Today's photograph from the Mullard archives shows Albert Pumphrey, a valve engineer from the Blackburn Works examining Hilda Brooks' efforts at delicately constructing an electrode cage in May 1952. The archive record does not record whether or not Albert subsequently asked Hilda to accompany him to see the new hit film Singin' In the Rain at the Savoy Picture House in Darwen some time after their Blackburn Valve Assembly tete-a-tete or if she said yes or no!
I was struck by the contrast between the current snowy weather conditions and this heartwarming nostalgic picture from the Mullard booklet of 1930, "How To Get Better Radio." Whatever you are up to this evening I do hope you are not sheltering in the lee of a wall whilst listening to the light programme via your SG Brown headphones.....brrrrrr.
Hah, you all thought you had escaped the photocell tech-o-fest but today, we are going to talk about them further and in particular 'gas amplification,' an interesting phenomenon shown by gas filled photocells.
Although the manufacturing methodology is the same as for a standard thermionic valve, in the case of the gas filled photocell, a precise aliquot of a noble (inert) gas was introduced after a vacuuum had been pulled from the envelope. The low vapour pressure of the injected gas means that it's molecules are distributed within the envelope and indeed the electrode cage. In operation, a portion of cathode electron emission will collide with the inert gas molecules resulting in ionisation. The secondary emission electrons so produced would then be attracted to the anode and conversely, the gaseous cations would be attracted to the cathode where they 'belt it' so hard that even more cathode emission occurs. The effect of this cascade reaction is that anode current would be greater than with an equivalent vacuum cell by up to a factor of 10X! And this my friends, is 'gas amplification.'
Gas filled photocells are very sensitive to low light levels and can discriminate very small changes in light as may be seen with sound-on-film equipment. Mullard produced a whole range of this type of photocell and so help you recognise them should you come across examples, I present below a photograph of the range, aren't they sweet: -
My webmaster called and said we need to do some website amendments so I was requested to decamp to sunny Shropshire - bring whisky and be prepared to do lots of work as he sternly ordered. Well, five hours work and half a bottle of Laphroaig later he turned to me and said "All work and no play makes Mr Mullard Magic a dull boy so I have lined up a treat for you. He's a hard task master, just like Your Honour Lord Sir Sugar and I wondered if the treat would be similar to one of Siralans.... well it was and I certainly wasn't disappointed.
He bundled me into the company Jaguar limousine and off we wafted to Bagington Airport at Coventry to see Ermintrude. Now I am sure that some of you know that Ermintrude got her name in 1972 after the cow character from Serge Danot's Magic Roundabout. To give her her full and proper name she is WR963 MR 2/AEW.2 AVRO Shackleton in service with the RAF from March 1954 until March 1989 at which point after 45 years of sterling service and 14957 flight hours, she retired into the custodianship of the Shackleton Preservation Society at Bagington Airport, Coventry.
You see, we had been invited by the Preservation Societies' Richard Woods to partake in a very special event which was the 4 engine run up of this aircraft's Rolls Royce Griffon engines for the first time since 2009.
Words cannot define how extremely enjoyable and emotional this experience was so my webmaster has recorded the proceedings for posterity in the following video:-
It was quite humbling to see the product of so many hours of selfless and devoted attention that has been put in by the Preservation Society members in conserving and maintaining this magnificent Cold War warrior. There are plans to return this grand lady to the skies and quite right too, after all, she looked after us for 45 years so it's only right that we now look after her. Unfortunately, this will come at a cost and it is estimated that between three to five million pounds will be required to make this dream a reality.
To this end, Mullard Magic, Crucial Solutions Ltd. & Building the Legend Ltd are proud to support this project and would hope some of our readers and followers may also help in any way they can to acheive this noble aim. Please watch out for future details of the Shackleton Preservation Society's new website and if you get a chance, please do go visit and put a pound or two into the return to flight fund.
These superb devices hailed from the Isle of Man and have been acclaimed by some as the best ever made which is quite an accolade as some lesser devices have earnt the sobriquet 'made from junk!'
Annoyingly however, is the fact that there is never a circuit diagram or instruction manual available for these devices on the internet, but there is now on this blog a circuit diagram, kindly provided and drawn out in in graphics programme by one of my rather clever customers Fred, G4VVQ: -
Just take a look at this photo of a vintage radio repair workshop during the early 1950s and of course the instrumentation ranged over two shelves and a bench top - can you identify them all?
Starting at the top and working L to R, we have a Muirhead decade box, a Mullard LCR bridge, a National HRO speaker, an AVO Mk7, an Advance SIgnal Generator.
Middle shellf working L to R we have a 'mystery item' - is this a Tannoy or GEC power supply? - please post a comment or write to me if you do know what it is - , an AVO All Wave Oscillator, a Marconi Signal generator, an EMI combined scope and voltmeter.
On the bench we have a Mullard Master Test board separated from it's younger brother the Mullard High Speed Valve tester by boxes of cards for each instrument.
What a simply super evocative photo, which remindes me, we have a number of pieces of test instrumentation to be listed on the website for sale, time to get weaving.....
The DM70 was quite a departure from previous tuning indicator/magic eye/ tune-on designs and was designed specifically with the portable dry battery receiver in mind but additionally was specified as an inexpensive device for the mains receiver too, not to mention as the reading indicator for a range of Negretti & Zambra high precision aneroid barometers made for the Royal Navy and Royal Air Force - in fact if you take a look at our prodict listings, you'll see a range of these barometers for sale!!!.
The DM70 has a subminiture envelope and is constructed as a wire ended triode with the control grid being a metal plate shaped shaped like an exclamation mark. The varying grid width gives this valve a vari-mu character. The anode is a plate coated with an electroluminescent phosphor and it is located behind the grid. The filament, rated at 1.4V @ 25mA is a single wire located in front of the grid. The configuration is shown in fig 2(b) in the photograph below.
The operation of this fascinating device is rather elegant which I will attempt to describe. With the grid at zero potential, electrons from the cathode travel through the 'exclamation mark' aperture of the grid to impinge on the anode hence causing it to fluoresce. VIewing from the filament side, the fluorescence is visible through the 'exclamation mark'. As the grid voltage becomes more negative, the anode current will decrease and hence the fluorescence will fade.
The fade commences as shown in photograph fig 2(a) above at area 'B' until only areas 'A & C' are luminous. At an even higher grid voltage, the electron flow will stop completely and the fluorescence wil disappear completely. The idea was that the grid would be fed from a resistive line in the AVC line of a receiver so that when the receiver was accurately tuned, the grid voltage would be at a maximum and conversely the fluorescent area would be at it's minimum so lo and behold, tuning indication was provided, simples as the meercats say!