Blog posts of '2012' 'May'


Yesterday I had a reminder of why I enjoy running Mullard Magic so much.   I had a call from Rob who needed some capacitors but was really bothered about exact size and shaping rather than capacitance and working voltage .   Not wanting to be rude by asking what he needed them for, I said we'd have a rummage and see what we could find for him.  As our conversation progressed, he admitted the reason for him being cagey which was that some vintage component vendors had been nasty and refused to sell capacitors to Rob as he as going to destroy them!!!!!!!!  Obviously he'd spoken to one of the rabid numpties that believe that every woodworm raddled poxy 1950s radiogram shpould be restored and worshiped.

No such problem for me though, a bit of fun with a steel rule and voila, 31 MetalMite 0.005uF 175V capacitors were soon on their way to their new owner - job done -  and we've helped out another customer.

What I could in no way have imagined is the use they were being put to for Rob is a precision scale modeller making high grade replicas of Star Wars vehicles and equipment.  The capacitors will be used as components in an Imperial Stormtroopers E11 rifle and Empire detention block (where Han Solo was incarcerated) power cylinders - I think that is absolutely marvellous and I feel priveleged to help out a customer for a non electronic usage for one of our products.............

That this ....................

According to plan, will become ...............

And here's  Rob, tooled up with one he made earlier........


Today, a question from South Korea - what is the best Mullard input valve for a Leak TL12.1.  Well, you can fit as per original, the EF36 or the improved version, the EF37 or the super improved version the EF37A or the mega special version the ME1400. 

The Mullard Type EF36 was designed and introduced to market in 1940 as a general-purpose RF/IF/AF pentode with a non variable mu characteristic.    There were two famous uses for the EF36, firstly, Colossus, Tommy Flower's computer at Bletchley Park used rows n rows of them then of course in 1947 the Leak TL12.1 used them as the input valve.  It was found that when used as a high-gain audio amplifier the EF36 proved to be microphonic.

The EF36 was developed further and a sturdier version, the EF37 was developed from it and introduced in 1946.  The problems still continued especially as the valve aged and gave an unacceptable  hum and a high noise floor so, it was back to the drawing board again.

 The EF37A was introduced in 1950 especially for made for sensitive applications and was hailed as a low noise non microphonic valve.  By careful redesign of the ‘hairpin’ heater such that it took a bi-filar form, induced mains hum was minimised.  The hum was reduced further to minimal levels with a beefed uo electrode cage in the designed for electrometer applications variant much loved by the GPO, the ME1400.

All three types have the same nominal characteristics and when Mullard's finally got their design and production problems sorted they made only EF37A's and supplied these as replacements for the other two types.



 An e-mail landed this morning from a customer wanting to know what Jones plugs he would need for his R1155 and T1154.    I didn't ask whether or not his station had em all ripped out, however, here is a nice little aide memoir to anyone needing to remind themselves of pin configurations of these connectors for this particular use BUT don't forget that shown below are the CHASSIS configurations and that you need the corresponding connector ie plug to socket & socket to plug to match....


 I had a very strange e-mail from a viewer in which he commented on my 'Mullard in the early 1950s' blog article, in particular my commentry about the Mullard LSD flash tubes.   He went on to lament that my article was just like most others on the 'net, a few words on these devices just copied from other websites!!!@*'?

I must admit, I was flabbergasted, what nonsense, why would this person act like a radio part - hint - the bit that you turn to get more volume.

Anyhow, in order to satisfy any other affectionados of the Mullard LSD flash tube here's a litle bit more about them - not copied from anywhere on the web but instead from my own personal experiences as well as Mullard official archive sources.  

All of the LSD series of flash tubes were fitted with a B4 base,  below is a photo of my own personal twin head LSD 3 flash system which I use with a period Hasselblad 1000F camera.      Each Xenon filled tube tube can muster a maximal energy discharge of 100J - quite a belt as this equates to half the energy a defibrilator outputs to kick start an ailing heart!     Each tube operates at a 2.7 KV anode voltage and utilises a minimum trigger voltage is 4 KV.     Flash duration is 100 microseconds with a light output of 3000 lumen-seconds. 

As a vintage flash system these are quite special, I can remember being captivated by photographs taken by Alec Perlman of a couple ballroom dancing which were published in a 1950s edition of The Rollei Way and knew I would have to have a set. Sure there's no thyristor control and you have to think in order to use my vintage flash but the rewards are a motion stopping well lit scene without a hint of harsh glare which go to produce some of the most beautiful silver images out there!

Some of you may be aware that the LSD series of flash tubes were developed in conjunction with Ilford Photographic who are still going today as the last bastion of Black & White silver image film and paper - look em up at

Below, I have an official Mullard photo showing the inventor (middle checking his wallet) being interviewed in June 1950 for the television programme Picture Page which was filmed as an outside bradcast at the Engineering Centre in Glasgow - is it still there, 62 years later in Sauchiehall Street I wonder?




Some of you may have seen the news report showing Her Maj boating on the Leeds & LIverpool Canal on the broad beam narrow boat, The Pride of Sefton in Burnley.    As she  cruised along, viewing  the old iron bedsteads, shopping trolleys and old tellies in the canal as she passed by, I wondered if these may have jogged her memory and did she then turn to Prince Philip and say, " I say, do you remember when we went to that valve making place nearby?"   Yes, they did visit the Blackburn site and details of this visit released from the Mullard archives will be the subject of a future blog entry, however, we are following a chronological traipse through Mullard's history here in this blog and as we are only still in 1950, you will have to content yourself with this picture of Queen Elizabeth visiting the Mullard stand at the 6th International Congress of Radiology in September 1950.

