Valve rectifier circuit and how it works.

This looks horribly intelligent.

Which means (by inference) that my coconut macaroon is fading into the sunset.

On the plus side, it’s not impossible I might get a laugh out of this. Not likely, just not impossible.

Onwards and upwards, bull by the horns, goldfish by the goolies……maybe not that…..

The first thing we need to supply our amp is something to plug into the socket on the wall. On the end of this cable, we go into an IEC (kettle) connector, and this has a line voltage, a neutral, and an earth. The line voltage and neutral connect to the mains transformer ‘primary winding’. This ‘primary winding’ is quite sensibly named. In a world of electronics bullshit, you should hang onto anything that might be sensible. Because there’s not much of it about.

This primary winding is the first thing that the mains voltage sees (so it is primary, or first) and it’s a winding, because it’s a long length of copper wire, wound around some metal former. In the drawing, that primary winding is represented by the block marked 240 volt. Also wound onto this same former is another (sometimes many) more lengths of wire. This is the secondary side of the transformer, and in this case we have two HT (stands for ‘high tension’ or ‘high voltage’) windings which supply the two anodes of the rectifier valve. American for ‘anode’ is ‘plate’. But this doesn’t make all that much sense because the cathode is still ‘the cathode’ in American. Don’t ask me??? We also have a heater winding, which is much lower voltage (5volts or 6.3 volts usually, depending on the valve. This heater winding supplies the rectifier valve heater (and possibly other valve heaters.) The property of ‘induction’ enables the transformer to function. The ac voltage applied to the primary winding has some fraction of that voltage ‘induced’ in the secondary winding, which is why a voltage appears at the secondary output without there being any direct connection between the windings.

The heater of the valve often (not always though) is completely separate electrically from the operation of the valve. All it does is heat the cathode.

Here we get to actually how the valve rectifier (and nearly all valves actually) works. By heating the cathode (usually made of a tube of copper with the heater inside it) we excite the free negative electrons on the cathode. I knew we would get to something exciting eventually. The copper cathode is often coated with something like boron, which gives off a lot more free electrons than copper, so making the valve more efficient.

This does next to nothing at all, other than to create a ‘space cloud’ which is a cloud of electrons that surrounds the cathode. However, if then we switch on mains voltage, the secondary of the transformer supplies a high voltage to the anodes, which is positive in one direction. Positive attracts negative, which means that the electrons from the space cloud are drawn towards the anode when it is positive. This has now begun an electron flow from cathode to anode (but only when the anode is on its positive cycle).  Conventional current flow is actually the opposite (positive to negative), because electron flow is negatively charged.

So what we have now is positive voltage appearing at the cathode (marked HT dc out).

On the end of this terminal, we have to provide smoothing for the HT output. That voltage is not usable as it is. So we will look at the choices of smoothing circuits next.

Tea. And not many laughs. Sadly.


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