《Articles on Valve Amplifiers》.pdf

NEW PUSH-PULL TUBE AMPLIFIERS MENNO VAN DER VEEN, AES member it. buro Vanderveen, The Netherlands fax: xx3l-38-4533-178, e-mail. mennovdv@noord.bar1.nl 1 INTRODUCTION With landmark design work such as the author presents here, the future of the tube amps, which reached their peak around the middle of this century, appears quite strong as we head into a new millennium. The first tube amplifiers were constructed with triodes, directly coupled to high impedance loudspeakers, with interstage transformers, but without output transformers. The demand for greater output power led to paralleled triodes and triodes in push-pull configurations with balanced output transformers. Based on the rejection of amplified radio frequencies at the control grid, the screen grid was invented, resulting in tetrodes tubes and later pentode tubes. The rapidly advancing tube technology made possible the high-power triodes and pentodes that resulted in amplifiers with greater output power. In the 1940s great inventions were made by Hafler and Keroes, Peter Walker and McIntosh, and many oth- ers, resulting in high advanced amplifiers with huge output power. Those days saw the invention of screen-grid feedback (ultralinear configuration) and cathode feedback (as still used in the famous amplifiers of Audio Research). McIntosh especially deserves a place of honor with his famous unity- coupled amplifiers, in which he used a combination of cathode- and screen grid feedback. STANDARDIZED DESIGNS After the 50s, the growth of new ideas in tube amplifier circuitry declined, and standardized designs became generally accepted, such as the Lafayette 2 x EL84 (6BQ8) push-pull power amplifiers in pentode mode with a rather large amount of overall negative feedback and a 12W power bandwidth from 20