To simplify very complex technology, a guitar amplifier ‘amplifies’ a signal from a guitar pickup through its preamp and power amp stages.
Similar to the construction of an old-fashioned lightbulb, a valve is a collection of different elements together in a vacuum, cased in a glass cover. One of the elements, the Cathode, is heated and electrons (electricity) flow from this end to the high voltage Anode on the other end. Another element, called the ‘grid’, controls the flow of electrons from the heated end to the high voltage end. It’s that fact that makes it an amplifier – wiggle the grid and a much larger flow changes amplification.
Incidentally, it’s the addition of the grid to the Cathode (heated element) and the Anode (high voltage element) that makes a Triode amplifier (Tri – three). With just the Cathode and Anode this would be a Diode amplifier (Di – two).
Preamps are the smaller-sized valves inside your amp. The preamp adds almost all of the “voltage gain” and the power amp multiplies the voltage gain with “current gain”. Voltage x current = power, hence the phrase ‘power amp’. The relationship between preamp, power amp and every component in-between can make a huge difference in your tone. Below we go through some of the most commonly used power amp valve types and how they lend themselves to different tones.