The thermionic valve

I wrote in a recent post that my father bought me some electrical bits and pieces from Mr Woolworth and thus set or more likely confirmed a course for my life.

Perhaps I was a strange kid. Back then the County Library rented the premises next door to my parents' home (it now occupies both this and my parents' home). I suppose most kids back then borrowed books of fiction but I searched out reference books on engineering and electronics. I do not think I understood all of what I read, mind you.

Back then I wished, oh how I wished, that I would meet a grown up who would give me a triode valve so that I could check out some of the stuff I had been reading about. How inaccessible can be the simple desires a child has. How vital is it that someone steps in to provide the answers or materials. How scary it is that a whole career might thus be at jeopardy.

Later on in my development I started to collect and dissect old radio sets and such like so I eventually got my triodes.  I went on to build my own oscilloscope based on the article "Investigator Oscilloscope" in Practical Electronics July - August 1967 see link.

The picture is from the link above and brings back happy memories. My version looked a bit prettier but the trace was as bad - very non-linear at the edges and appalling bandwidth, but at least it worked and it was useful. And it used valves, in particular the ECC81 double triode.

Later still a radio enthusiast who was a friend of a friend of the family died and I was given some of his stuff including boxes of unused valves: sad that I gave them away when I left home as they might have been very valuable now.

Now-a-days valves (vacuum tubes to our friends across the pond) have made a come back with audio freaks who claim to have golden ears. I made mention in a previous post of a balanced pentode audio amplifier I extracted from some equipment: it gave such wonderful sound quality. The two ingredients of valve and balanced (push pull) are hard to beat when it comes to audio reproduction.

I still create valve circuits sometimes when I doodle.  Along with maps of roads and railways, or cross sections of tunnels and caves.  They are that dear to me.

A valve is a pretty near perfect amplifying device. Much nicer in operation than a bipolar transistor. The input impedance is almost infinite. The transfer characteristic is at least smooth even if not as linear as one might desire (but a bipolar transistor is horribly non-linear).  And so cool to think of all those electrons doing their stuff like honey bees in a hive.

In operation the cathode 'k' is heated to red hot by the filament 'hh'. The electronics in the cathode are thus vibrating a lot (their increase in vibrational energy IS what we call increase in temperature) and some 'boil' off. The valve or 'tube' is evacuated so there are no air molecules to get in the way of those electrons moving. And there is a big metal plate a few mm away called the anode 'a' which is at a 'high tension' or high positive potential (voltage) with respect to the cathode, so it attracts those electrons and hence am electric current flows. Between cathode and anode there is a potential gradient (aka electric field). Very much closer to the cathode than the anode is a grid of thin wires 'g1'. If a relatively small negative potential is applied to the grid, because at its point the potential otherwise due to the anode is not yet that great because the anode is much further away, the negative voltage on the grid is enough to turn back some or maybe all the electrons. Thus a small voltage on the grid can control the electron flow.  Moreover, because the grid is negative, it repels the electrons and thus does not pick them up thus no grid current flows thus the input impedance is very high, or put another way no work is done by the grid.

The potential drop across the cathode resistor 'RK' due to the finite anode current provides the negative bias required to operate the grid. Some of the valve radios I had were old enough to require a separate grid bias battery.

In those early days I saved up my pocket money over many weeks to buy a 120V battery.  90V batteries were comparatively common back then but I had convinced myself (I know not how) that I needed the rarer 120V kind. The cruel thing was that, when I eventually got it, it lasted hardly any time before it ran out of juice. I rather think it was old stock by the time I got it.

A water analogue for the triode? The best I can come up with is a mountain stream falling over a high cliff. The height implies a lot of potential power and is analogous to the high tension. And by inserting a relatively small dam (the grid) upstream of the fall you can control this power.

The George Yard and Groundhogs

My childhood home was, I am told, once part of the George Inn which, hundreds of years ago, straddled the archway that leads into what is now called the George Yard.  My parents now both being deceased, the house was sold and converted into the County Library.  When I last visited the Library I found that my old bedroom and the bathroom next door still exist much as I remember it.  Even the bathroom suite, and the towel rail with its secret fixing which I made myself, is still there.  This is surprising because much of the rest of the house was gutted in the conversion.  My bedroom was where the yellow square is in the Google Earth image below.

The view from my bedroom window looked down on Smith & Cockford, Builder's Providers owned and run by my parents (now Alresford Builders Supplies).

You can see from the earlier picture below (which predates me) that my parents did some improvements, but some of the sheds to the rear were still in bad shape and, with their rusty tin roofs, served as my childhood playground.  My friend from next door and I clambered over many of the roofs in the vicinity, often against the express order of the owners. The goal was to get from A to B without touching the ground and without being seen by any adult.  Or we would (from an allowed vantage point) hurl abuse at my father's workmen on their way to or from the very suspect convenience (which stank) provided for them.  In one shed there were two layers of stacked timber which you could crawl between to hide or to, once again, get from A to B without being seen when playing tag.  There was a large rectangular rainwater tank for playing submarines - you get a suitably shaped piece of wood and leave it for several days to become waterlogged, then drive in nails until it is on the verge of sinking.  The game is to see who can make a submarine that stays down longest (without aide) although which eventually surfaces.  As a result the bottom of the tank became filled, after a time, with failed missions and other debris which my father had to clean out to his chagrin.  This tank is the same one that supplied my venturi pump described in an earlier post.

