I think we really need some diagrammatical stuff soon. But as I can’t find a pencil in this dump, I’ll leave it for now.
We finished up last time discovering that you don’t get summat for nowt. (Something for nothing in English). And this, in amp terms anyway, means that all this power we have coming out of the output stage, which has multiplied the input from your guitar by maybe a hundred, say, has to come from somewhere.
This ‘from somewhere’ is the power supply, and it’s the function of this that we need to get to know about.
For a start, the stuff that comes out of your wall socket at home, is of no use to your amp, as it is. It comes out of there as ac. These two letters stand for ‘alternating current’ and what that means is that the stuff that comes out of the socket continuously alternates between positive and negative, 50 times per second, and our stuff (UK mains voltage)alternates about 336 volts positive and the same negative, which works out to a value of 240 volts rms. The rms bit is another serious source of bullshit that I’d really like to bomb out of existence, but this isn’t the place to do it. I might finish up generating my own bullshit, and there’s enough around without me joining in.
Rms is actually a really useful term if we steer clear of all the rubbish. Without going into it in any depth, if you connected a 9 volt battery (which is a dc, means direct current, or non-alternating current source) up to, say, a bulb, you would get exactly the same light/power out of it as if you shoved an ac voltage of 9 volts rms into it. That’s where the idea of rms (which means ‘root-mean-squared’ and is something we’ll look at somewhere else) came from.
The reason for that agreement between ac and dc, is largely a historical one. If we go back to the 1950’s and before, the ‘mains voltage’ (in UK at any rate) that came out of your wall socket was d.c. at a lethal 240-250 volts. When the system began to be changed to ac, it was important to be able to have some means of comparing the output voltage/current in ac to the equivalent as it existed in dc. That way, your 1kw (dc) electric heater was still 1kw (ac rms) after your home supply had been changed.
Phew, that went on a bit.
Anyway, back to the plot, if I can remember what it was.
Inside your amp, is a transformer; almost, but not quite, always. If it hasn’t got one, it’s either battery driven, or goes back to the 1940’s (ish) and was designed to operate on ac or dc. They were called ‘universal’ at the time, and were, if they developed a particular fault, universally lethal. Or it might have a ‘switch-mode’ design. This is a relatively new technique and if I get onto that subject I’m going to sound like an old git pretty quick. I don’t like them.
The transformer is the first bit of the power supply in your amp, and it has two functions. It completely isolates the amp from the mains voltage, (that voltage is connected to the ‘primary’ or input side, of the transformer), and also modifies (transforms) the voltage that goes into the amp. This ‘secondary’ voltage comes out of the ‘secondary’ side of the transformer, and it can be more volts (often in the case of valve gear), or less, maybe, if we’re supplying semiconductors. The stuff coming out of the secondary ‘winding’ of the transformer (they’re called windings, because they are actually coils of wire wound on a chunk of laminated metal, called a ‘former’) is still ac, and as such is no earthly use to the amp as it stands. It has to be ‘rectified’. It’s alright (I suppose) calling an amp a ‘double’ or ‘triple’ rectifier, like Mesa Boogie do, but if we aren’t told what it is or does, then it’s bullshit again.
This is what a rectifier does. It converts ac to dc. That’s it. That’s all it does.
It can do it in a variety of ways, using a number of different devices and circuits, but the voltage goes into it ac, and comes out dc. This dc voltage is connected to the dc ‘rails’ in the amp, that often used to be solid copper ‘rails’ at one time, but now are usually tracks on a pcb (printed circuit board), and their function is to carry this dc voltage around to various places in the amp.
Why do we need dc voltages? Well, when it comes right down to it, the valve, the transistor, semiconductors generally, and mosfets, all do the same thing. They are all devices to control the flow of current.
There is a particular element in each of these devices which more or less defines what it does. In the case of a valve, (this is simplified: there are a lot of different valves) this element is called the grid; for a bipolar transistor, it’s the base; in a mosfet it’s called the gate. This, in each case, is the ‘control’ element.
By applying a small voltage, say from your guitar pickup, to the grid of a triode valve (say an ECC83: this is a actually double triode but most folks know its name. It’s a 12AX7 in American-ese), we can control relatively big currents and voltages going through the valve. These currents/voltages come from the voltage rail(s), and so for these relatively big currents to accurately reflect the small signal we applied to the grid, we need a stable and noise free supply voltage. This is what the power supply, supplies, and is carried to the valve via the rails.
So, what comes out of your amp, is just a controlled version of the voltage rails as supplied by the power supply, and controlled by the valves or what-have-you, in your amp.
The kettle calls. The next blog will be concern itself with clarifying all the previous stuff, by drawings or pics or whatever. Then we might look at the ‘schematic diagram’, that piece of black art that shouldn’t be.