THE GREAT MULLARD MAGIC BLOG

WHAT'S INSIDE A VALVE? (v)

Posted by STEVE MYCIUNKA on

WHAT'S INSIDE A VALVE? (v)

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...

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WHAT'S INSIDE A VALVE? (iv)

Posted by STEVE MYCIUNKA on

WHAT'S INSIDE A VALVE? (iv)

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...

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WHAT'S INSIDE A VALVE? (iii)

Posted by STEVE M on

WHAT'S INSIDE A VALVE? (iii)

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...

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WHAT'S INSIDE A VALVE? (ii)

Posted by STEVE M on

WHAT'S INSIDE A VALVE? (ii)

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. 

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WHAT'S INSIDE A VALVE? (i)

Posted by STEVE M on

WHAT'S INSIDE A VALVE? (i)

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...

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