Date of Defense

4-13-2004

Department

Electrical and Computer Engineering

First Advisor

Dr. Frank Severance

Second Advisor

Dr. Damon Miller

Third Advisor

Dr. John Gesink

Abstract

This project consisted of designing and fabricating an electron vacuum tube guitar amplifier. Vacuum tubes have become obsolete in modern electronics due to their size, cost, and inefficiency. However, vacuum tubes are still commonly used in guitar amplifiers. Vacuums tube's nonlinear amplifying characteristics are ideal for distorting a signal. Hence vacuum tubes are used in distorting guitar amplifiers commonly used in rock music. While guitar amplifiers consist of many subsystems, the focus of this senior design report is the distorting preamplifier.

Imagine a guitar signal as a pure sine wave (although in reality the signal is a complicated series of harmonics). It is possible to enhance the signal with distortion. This can be done by biasing gain stages of the preamplifier in nonlinear regions of the vacuum tube's plate characteristic curves [2]. When distorting the signal, harmonics of the fundamental frequency are generated [2]. By "clipping" the sine wave in several gain stages, the sine wave will eventually start to look like a square wave [6]. The square wave will consist of the fundamental frequency and mostly odd harmonics. Some of the higher harmonics are filtered out to avoid a brittle and bright sounding amplifier.

The preamplifier designed in the Widman VT-2 consists of four gain stages that clip the guitar signal to add harmonics to the fundamental signal. The bandwidth of the amplifier is 80Hzto 3.5 kHz. All specifications were met except the actual stage gains were lower than the designed stage gains. The error was caused by a graphical method used to determine the designed stage gains. The second harmonic distortion could not be measured acurately due to limitations of the spectrum analyser. Thus the second harmonic distortion specification was omitted. The amplifier was completed in a timely manner and demonstrated at Western Michigan University's senior design conference on April 13th 2004.

Access Setting

Honors Thesis-Open Access

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