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RADIUM IT'S HEALTHY STUFF - GLOW IN THE DARK OR WHAT!

Well, I had a query from Vicky today who was interested in buying a watch from us but  was concerned that the dial might be radium luminous.  Most pre war watches are of course radium lumed however, post war, we moved towards dials using the 3H isotope of Hydrogen, Tritium, to give luminosity through a Jablonski decade emission, today, on watch dials we use a proprietary compound known as Luminova which is a strontium aluminate photoluminescent compound invented in the early 1980s.

Anyhow, Vicky, just look at how well rated radium was in the 1920s and the health benefits so claimed - simply super stuff: -

 

GLAMOUR OR SID - WHICH ONE FOR THE 1955 MULLARD CALENDAR

As early as 1952, Mullard top brass were considering how best to promote their product range with the expounding of an idea which became all too prominent some 10 - 15 years later.      The idea was of course a calendar and not just any calendar but a glamour calendar, much board room discussion was made of the merits of Roedean-esqe girls showing cheeky smiles, a shapely ankle or even a svelte swimsuit with a plunging back line but the eventual conclusion was that such an approach might sully the wholesome Mullard image - the phrase 'sex sells' wasn't yet coined by Madison Avenue though I am sure Don Draper and Roger Sterling must have thought of it!!!!!!

Instead, a compromise was reached, the calendar format was used with a rather fetching presentation of Sid the Serviceman on the front cover and handy valve equivalents and operating data inside for common valve types.  Here we see the Mullard Top Brass at the Mullard Dealer event in 1954 perusing an advance copy of the wall chart with the chap on the right, Mr D M Hall, Manager of the Mullard Valve Sales Department appreciatively commenting, sotto voce, "I say, phwoarrr, look at the getter bloom on that!"

Today, these wall charts do occasionally turn up with their page edges crinkled and worn, a testimony to their use as an aide-memoir in some long gone radio service workshop.

MULLARD MAGIC RACE AT THE DONNINGTON CLASSIC 2014

FM RECEPTION

I had a call from Chris who was asking how his Leak Troughline worked so I decided that maybe it was time for a quick blog article on FM.  Frequency Modulation or FM is a system of radio transmission where the carrier amplitude is kept constant whilst the frequency varies about the nominal value.  

The frequency variation has to convey two pieces of information: - 

i) Audio frequency which is the number of times per second the carrier frequency oscillates or wobbles either side of it's nominal value.

ii) Audio amplitude which is the amount the transmitted frequency shifts above and below the carrier frequency fc and is known as the frequency swing fs where fs is proportional to the amplitude of the original sound and has a maximal value of 75KHz

The simplest method of recovering the audio frequency oscillation is shown below: - 

Above you can see that an RF circuit has been set 'off-tune' to the carrier frequency such that the oscillations correspond to the straight part of one half of the response curve.  As the frequency of the FM signal oscillates from fc-fs to fc-fs and back again the response changes from point A to B and thence back to A.  This completes one cycle at audio frequency.

The amplitude of the AF component in the output depends on the magnitude of fs and also on the slope of the response curve. The repetition periodicity of the waveform (= 1/frequency) is the time taken for the signal to swing fc-fs to fc+fs and back again.

To give a worked example consider a carrier frequency of 90 MHz being modulated by an AF signal of 1KHz showing a frequency oscillation of 50KHz - this will show a signal oscillalation of 1000Hz between the limits of 90.05MHz to 89.95MHz and will reach these maxima 1000 times per second.

The bandwidth required for FM transmissions is much larger than that needed for AM transmissions.  If the modulating (audio) frequency is denoted by fm then sidebands are formed at fc+/-fm dependant upon the modulation index fs/fm.  Theoretically, there will exist an infinate sequence of sidebands but the range can be reduced by operating with minimal frequency oscillations, the maximal allowable of which is denoted the frequency deviation fd.   In broadcasting, the magnitude of fd is typically several times greater than the highest transmitted audio frequency.  fd can be characterised further by the deviation ratio a value of 5 or higher being preferred where the frequency deviation of 75KHz results from a maximal modulating frequency of 15KHz.

