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.
