The Complete Handbook of Musical Instrument Amplifier Tubes

Chapter 1: Understanding Musical Instrument Amplifier Tubes

The electric guitar amplifier is, in most cases, a simple machine with a complex heart. On one side, you plug in a guitar that produces a signal measured in millivolts. On the other side, you have a speaker that needs tens of volts to move air and create sound. The component that bridges this gap, and in the process imparts the character that defines the instrument's voice, is the vacuum tube. To understand a musical instrument amplifier, you must understand the tubes that drive it.

At its most basic, a tube is a glass or metal envelope containing a vacuum, with a set of internal electrodes. The most critical of these is the cathode, a filament that is heated to a high temperature, causing it to release electrons. This process is called thermionic emission. The electrons, being negatively charged, are drawn toward a positively charged plate, or anode. Between the cathode and the plate sits a control grid, which is typically a wire mesh or coil. By applying a varying voltage to this grid, you control the flow of electrons from the cathode to the plate. A small voltage change on the grid can cause a much larger change in the flow of electrons, which is how the tube amplifies a signal.

This is a simplification, of course. A real tube is a precision component. Consider a simple preamp tube like the 12AX7, which is the workhorse of nearly every guitar amplifier's preamp