How Valves Work - PART I - barebonesAmps.co.uk

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How Valves Work - PART I - barebonesAmps.co.uk
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Valves

The magical guitar amplifier valve, or tube if you prefer - I can’t think of any other musical component which causes so much confusion or interest.



One thing is for sure, they look great. They glow in such a nice way that I’m sure we think they sound classic and warm before we’ve even heard them. I have not met a single person who does not like valves, whether from nostalgia or a desire to one day own a valve amplifier. Valves also find their way into microphone pre-amps, mixing desks, effects units and I’m sure many, many other devices, all designed to add that touch of magic.


Why not use Transistors?


Transistor or solid-state amplifiers have a poor reputation that is in some ways undeserved. I guess that had Jimi Hendrix called Jim Marshall and asked him to sort out a few stacks of ‘tranny’ amps for his concerts around the world, then we’d all be lusting after those instead. What we really want to hear from our own guitars and amps is the sound that we hear on our favourite recordings. As the only feasible technology available to Jim Marshall at the time were valve amps, that spot in guitar history has been fixed in time and space, so not even Doctor Who could shift it.
So why do valve amps sound better?
The simplest answer is that valve amps sound better because they are made to sound better. The market is flooded with budget ‘tranny’ amps, made with cheap components and designs. Over the years, these budget amps have muddied the reputation of ‘tranny’ amps to such an extent that even with the amazing ‘modelled’ amplifiers available today, most guitarist still dream of owning an all-valve amplifier.

However, that is not the whole story. Valves, when pushed hard, are non-linear amplifiers. What this means is that the output from the valve is not simply a larger copy of the input but is coloured by the valve. The output is in fact compressed and distorted. In theory, this makes the valves a poor amplifier. In practice, however, this makes our guitars sound louder and adds harmonic distortion to the original sound – forming the basis of the classic guitar sound we all seek.


So what is a valve?


Consider a typical water tap found in every home. This ‘valve’ controls the flow of water in a pipe. We can control the temperature of our shower or the speed at which a bath fills by making a tiny adjustment to the tap. In the same way, our guitar amplifier controls the large signal going to the speaker by using the tiny signal from the guitar.
Let’s start by looking at current flowing in a simple circuit
 

 
We are taught that the current flows from the positive end of the battery to the negative. It was discovered that the electrons actually travel from negative to positive. In most situations this makes no difference to the way we design circuits, however, for valve amps it is important so I’ll draw the electron flow.

If we make a break in the above circuit, the current will no longer flow. However, if two plates are placed in a vacuum and heated, then the electrons can ‘jump’ across this gap.
 

 
This is nice to know and makes for good light bulbs but is not much good for us. However, by adding another filament, known as the grid, between the two plates, the flow of electrons can be controlled by applying a small signal to this grid.
 

 
That’s it. It is as simple as that. We plug our guitar into the grid element and get an amplified signal from the heated filaments.

Here is what that looks like in practice-
 


 This is the first stage of the barebonesMonacoLite.
The yellow line shows the guitar input signal (in this case a 12th-fret harmonic on the low E-string). The blue line shows the output from the valve. The signal level has been increased from a peak-to-peak of 0.1V to 2.78V. That’s a ‘gain factor’ of almost 30. You may also notice that the output is ‘upside down’. This is of no concern and can be used to our advantage in more complex amplifiers.
 
Of course, amplifiers have lots of components and are not battery powered so we’ll explore amplifier design in future articles.
 
Matt Green
barebonesAmps.co.uk

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