On the last blog, the construction of an ECC 83 valve was discussed, and, in a nutshell, it’s a triode; it consists of a vacuum sealed glass envelope, inside that a rectangular section tube called the anode, inside that a coil of wire called the control grid, inside that a tube called the cathode, and inside that, the heater, which is also a coil of wire. So I suppose, 3 concentric tubes with the heater in the centre. All that, twice, because the ECC83 is a double triode.
There is one valve that is simpler than the triode, and that’s diode. It has an anode, a cathode and a heater. In some diodes the heater is the cathode. An example of this kind of valve would be the GZ34 or 5Y3 and they are diode rectifiers. They’re a bit different because they’re bridge rectifiers, but the same principles apply.
The diode is the simplest valve to explain and it goes a long way to showing how they all work.
If we stick a positive voltage to the anode, and a negative to the cathode, it won’t do anything. The anode is the outer box and the cathode the inner tube. Basically all there is inside the valve are two unconnected pieces of metal, so it’s like an open switch. If, though, we heat the cathode, a current will flow from positive to negative. Conventional current flow is always positive to negative so that’s as we’d expect. Water doesn’t flow uphill, and it’s as normal as that. But what use is that, and why does it work when the cathode is heated, and not when it’s cold?
If we put an alternating signal (current) (this is the stuff that comes out of the wall socket) across the anode and cathode, the valve will allow current to flow when the anode is positive, and not when it’s negative. So we get current flow in one direction only, which means we can convert the alternating current (a.c.) to direct current (d.c.), and this what electronic gear of all sorts needs. So the first thing that happens in an amp is that the mains in, is converted to d.c. by the rectifier.
How does it work? Well, you can take this into the realms of quantum physics, but they’re all theories, and only last until another comes along. However, the conventional explanation is probably not too silly. The heated cathode emits electrons; a lot when it’s hot, and hardly any when it’s cold. So when it’s hot there are a lot of electrons hanging around having been thrown off the cathode by the energy supplied by the heater. If then we stick our positive voltage onto the anode, this has the effect of drawing the electrons towards it, because positive charges attract negative charges, and as electrons are negatively charged, they are pulled up to the anode, if the voltage is high enough. The electrons flow though the valve by being emitted at the cathode and attracted to the anode. So it’s a bit like a one way switch (the electrons won’t go back the other way, because the cathode negative charge repels them) but not quite. The valve can only deal with so much current, and beyond that, the output ‘sags’ and this is why a valve rectifier sounds so different at high power outputs. It behaves as a very complex compressor. And, even worse than that, so far as digital modelling is concerned, a very unpredictable complex compressor. Digital does not like unpredictable, so it’s impossible to simulate accurately. Mainly because there’s little that’s accurate to model.
So that’s more or less it for a diode valve operation. (ish) Next up we’ll find out what’s happening in a triode. Tea time.