Circuit Analysis of a Legendary Tube Amplifier

Book excerpt: This book moves beyond general principles of tube amplifier design to carry out an intense examination of one of the most famous circuits of the rock era. The author begins with the 5F6-A's relatively simple triode preamps (bypassed cathode resistor, unbypassed cathode resistor, and cathode follower) and then progresses through the mathematically challenging tone stack, long-tailed-pair phase splitter, and push-pull power amp. Every formula for every tube is derived in all its gory detail, including voltage gains, input and output impedances, frequency responses, dynamic power supply loads, and interactions with the rest of the system. The author's methods include the classic load lines and composite characteristic curves of Frederick Terman and the Radiotron Handbook as well as more modern techniques like linear regression and the Discrete Fourier Transform. Special attention is paid to quantifying the push-pull amplifier's nonlinear response and to analyzing power supply voltage sag as it reacts over time.The Bassman 5F6-A circuit has inspired guitar amplifier designs for over four decades, so sharpen your pencil, fire up your calculator, and find out what makes this amp rock!Richard Kuehnel is a member of the Circuits and Systems Society of the Institute of Electrical and Electronics Engineers.