Before you all cough and splutter, yes, it is actually the previous Queen Elizabeth, no, not QE 1 but The Queen Mother...........


 It is said that a picture is worth a thousand words so for this blog entry, here we are - a picture which lists some of the valve failure modes discussed below as well as some extra ones:-



Over-running output valves and rectifiers or allowing them to operate at extremely high envelope temperatures, can have another undesirable effect as the valve ages where electrolysis of the the envelope glass can occur.  The electrolytic action attacks the glass envelope in which lead (Pb) is extracted from the glass  as an anion which in turn is then attracted to internal wires having a negative charge resulting in metallic lead deposition around the negatively charged lead in wires -  especially where they enter the pinch or base button.   The effect of these 'Lead lead trees'  - which are often only of monolayer thickness - is to provide a leakage pathway which plays havoc with normal operation hence rendering the valve useless. 


This is an interesting one as a number of insulation failure modes exist for ageing valves.   Often poor insulation may be caused due to cathode emissive material or metallic deposits which have evaporated from the  electrode cage being deposited on various parts of the valves interior.  Deposits can build up on mica separators or the glass pinch where the connection pins/leads enter the envelope and even sometimes the inner envelope glass - remember those manky EL84 which had been run hard and hot and built up those yukky black deposits on the glass.........?

In manufacture, the mica separators are coated with a solution of Magnesium Oxide to iimit potential leakage path length  and provide a repellant mono layer to prevent surface deposition, however, the user can take measures to prevent deposition by always running their valves within recommended maxmal limits such that they do not get overrun and overheated - remember those manky EL84!   Rectifier and output valves by nature of use have high anode current loadings and consequently can run very hot, however, manufacturers had designed in a number of mechanisms to combat excess heat - did you admire those sexy black anode coatings in your rectifiers ... boutique or not, alas NOT because although it looks pretty, it exists to dissipate heat more effectively - what about that PX4 with a fin mounted orthogonally against the anode ... not just there to look nice and provide a certain sound but yes, you've guessed it, it's there to help dissipate heat more effectively



When a valve is manufactured, it undergoes a process called evacuation where successive pumping operations remove air from the envelope prior to sealing.   However, even after evacuation, a small amount of gas remains - one part per one-hundred-thousand-million - as Cilla would agree 'That's NORRA lorra, lorra gas!'    More gas may be released from the metallic components that comprise the electrode cage of a valve, gas that is trapped within the interstices of the metal structure especially if a valve is run under arduous operating conditions such that it gets extremely hot.

Valve manufacturers include a component within the valve that 'mops up' residual gas - this is the getter flashing - that pretty silver mirror like coating seen on the inside of a valve's glass envelope.  The flashing is a deposited barium metal mirror, which originates from the getter rings and pans that form part of a valve's structure.  After evacuation the sealed valve is subject to an inductive RF heating pulse which evaporates the metallic barium from the getter ring/pan onto the envelope.

This mirror is sacrificially consumed as it 'mops up' gas within the envelope and many of us will have seen an elderly valve where the getter flashing takes on a brown and then a pinky-white hue as it's absorbtive properties have exhausted - all schoolboys of a certain age will also be familiar with the silver getter flashing disappearing to a dull white powdery coating as the valve envelope is broken either by stones, airgun pellets, Dinky toy collisions  or other means thus letting atmospheric oxygen in to consume the getter flashing!

OK, so we have got a bit of gas left in a valve and something to sort out any that's left, what does the gas actually DO, can it be a problem???? 

The answer is a resounding YES for as the gas molecules 'bimble' and 'gambol' in Brownian motion within the envelope, electrons from the cathode 'belt' into the gas molecules causing them to shed their own electrons and become positively charged cations.  These positively charged gas molecules are then attracted to the negatively charged grid where they tend to nullify the grid bias making the anode current increase.  Unfortunately this can start a cascade effect as mure current causes more gas molecules to ionise and the anode current can continue to rise such that a sort of meltdown occurs in which the valve destroys itself. 

In reality though. often long before this happens, the positively charged gas molecules are simultaneously attracted to the cathode where they bombard the emissive coating, forming a non emissive layer on the cathode surface called  'poisoning' which reduces emission to very low evels and renders the valve useless.  Could that be what has happened to that bargain valve with 5% emission - is it a victim of gas attack?


 If a valve passes an anode current of 1mA then this equates to 650, 000, 000, 000, 000 electrons a second.  When you consider that an output tetrode like a KT66 can pass up to 100mA in anode current then as Cilla Black would say 'thats a lorra, lorra electrons!'    I think this little fact demonstrates quite well that over a period of anywhere between 2000 and 8000 hours on average, a valve may loose a proportion of it's emissive properties.

During the early life of a valve, the effect of emission loss is realtively imperceptible but after a while, the multual conductance, gm, falls off significantly.  The following graph shows the change in gm with negative grid current at three points in a particular valve's life; curve A is the valve when brand new; curve B after burn in and curve C when worn and showing less than 50% emission.

You can see also that the slope of the curve changes too and the sensitivity of the valve is measured by the steepness of this slope.  The effect of shallowing of the slope means loss of volume, loss of sparkle and increase in distortion due to overloading.   So you can see now why an emissive limit of approx 50% may be considered the bare minimum for continued usage and typically an emissive limit of 70% for a valve to be considered to show reasonably good emission.

In our next blog entry we will look at some other things that happen as valves 'wear out'.