Looking back from where this photograph was taken, the next two pictures are recent and show our house on the left, and our neighbour's on the right.

It you walk up the George Yard passing what was Smith & Crockford on the left, there is now the plot of a new house occupying what was our "Top Garden".  My workshop, marked by the red square is in the aerial view above, was in the corner of this garden and is where I would retreat to make my inventions.  My father built the swimming pool which you can see in the aerial view.  I plumbed in the filtering system and made a solar heating system (by diverting some of the pumped water to trickle down the plastic roof of the changing room  / shed).  The picture below is recent and was taken by slightly trespassing into what is now the garden of that new house.  Had anyone been there to ask I would have done so and I am sure they would have been happy to show me around.

An advantage of owning your own swimming pool is that you can swim naked.  It is better in every way - you are more streamlined, there is none of that clinging of wet material to the skin, the water does not feel as cold, no wet swimming costume to peel off and deal with afterwards, etc.   There are locations on the river close to where I live now where one can (with some care) get away with swimming naked, but I am not going to tell you where they are.

The pallisade fence is new but the low brick dividing wall, the fruit tree and the high flint wall at the rear with its inset arched seat are all as I remember them.  The wall was built by Bill Smith (no relation of the Smith in Smith & Crockford who died long before my time).  Bill was an old-timer.  He and his family lived in what seemed to me, as a child, little more than a shed with a tin roof.  They kept chickens and my mother would buy free range eggs from them.  It used to be said that anything Bill Smith built would last a lifetime.  The fact that this wall still exists and is in perfect condition is testimony to this, although Bill himself is long gone.  He used to let me "help" him and there is, in that wall, several pieces of green glass that I inserted all by myself.  If the present owner reads this, they can find the glass not far from the inset arched seat, to the left if I remember correctly.

A little further up the George Yard there were trash heaps to left and right.  These were doubtless meant to be for builder's rubble but sometimes there was more - so they were wonderful places for me - very, very occasionally I would find a prize - an old radio, or a discarded vacuum cleaner.  These I would haul back to my workshop, like an ant hauling food for its menagerie, where they would be stripped down and any useful components kept.

In my earlier years most of my "work" was in disassembling (parents should not necessarily discourage this activity, negative though it may seem - it is how I learn.  Adults call it "reverse engineering").  But later I did make stuff - here are some examples.

My earliest memory, having previously decimated various valve radios and stuff, was to construct my very first radio receiver.  Or so I hoped.  I figured that these parts once made up a radio, so my connecting them together I must surely stand a chance of making a radio.  This exercise was not completely without knowledge - I had a battery and earphones.  But it didn't work.

My other examples were later on in my career...

An oscilloscope - this was heavily based on a magazine article.  It used a 2.5" CRT, a special mains transformer, and valves.  Valves (vacuum tubes) are ideal for an oscilloscope because of their naturally high input impedance.  I did all the metal work myself, the source of aluminium sheet being an unused Green Shield Stamps placard  (bare aluminium on the reverse) which was surplus to my father's requirements.   .

An audio signal generator - again heavily based on a magazine article, but I could not afford the thermistor stabilising device so instead had a potentiometer which had to be carefully tweaked for lowest distortion.

An X-Y plotter.  This used a platen roller from a type-writer for the Y-dimension, and a pen drawn along a brass rod for the X-dimension.  The rod was milled (a friend of my father's who built model steam engines allowed me to use his milling machine) with a slot and turning the rod lifted the pen so gave the Z-dimension.  Z was actuated by a solenoid.  X and Y were actuated by d.c. motors driven by AD161 / AD162 power germanium transistors in a servo loop.  The position coder was a standard linear potentiometer in each case. The design was largely my own this time.  The chassis, this time, was made from Perspex (acrylic).  I had discovered that chloroform is a solvent for Perspex so, accompanied my my father, we had to sign the dangerous substances register at the local chemist to purchase a small bottle of the stuff, which promptly evaporated if you didn't screw the lid back on tight after use.

My crowning achievement was to use this device to plot a groundhog.  Dunlop had been running adverts for their tyres featuring this creature, and I had a younger friend Nigel at school whose claim to fame was the ability to draw them.  So my hand-drawn groundhog became a sort of icon for any graphics project.

Later - or was it concurrent? - I discovered the digital plotter at college and my first software program to make use of it had to have it drawing groundhogs subject to various geometrical transformations, but that is another story.

And then there was the railway room.