Guitar Amp: how it works. Part 1

Guitar amplifiers - Why?


I remember the first time I had an electric guitars on my hands. As per his "electric" adjective, I thought I should plug the guitar into the wall's mains plug but, that kind of cable wasn't appropriated for such a task.
Second thing that surprised me a bit was that the guitar didn't sound so exciting when I played (unplugged) her.

Well, that's why we need guitar amplifiers, to make that guitar to sound louder.
The signal generated on guitar is really weak, around 300 mV in the hottest humbucker pickups, by example. That is 0,3 V but, to drive a speaker, we need about 220 V (or 120V in other countries) so, a voltage that is 733,33 times greater than the original one!.
And not just voltage must be increased but current also.

Since voltage multiplied by current is named power (P = VI), what we need is a significant increase of power to make a guitar able to move the cone of a speaker.

But, how an amp works, then?.


Guitar amplifiers - How?

Along several blog entries, we will try to describe how the different parts of a guitar amp work together to produce that exciting sound but, please, don't expect a high tech discussion here but, just the few drops of tech stuff to have an overall idea.


Power Transformer (PT)

The Power Plant that delivers electricity to your home, it's doing it in a tricky way.
First, power is being generated with Generators. Generators are some kind of giant coils with moving giant magnets that, when rotate, create fluctuations in the magnetic fields, inducing electricity.

Those generators create electrical fluid with very low voltage and very high current values but, it seems that the most efficient way to transport electricity along long distances is with high voltage and low current, instead.
So, that low voltage / high current electricity is being transformed to high voltage / low electricity before sending it to the distribution network (High Voltage Lines).
This step is being made by a Transformer.

A Transformer is some kind of two faced coils with different wind turns. When electricity travels thru the first coil (Primary), electricity is induced (without contact) in the second coil (Secondary).

At the end, most of the original power (some is dissipated as heat, by example) is transferred from the Primary to the Secondary and, depending on the relationship between the windings of both coils, voltage and current differ.

Just a simple and not necessarily true example; imagine that a Generator is creating electricity at 100V / 100A, that means a power of  10,000 W (= 100V * 100A). Imagine now that the transformer transforms those magnitudes to 100,000V * 0.1 A. You can see that the original power is maintained, because 100 * 100 = 100,000 * 0.1 = 10,000 W but, we achieved a way higher voltage, dramatically decreasing the value of current.

That High Voltage usually cable will be lately split in thinner cables and transformed down again, until you get those 120 V or 220 V at home, with the maximum amperage that you agreed on your contract with the company.

Well, one of the parts of a guitar amplifier is just the Power Transformer (PT).

The PT will get the electricity coming from your wall plug and will transform it into several different electrical values. This is achieved because during the winding of the Secondary, some conductors are pulled off, at different wind turns, to achieve different electrical values. Those different values are named tappers.

So, we can have a PT with a Primary of 230V and a Secondary with tappers of 450V, 150V and 5V by example.

At this point, you could be tempted to think that, since the transformer is able to increase the voltage, maybe it is enough to amplify guitar signal up to 220V and then to move those speakers.
Then... not!. The issue is that the original power generated by guitar's coils is very very low and, speakers need a good amount of current as well. By example, a 100W Speaker, running at 220V would need 0.45A.
If the voltage and current originated by coils were of 0.3V and 0.1A then, the available power should be 0.3V * 0.1A = 0.03W. It doesn't matter if we can increase voltage up to 220V with the aid of a transformer, because we need 100W to drive the Speaker and we have just 0.03W to transform.

Therefore, the PT helps to provide the right amount of voltage and current to every part of the amp.
But, why does the amplifier need different electrical values?.

Later, we are going to describe the rest of parts that make an amplifier and then, we will see that every part is working in a very different level of energy

Event that the PT is not directly affecting the sound of the amp, a good transformer will be more efficient and will provide the right amount of energy to the rest of the amp's parts. The PT should be correctly sized, to support any peak demand of energy but, just of the right size to avoid a waste of energy.

How much consumes an amp of 100W?. You could be tempted to say: just 100W!.
Then... not!. A guitar amp is usually consuming more than double the energy that delivers so, a 100W amp can be consuming at least 200W!. The relationship between power consumed / power delivered is what is known as Amp's Efficiency. An efficient amp design will consume less energy to deliver its own energy.
We will see that even that in some parts we need very low wattage, in other parts we can reach more than 400V and, that increases the need of source energy.

Finally, the PT can have different tappers for different electrical environments (120V or 220V). We can see how much current a transformer will consume, depending on the voltage of the mains.
Remember, Power = Voltage * Current so, Current = Power / Voltage.
For 120V, Current = 200W / 120V = 1.67A.
For 220V, Current = 200W / 220V = 0.91A.
Old amps hadn't that kind of input tapper so, they worked just for 120V or 220V.

The value of the Fuse that is being used to protect the amp from overload is usually higher than the current that the amp really needs, allowing some peaks but avoiding excessive high peaks, during a certain time.
Once a peak level has been hold for more than the time that the fuse is designed for, the fuse breaks and the amp goes off. There are slow fuses (that support peaks for longer times) and fast fuses (a single peak can blow it).

Modern amps, have at least one fuse for the mains plug, protecting the PT. Other amps include one or more fuses to protect the Power Tubes and Output Transformer, also.

Let see how efficients are some amps. By example, a Marshall's 1923C consumes 175W to deliver 50W RMS so, it's delivering just a 28,57% of the original power, rest is being lost inside the amp (as heat dissipation and electrical work, by example). A Fender's '65 Princeton Reverb reissue consumes 125W to deliver just 15W so, it is delivering just a 12% of the original Power.
As you can see, guitar amplifiers are usually very inefficients. Even that the design of the Marshall's seems to be more efficient than Fender's one, an amp is considered very efficient when at least transforms the 50% of the input power in its output power and, this is very uncommon in guitar amp's world.