technology

Introduction

 Warning - repairing and restoring vintage valve radio receivers is a fascinating hobby. Locating and rectifying faults in such a radio is a uniquely satisfying experience. You may find the hobby becomes an obsession!

IMPORTANT - BEFORE ATTEMPTING ANY WORK ON A VINTAGE RADIO, PLEASE READ THE NOTES PROVIDED ON THE SAFETY PAGE (LINK ON LEFT NAVIGATION)

Produced in response to many requests, it is hoped that the reader will be encouraged by the straightforward, practical treatment of the subject. The explanations are concise and for the most part free from maths and jargon. As a result, it is fair to say that inevitably some of the explanations are to an extent approximate. I believe this approach is entirely justifiable – this is an enjoyable and absorbing hobby and I have no wish to over-complicate issues. If the reader wants to know more, there are many texts and education courses available. Suggestions for further reading are provided on the BIBLIO page where the are sub-pages for magazines and used books.

If this approach is to your liking, you should enjoy 'The complete guide to Vintage Radios' (CD-ROM). To take a look at the contents, click on 'COMPLETE GUIDE' in the left navigation pane.

The CONDUCTOR AND INSULATOR 

Presented here is a basic 'model' of electric conduction. This is based upon popular science and it should not be inferred that the author thinks of the world in quite these terms; the electron, for example, cannot be located exactly but occupies at any moment a 'probability' of being in a given place. The electron possesses both the properties of a wave and the properties of a particle and which it is depends upon the test applied. This is deep water and this is not the place to discuss quantum-mechanical uncertainties. All we need is to know that this model WORKS well enough to allow practical application.

All matter is made from atoms. Atoms are so small that even the most powerful microscope cannot see them. Grouped together, atoms form molecules from which everything in the universe is made. 

The simplified view of the atom (left) shows a central nucleus built up from protons and neutrons, around which spin a varying number of electrons. Protons are said to carry a positive electrical charge, neutrons no charge at all, and electrons a negative charge. These charges are shown graphically at the bottom of the left navigation pane.

Typically, the number of protons in the central core balances the number of electrons and as the protons are positive, there exists an equal and opposite charge to that of the negative electrons. The result, on the average, is zero charge on the atom as a whole because the equal and opposite polarities cancel each other.

  Materials classed as insulators have tightly packed clouds of electrons and there are very few electrons capable of leaving the ‘parent’ atom. In conducting materials such as metals, electrons are more loosely bound to the individual atom and those in the outer orbits are free to move at random from one atom to another*.

When an atom ‘loses’ an electron to another atom, it becomes positively charged due to the loss of that electron’s quantity of negative charge. Atoms so charged are said to be ‘ionised’. Because opposite charges attract and like charges repel, another electron will be attracted from a nearby atom. This happens continually with all the atoms of conducting materials. So, although there may be individual atoms with a positive charge, over a group of atoms there is effectively neither a positive or negative charge. This is a state we call ‘neutral’.   

*see valency, valence electron in any good dictionary

Remember that a positive charge (a lack of electrons) will attract electrons. If by some means such a charge is placed on one end of a conducting wire, it will exert an attractive force on the electrons surrounding other atoms in the wire. This lack of electrons is called a difference of electric potential (PD). However, the term ‘Potential Difference’ is best used to describe the function of a resistor, which, when a current is flowing through it has a difference of potential – which we measure as voltage - across it.

  Let's consider what takes place when we touch a short length of copper wire to the positive terminal of a battery. As I've already said, there will be a movement of electrons toward the attracting positive charge (positive = a lack of negative, remember).  Equilibrium is reached when the atoms in the wire that are deficient in electrons – therefore positively charged - exert sufficient ‘back’ pull on the electrons. This means that only a momentary shift in electron position has occurred, like the tug-of-war rope being pulled tight, resulting in a strain or static charge but no actual flow of electrons.

  You can create static charges by combing your hair with a plastic comb, rubbing a balloon on your clothing, or sometimes even walking on certain kinds of man-made carpet. Even wearing insulated shoes can create static charge, especially on hot, dry days. Lightning is an extreme example of a static charge.

  These charges are interesting, but their uses, as far as the subject matter here is concerned, are limited. To make electricity work for us, we need to create the continuous flow of electricity, which we call ‘electric current’. One way to do this would be to connect the free end of our length of copper wire to the negative terminal of the battery. Because the battery acts like a pump, by chemical means constantly creating an imbalance of charge, electrons will now flow in one direction around the circuit, toward the positive terminal, with replacement electrons flowing into the circuit from the negative battery terminal. With our length of copper wire forming a complete circuit with our battery, we have the two essentials to create a flow of current – a source of electric potential and a circuit to convert the potential into current. It is not a sensible thing to do, however, as we have no control over the resultant current. We have turned the tap full on and a heavy current will flow, limited only by the capacity of the battery – its internal resistance - to supply electrons at its negative terminal to replace those flowing through the wire into the positive terminal. A timely warning would not be out of place here. All the above is theoretical. I DO NOT ADVISE you EVER to deliberately place a conductor across a power source. A small dry battery will rapidly overheat and fail under such uncontrolled conditions but far worse, a car battery or wet-cell accumulator could instantly burn out the conducting wire or - even - accidentally explode.

Whenever a conductor, such as our length of copper wire, is connected directly from positive to negative it is said to have created a short-circuit, meaning a circuit with effectively no resistance to the flow of current.

Rather than wasting the power of the current in heating up the battery and the wire, we want it to do something useful so we need to have control over the flow - we need something to limit the movement of the electrons. Resistance comes in several forms. We have already met it in the battery and the wire. It was resistance that caused them to heat up. This is frictional resistance. No conductor of electricity is perfect and there is always some resistance and this can be useful, for example in the electric kettle, cooker, fire, toaster or any appliance that uses an element. It is the resistance of the element, due partly to the kind of metal used and partly to the thinness of the wire, that causes the heating effect.

  Light bulbs, too, work in a similar fashion only in this case the wire inside the bulb - the filament - is very fine and would normally burn out. The bulb has no oxygen in it, however. This means the filament cannot burn in the sense of combining with oxygen, even though it will become very hot. It can glow white hot, so hot it gives off light.

What if we want to prevent current from flowing? That’s where insulators come in again. Most plastics, dry wood, ceramic and rubber are effective insulators and are used to prevent us from getting electric shocks. Plastics of varying kinds form protective coatings around conducting cables.

Fuses are safety devices, designed to fail in the event of excess current flow. 

So far, only direct current flow has been described. Direct current (DC) flows in only one direction around a circuit. If we arranged a switching system to continuously reverse the polarity of the power, the direction of current flow would continually change, too, and current would flow back and forth around the circuit at the switching rate. In other words, it would alternate. The mains supply is alternating current (AC) for good reason: AC can be stepped up or down in voltage at will, via transformers. Although DC mains were supplied to parts of the UK in years gone by, it is much more efficient to distribute very high voltage AC via the national grid, stepping down where convenient to 240 volts, the nominal supply voltage pressure in the UK. The direction change occurs 50 times a second (50 cycles, nowadays called 50 Hz, after Hertz, the German physicist).

The mains supply is not ‘switched’ rapidly in polarity, though. The AC generators that create it cycle more or less smoothly through from a peak in power in one direction, falling through zero voltage/current to rise to a similar but opposite peak. I stated ‘smoothly’ but that’s an ideal and the actual wave shape created may fall some way short of that. It is direct current (DC) that valves need. This leaves mains powered radio designers with the need to convert the AC power into DC, of course, because amplifying valves cannot function on high voltage AC supplies.

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