at the production and inspection process in a little detail in our next two blog entries - what a scintillating way to enter February 2013!

 

 

A PHOTO OF ME AS A BOY SHOWING MY CUTE LITTLE ONE

You know, this could be me, in my Rosebank school uniform, peaked cap, short trousers, sturdy satchel and a pair of Clarks shoes with a compass in the heel - happy days.... but it isn't.

The little lad you see in this 1953 picture is the 5 year old  Norman Skentlebury at the Earl's Court Radio Show of that year.  He appears clutching a typical CRT of the era whilst being dwarfed by the giant model television tube on Mullard's stand because chaps, as we know, size is everything.

MULLARD BATTERY VALVES (ii)

 

Today's blog entry deals with the Mullard miniature battery valves that have use in the AF side of a receiver or battery amplifier.

For detector and AF voltage amplification applications, the DAF91 diode-pentode may be used. The pentode section is a short-grid-base valve specifically designed for audio amplification whereas the diode section, located at the negative end of the filament, is designed for speech detection.  Using an HT of 90V a stage gain of 100x may be achielved and the performance is no less impressive with a voltage drop as HT at 45V  which will provide a stage gain of 50x.

For output stage duties, there are two choices, types DL92 and DL94.  Both of these devices have centre tapped filament which can be operated either in series or parallell.  LT drain is 50mA at 2.8V for series working and 100mA at 1.4V for paralllel working.   Type DL94 is designed to operate with equal voltages on both anode and screen and with a 90V potential, will provide a 270mW output at 7% THD with a drive voltage of 3.2V.  Type DL92 can operate with unequal voltage on anode and screen and like it's bigger brother, is capable of an output of 160mW under similar working conditions.  Although some of you will be thinking this ain't exactly hi-fi, these valves were used in a number of commercial battery operated gramophone amplifiers which were well received at the time.

HAPPY FIRST BIRTHDAY TO MULLARD MAGIC!

Normally, the 13th tends to be a less auspicious day than normal, however, not for Mullard Magic and especially this month's 13th day as yes, TODAY is the first anniversary of the Mullard Magic website going live.  

Many thanks to all of our 690 loyal registered customers, some like Andy, Ray and Chris who have come firm friends.  Thank you all for helping make our first year a success and also for making our business so enjoyable to run.  

Here's looking forward to our second year and I really must get my finger out with more lines of stock listed from the treasure stash.

BLACK & WHITE TUBE PHOSPHOR APPLICATION (ii)

 

Just in case you thought Sebastian and his co-workers had to heft these long and wieldy tubes to remove the supernate, actually, no, they were placed on a rotating carrel that was tilted at such a rate as not to disturb the accreted phosophor film that sat on the inner face of the tube.  Here you see a laboratory coating system, however, the full size plant used a similar principle  - such fun!

BLACK & WHITE TUBE PHOSPHOR APPLICATION

Here we have a nice photograph taken at Mullards in 1952 of Sebastian Firkle who is laying the luminescent screen in a Mullard picture tube.  The screen phosphor was suspended in a liquid carrier and introduced into the envelope. After settling, the supernatant liquid was carefully decanted off leaving an even distribution of phosphor which was then dried under a gentle stream of cold air.

MULLARD BATTERY VALVES

Even though the National Grid came on stream in 1947, the public electricity supply in the UK was a real hotch - potch of AC/DC offered at differing voltages - indeed my own parents came cropper of this in their flat at Arnold Place, Whetley Hill,  Bradford as one plug was DC yet another AC resulting in a prized radio going BANG - what a mess!  

Little wonder then that 20% of all radios fittted with Mullard valves in the early half of the 1950s were fitted with battery valves.   The Mullard battery valve series comprised of five types of valve: -

DL92 - a heptode frequency changer.

DF91 -  a vari-mu HF pentode to act as RF or IF amplifier.

DAF91 -  a short-grid base pentode t act as AF amplifier combined with a diode detector.

DL92 - an output pentode for portable battery powered receivers.

DL94 - an putput pentode for domestic battery receivers.

All of these valves were designed to be B7G based with a footprint of only 19 x 50mm meaning they were very portable device orientated.  The filament current for all types was 50mA @ 1.4V.

These devices were a masterpiece of miniaturisation, allbeit developed from the American 1S & 1T series, these devices meant a freedom from accumulator and mains supplies that "kids of today" take for granted 60 odd year later with their new fangled i-Pod -  awww, the ittle darlings..

Eeeeeh, progress tha knows!

A little more on these enigmatic devices in a future blog..................

 

GORDON AND HIS AMAZING ORGAN BEING HELPED BY MULLARD WELCOMES IN 2013

Wahey, here's the New Year blog entry!    

It shows Gordon Twyford Fossdyke and his amazing organ school.  The early 1950s was a time when organ playing was much in vogue with musical young ladies, we don't know why, it may be the mighty swell of tremolo that an organ gives as it thunders towards a crescendo gave these impressionable young things a sense of power in an age of post war austerity.  

Whatever the phenomenon was, it presented Gordon with an interesting problem as even though he had a big organ and knew how to us it as well as being a very capable teacher, he was still unable to satisfy the large number of young ladies that clamoured for his expert tuition as his traditional pipe organ did not lend itself to multiple players and then there was the noise level as well.   Even going down in scale with a harmonium wouldn't have been much help and if he resorted to just a little virginal, well, that wouldn't have been much cop either.   Whatever could he do?

Well, Gordon turned to Mullard for help who designed for him a circuit for a Multi-Keyboard SIlent Organ.  This fascinating device was of the keyboard type and was capable of full harmony with each tone generating unit comprising of one oscillator valve per note.  The valves used were the Mullard EF50, of which there were many as ex wartime stock as well as contemporary production freely available at this time.  

The keyboards were very simple, each consisiting of a series of key switches feeding a pair of headphones and a pedal operated swell pedal was provided for each keyboard.  Up to 24 keyboards could be simultaneously connected to the unit and hence Gordon could satisfy the teaching demands made upon him.  It was possible for Gordon to listen in to any of the players or even connect groups of players for duets or quartets and via a microphone, he could even speak to each pupil without disturbing other players.  Pupils liked the system and they found the DLR 5 & WS38 set military headphones, ex Govt Surplus to be very comfortable indeed.

I am sure you all yearn for that halcyon time when we didn't have a nanny, loony, multiculturally embracing, wafty, do-gooding, PC, elf and safety conscious mindset, the likes of which are amply illustrated by the following photograph showing Gordon fiddling with his unprotected bits.  How he never got a shock I'll never know and history does not record whether or not the gubbins was ever enclosed in a casing to protect fingers accidentally or even deliberately contacting the circuitry and frying anyone feckless enough to fiddle: -