Blog posts of '2012' 'July'


After a little diversion, let's get back on track looking at the inside of valves, today we'll look at the ANODE.

A valve anode typically is shaped as a hollow open topped cylinder, which surrounds a central cathode and any other electrodes. Although the cylindrical type is most commonly encountered due to it’s efficiency in both electron capture and heat dissipation, oval or flat pates may also be used.

The anode’s purpose is to receive the electrons emitted from the cathode. This happens because the anode has a positive applied potential, which attracts the negatively charged electrons. Indeed, often the number of electrons drawn from the cathode are in part governed by the applied anode potential.

In small valves, the anode is made of nickel or nickel-plated steel. The power represented by anode current x anode voltage is under a no-signal condition converted into heat as electrode impingement of the anode occurs.

In a power tetrode such as the KT66 operating at no-signal conditions with an anode voltage of 250V and an anode current of 100mA, the power dissipation at the anode is 25W. When you consider that this wattage would boil a tablespoonful of water in just 45 seconds, you get some idea of just how much heat needs to be dissipated for if it isn’t, the anode temperature will progressively rise until the valve is destroyed.

There are a number of heat dissipation methods used, the most commonly seen being anode carbonization in which an anode is first keyed by sand blasting and then sprayed with a colloidal carbon and cellulose admixture, the increased surface area and black body radiation allow enhanced heat dissipation. Alternative heat dissipation methods include making the anode from a wire mesh, fitting the anode with radiant fins or even manufacturing the anode from a less thermally labile and resistive material such as molybdenum, carbon, tantalum or zirconium. In output valves used specifically for transmitting, the anode may form part of the external envelope of the valve, which may then be readily cooled by thermal conduction to an external heat sink or by a circulating-water jacket or an air blast.

In the photo below, you see the deft fingers of Fanny Pincher using a spot welder at the Mullard Blackburn Valve Assembly Department to attach the anode of an EL37 by spot welding: -



In early 1951, Mullard introduced a handy steel container finished in 'Dimenso' - that's posh for silver hammer finish paint. The container was designed to hold 600 cards in six separate compartments.   This number of cards would test 750 different valve types.    The container featured rubber feet and was sized to be equivalent in length to the depth of a tester so it could sit alongside - provided enough room was left to allow the clamshell top to open!   And the price for this marvel of modern engineering in March 1951 was 45s - a snip!


Eh up peeps, it walks.... it talks.... it's all-ll-ll-most human AND it's got a Nixie tube watch, yes it's me Mr Mullard Magic with my genwine thermionic watch.   It comes to me courtesy of my good friend and even better customer Andy Smith, many of you here will know Andy as a Head-Fi big cheese and organiser of the 2012 Head- Fi meet which is occuring on September 15th near Cambridge.

Check Andy and the Head-Fiers out here:-

HEAD-FI 2012

But before you do, check this watch out, titanium case, big enough to make an Olympic shot putter's wrist look puny and big enough for my 51 yo eyes to see....

All I do in bed is tilt my wrist (oooo-eeer!): -

And I am in good company because Steve Wozniak, co-founder of Apple wears a very similar watch and Woz goes on to say  "My coolest, most attention-getting gadget is the coolest watch made. It's a nixie tube watch from Cathode Corner."   The old-school nixie tubes run on 140 volts inside this watch. The screw-on cover makes it water resistant. When I flip my wrist, the two nixie tubes display first the hour, and second, the minutes. You can even program the angle at which the time gets displayed. 

If you fancy a top day out wanting to see some smashing kit - including valves -  along with me and my Nixie watch then pop along to the Head Fi Meet on Saturday 15th September - we'll see you there!





Last time, we only covered half the story about the CATHODE and today we'll look at the indirectly heated type.  These have a construction in which a pure metallic nickel tube is sprayed with an emissive coating admixture comprising of barium and strontium carbonate.    In the picture below, you can see the various stages taken to produce an indirectly heated cathode tube, and working from L to R these are: - cut; reamed; end swaged; pointing for mica location, flattened; lower end pointed; connecting strip welded; etched to allow emissive coating adhesion.