Consequently, medium waves are unsuitable for FM transmissions as the output would be ruined by phase selective fading,   conversely at very high frequencies (VHF) fading and selectivity concerns are much reduced as tropospheric bounce has little effect as only the ground wave reaches the receiver. 

MULLARD VALVES, A WORK OF ART

I recently met Steve Burke - Hare who is a Mortuary Manager and Medical Enbalmer with a penchant for valves.   Steve has engaged the talent of Monika, a Polish tattoo artist working from a studio in Macclesfield to illustrate his devotion to the thermionic cult.    

Here you can see Monika at work and Mrs. Mullard Magic is a little in awe of Monika as she likes the idea of ladies inflicting pain and discomfort on a man: - 


And now, you can see the fruits of her labours and I think you will agree, the results, in startling clarity, detail and colour are both beautifully executed and very impressive indeed: -


I remarked to Mrs. Mullard Magic rather smugly that I might get a valve tattoo done on some part of my body,  but then decided perhaps not as with my being a stickler for detail, I wouldn't like to make a KT66 look like an 813.  

Quick as a flash she retorted, " More like making an ECC83 look like a 956 acorn!."  Thus humiliated, that was an end to that particular idea, dejectedly, I am destined to remain unadorned.

RASPBERRY RIPPLE AND PIE OR AC RIPPLE AND PI - YOUR CHOICE

Today, I thought we would talk about AC ripple and Pi section power supplies for as we all know, one of the most important considerations in an amplifier is the minimisation of AC ripple that can be superimposed on the DC voltage supplied by a rectifier.

The value of ripple is dependant for a unit voltage on the reservoir capacitance and the smoothing network - let's take a typical Pi network comprising of C1, a reservoir capacitor, and C2, a smoothing capactor with either an inductance L or a resistance R: - 

The ripple voltage is inversely proportional to the values of C1 and C2 - so if you want low ripple then get BIG capacitors.  Some people economise in the design of equipment by substituting a resistance in place of a more expensive choke with little appreciable rise in perceived ripple current.  Of course in this world 'tha gets nowt for nowt,' and the resistance unfortunately reduces the DC output voltage dependant upon current delivery loading and resistance value.   This disadvantage can be minimised by feeding the PA stage output valves from C1 rather than C2 with the ripple being nullified by a push-pull output stage, particularly if this stage is balanced.

Taking the above example: -

For C1 = 8 uF; C2 = 16 uF; L = 10 H then Irip = 210 mV for V0 = 251 V @ C2

For C1 = 50 uF; C2 = 50 uF; R = 500 Ω then Irip = 210 mV for V0 = 221 V @ C2 or 265V @ C1

So there you have it but my take on it is don't be cheap, use a choke and preferably one bought from Mullard Magic!!!!!!

 

 

SISTERS DOING IT FOR THEMSELVES AND KEEPING IT IN THE FAMILY

Today's blog entry shows a press photograph and article from the Wandsworth Beagle newsheet from 1954 and was taken at the Mullard Equipment Factory at Wandsworth.

It shows Betty Calf, Hetty Calf and Lettie Calf assembling Mullard High Speed Valve Testers - truly a family affair - and not a tin of pineapple rings to be seen!  I wonder if anyone will 'get' that last comment - answers on a postcard............................

 

 

 

MULLARD HIGH QUALITY TEN WATT AMPLIFIER THE 5-10

'Technical advances have effected considerable increases in the quality of broadcast and recorded sound with television sound and FM transmissions including the full range of audio frequencies.  It is thus essential to have an amplifier which does full justice to a high quality audio signal.  Details of such a new amplifier have been released by Mullard in the booklet "Mullard 5 Valve 10 Watt  High Quality Amplifier Circuit (ref. no. MV8104).'

