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.
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.
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............................
Today's Mullard archive photograph shows the evacuation of television picture tubes at the Mitcham works some time in 1953.
The operator is placing the tube on a diffusion pump. Diffusion pumps use a high speed jet of vapour to direct gas molecules in the pump throat down into the bottom of the pump and out the exhaust, nowadays we use a polyethylene glycol (PEG), PEG ethers or more commonly, a silicon oil, however, in 1954, the preferred vapour was mercury as it didn't char, didn't contaminate the tubes with vapor backflow and of course was readily available.
The tube passed through a heated tunnel where any gaseous emissions were pumped out prior to sealing once the hi-Torr vacuum was pulled.
Mullard Magic went on tour last week and on one day rolled up at the Morgan Cars factory for a guided tour. Now, I once ripped my trousers getting into a Morgan when I was younger (and thinner!!) so I have never been a keen fan of them but I am very impressed with the newer breed of Morgan Aeromax with it's Talbot-Lago-esque rear end and at £107k every household should own one! With the passing of Bristol Cars into foreign ownership last year, Morgan remains the only wholly British owned private car company left.
Interestingly Dixon, our guide at Morgan bought his Morgan in 1955 and had turned down an offer of £120k from a keen prospective purchaser last year, talk about a gold plated investment, now that really is an impressive return!
Feast your eyes on the following video so you can see what the tour was like: -
In late 1952, a new range of Mullard B9A Noval based valves having 6.3V heaters were introduced aimed at use in audio amplifiers.
PRE-AMPLIFIER - type EF86 now replaced the EF37A or EF40 having similar charcteristics to the EF40 with a gain of up to 140 being achievable. The heater of the EF86 is rated at 6.3V @ 200mA.
DOUBLE-TRIODES - three new devices were made available - the ECC81 being a medium impedance valve, the ECC82 a low impedance and the ECC83 a high impedance. All designs have independant cathodes for each section, centre tapped and capable of operation at 12.6V @ 150mA or 6.3V @ 300mA.
OUTPUT PENTODE - the EL84 meets the demand for an output valve with 30% more output than the EL41. A single EL84 was designed to give an output of 5.4W with 10% THD. A pair of EL84 in Class AB1 push-pull could deliver 16W. The heater of the EL84 is rated at 6.3V @ 800mA.
FULL-WAVE RECTIFIER - the EZ80 has identical characteristics to the EZ40 and is capable of a rectified output of 90mA at a maximal voltage of 2x 350V RMS. The heater of the EZ80 is rated at 6.3V @ 600mA.
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!
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.
In 1951, Mullard were made aware of an absolutely hilarious occurence. A proud owner had bought a Bush TV22 bakelite TV - you know the one, I think we have one on sale here.
Anyhow, he was moving it to try and find the best spot for it and guess what, in typical Mr Butterfingers Numpty style he dropped it and shattered the cae, however, all was not lost and the chassis and tube were quite fine and our slip fingered gent wrote to Mullard moaning about the fragility of Bush bakelite but praising the enduring qualities of Mullard glass.
The TV22 bare chassis was retrofitted with an alternative casing and went on to provide sterling service safely ensconced in an orange box, tastefully painted in battleship grey, using a type of mica loaded paint designed for electricity pylon protection.
Could this be an early example of 'UPcycling' - goodness how I HATE that term! Thus clothed, the set allowed a grateful family to watch the 1953 Coronation - huzzah, and very topical at this moment some 60 years on!
This episode prompted Mullard to publish the following cartoon in their in-house magazine featurung the eponymous Syd who espouses a salutory lesson to all of us who choose to monkey around with delicate thermionicalia (is that a real scrabble-able word I wonder??).
This wasn't the only accidental event extolling the longevity and hardiness of Mullard devices, so stick with watching this blog for other jolly japes of a similar kind - priceless@!