Once the cathode tube is coated, a heating filament which is insulated from the cathode tube is inserted into it.  the filament is made of pure tungsten wire which is coated with magnesium oxide which acts as an insulation barrier between heater and cathode.  In the picture below, working from L to R you can see in turn, the coated cathode tube; the insulated heater filament; the assembled indirectly heated cathode.

Typically, the indirect cathode type is used in amplification and other mains powered equipment as this method of cathode construction reduces the risk of mains borne 50/60Hz hum.


Let's look at the CATHODE today.  Of the two types of CATHODE construction we commonly see, I am today going to describe just one - the directly heated cathode.    

Directly heated cathodes are often seen in battery valves and power rectifiers, these tend to be filament wires or filament tapes coated with an emissive cathode material.  Just take a look at the shadowgram below where we have, from L to R:- a human hair, a coated directly heated filament for a DF66 valve,  an uncoated filament for a DF66 valve.

The filament wire used for diretly heated cathodes tend to be either pure metallic nickel, pure metallic tungsten or a nickel alloy.   The emissive coating is a proprietary admixture of barium and strontium carbonates which are oxidized to barium and strontium oxide as in the firing process of a valve's manufacture. 


Let's look at the PINCH today.  The PINCH is something borrowed from incandescent lamp manufacture and is where lead-in wires pass through a valve envelope in a cleverly wedge shaped piece of glass which not only provides a perfect impermeable seal but also provides a very nice mount upon which an ELECTRODE CAGE can sit.

As can be seen from the photo sequence below,  as we work from A to C, in A you can see the sectioned PINCH at the base of the envelope showing clearly the transit of the wires through the impermeable glass seal.    In B we can clearly see an electrode cage being mounted atop the PINCH.   In C we can see moving from L to R, the support structure and thence the cathode being added as the mounted electrode cage is successively built up.

This next photo shows the semi-molten glass stem being made into a PINCH on a Mullard glass blow-form machine.

So, there we are, we now know all there is to be said about valve PINCHES!


Continuing our journey to look at what's inside a valve, today we are going to look a little further at our EL37 pentode with the envelope broken open.  After a little more surgery on the ELECTRODE CAGE we can see in the centre. the white vertical cylinder of the CATHODE, surrounding that are the three concentric grids, the inner being the CONTROL GRID, then comes the SCREN GRID and finally the SUPRESSOR GRID.


I thought I would write a series of blog articles on what is inside a valve.  Famously, the head of Mullard UK, the ex-Philips SS Eriks once stated that the only British thing inside a Mullard valve was the vacuum and after I recently waffled on to some poor chap about electrode cage construction, pinches and getter flashing, they were bemused so I thought that I had better expand on the great Dutch master's scathing comment, so, without further ado, let's talk about......... what's in a valve..................................

The picture below shows a sectioned Mullard EL37 output pentode, as you can see, this valve is typical of those which have PINCH consttruction.  The EL37 valve has an OCTAL BASE and you can clearly see the glass PINCH upon which the ELECTRODE CAGE is mounted then secured between MICA plates.   We will go on to discuss each of the components mentioned and highlit in upper case text in future articles but for now, just enjoy this scintillating picture of an undressed EL37!



The advances in television since WW2 and the extension of the British TV service and indeed those of other countries brought with it the necessity to develop a more effective series of valves for television manufacture.  In order to reduce the size, weight, customer cost - and manufacturer profit margin, British TV sets were now being built using the 'transformerless technique' pioneered first in 1948.

So, in 1951, Mullard introduced their World Series of TV valves comprising of the ECC81, EF80, EB91, EQ80, PL83, PL82, ECL80, PL81, PY80, PY82, EY51.  As you may deduce from the list, all were miniatue B9A valves except for the EB91 double diode and the EY51 EHT boost diode.


Here we see a wonderful evocative photograph of operator Dabney Grimble attending to CRT bulbs at Mullard Mitcham. Pictured is the manufacturing stage where the neck tube has just been welded to the bulb.  To avoid heat induced stress at the weld junction, the bulbs are placed in an annealing furnace for 150 minutes along a graduated tunnel furnace which controlls the rate of glass cooling.   The bulbs travel by carrel to the annealing furnace whilst suspended in a vacuum chuck which is kept at the same temperature as the bulb to prevent breakage.    Placement of the vacuum chuck required a firm and precise hand and Dabney was a "dab hand" at this task as you can see.........