The above entry is from a press release by Mullard dated October 1954.  Today, the 5-10 is much revered  and not at all in the shadow of it's larger brother, the 5-20 - and so it should not be when it boasted an output of 10W showing a THD of < 0.4% with hum and noise being inaudiable being -74dB below maximal output.  All this and a flat frequency response to within +/- 0.5dB within the range 10 - 20000Hz to boot!

Of course, this amplifier was designed to showcase a range of Mullard valves, first, the first stage voltage amplifier used the low hum/low microphony EF86 pentode which was in turn coupled to a cathode coupled phase splitter, courtesy of an ECC83 dual triode.  An elegant design point was the g'' of the ECC83 being capacitively earthed with the bias for the second section being applied due to current flowing through a common cathode resistor - a nice touch that prevented motorboating or any form of instability at normal driving conditions.  

Interestingly, the output stage was configurable with an 8K or 6K secondary to adjust loading such that high power or lower power with a consequently better transient response could be enjoyed.

The powerhouse for all this was supplied by a GZ30 rectifier or if you wished to stay with an all B9A arrangement then alternatively, the EZ80.   THe PSU was of a resistance-capacitance design utilising a comparatively large reservoir capacitance of 50uF to reduce ripple current.

And here is Mullard Laboratories photograph of "one they built earlier." : -

 

 

THE MIGHTY ROHDE & SCHWARZ EK-07

 Well, evidence of my advancing years crept up on me and was revealed today when I ran out of puff trying to move my Rohde & Schwarz EK-07 receiver.  Widely acclaimed by some as the best valve communications receiver made, I am priveledged to have one courtesy of my friend Norman Varnes (G4YXX) who tells a tale of spirited bidding over a glass or several of jenever at the famed Helmut SInger emporium to liberate it with a terrible sacrifice not only in pounds sterling but also a most horrendous hangover courtesy of much falling down water consumption during the lengthy negotiations necessary to secure this leviathon. 

For those that are not 'in the know', here's a little history lesson.......  The year is 1957. Rohde & Schwarz, founded in 1933, introduced the EK 07 shortwave receiver. This milestone set the benchmark for precision engineering in the radiocommunications market. Frequencies could be read with high accuracy, image frequencies were a thing of the past, the radio was immune to overload, and sound quality was above average. This was reason enough for the German Armed Forces to adopt the EK 07 as its standard communications intelligence receiver beginning in 1962.

Indeed, my EK-07 has an interesting provenence as it came from the renowned Norddeich DAN German maritime radio station and did sterling service until being demobbed on station closure in 1998.  Here is a nice photo of a bank of EK-07 in action at DAN: -

 

And here is an even nicer photo of the EK-07 production line at R + S in 1961: - 


Finally, here is my baby - a true representation of a boatanchor weighing in at 75kg: -

 

Thanks are also due to my friend Brian Harrison (KN4R) who has helped me out with an English translation of the weighty EK-07 manual so I now have no excuse to not check this over to ensure all is in spec for the next 50 years of operation - time to get weaving!!

 

 

 

I SAY, BASE ADAPTORS FOR YOUR VALVE TESTER - SIMPLY SPIFFING!

 

May 1854 brought a useful accessory addition to the Mullard High Speed Valve Tester (MHSVT).  Developed in conjunction with Messrs. Spear Engineering of Warlingham the range comprised of eight adaptors to facilitate the testing of valves for which no existing valve base was provided on the instrument.

These were precision items, with the bodies of the adaptors being trned from duralumin - usually reserved for aircraft bodies - and just look at the adaptor below for testing the EY51 boost diode - impressive huh?

The adaptors were engraved 1-8 with the figures clearly identifying each adaptor body and were priced at £4. 2. 6d. (£4.125)  the set complete in a holding rack.   otherwise the adaptors were available idividually at prices ranging from 7s. 6d (37.5p) to 15s. 0d. (75p)