Amp Basics

By Tonestack

Rock and blues guitarists tend to love vacuum tube guitar amplifiers.  There is something about a set of glowing “fire bottles” that brings out the best in a guitarist.  It could be the tactile feedback, or it could be the harmonically rich tones that only vacuum tubes seem to be able to produce.  One thing is for sure, and, that is, vacuum tube guitar amplification has outlived almost all other commercial applications of this obsolete technology.

This posting is the first installment in a series of articles covering the fundamentals of vacuum tube guitar amplification.  We will kick this series off with a high-level overview of the technology.  The members of DRG will determine how far we go with this topic.  The use of higher-order mathematics will be kept to a minimum; however, there will be a few concepts that will require one to possess a basic understanding of algebra and trigonometry to fully comprehend.  Let’s start by discussing the basic layout of a vacuum tube guitar amplifier.

Vacuum tube guitar amplifiers are not complex devices.  In fact, the circuits found in the average vacuum tube guitar amplifier are less complex than those found in the average solid-state guitar amplifier.  A vacuum tube guitar amplifier is basically composed of a preamplifier, a power amplifier, and a power supply. Together, these building blocks take a small signal and make it bigger! 

The preamplifier’s job is to take the low voltage signal produced by one's guitar and make it big enough to drive the power amplifier.  The vacuum tube triode (three-element vacuum tube) is the small signal amplification device of choice for most vacuum tube guitar preamplifier designers.  The basic building block used in designing preamplifiers is the “gain stage.”  Most vacuum tube preamplifier designs employ multiple cascaded gain stages.

In a vacuum tube guitar amplifier, the power amplifier’s job is to take the voltage signal that was amplified by the preamplifier and translate it into a current source large enough to drive the load (a.k.a. speakers).  The current supplied by the power amplifier changes with respect to the voltage signal supplied by the preamplifier.  The mechanism through which a power amplifier performs this feat is known as “transconductance.”  American power amplifiers tend to be designed around the use of beam power tetrodes (four-element vacuum tubes with beam-forming plates) whereas British designs tend to employ power pentodes (five-element vacuum tubes).

The power supply is responsible for converting alternating current (AC) to direct current (DC).  It performs this feat via rectification and capacitive/inductive smoothing.  The typical power supply provides several different DC voltage levels. The DC voltage level that one will hear referenced most often in tube amplification is known as the “B+" voltage.  The B+ voltage is the DC voltage level that is applied to the power tubes.

Summarizing, a vacuum guitar tube amplifier is composed of a preamplifier, a power amplifier, and a power supply.  The preamplifier takes the low voltage signal produced by one’s guitar and amplifies to the point where it can drive the power amplifier.   The power amplifier converts this amplified voltage signal into a current source large enough to drive the load.  The power supply coverts AC voltage into several different DC voltage levels.