Altec 1566A:Vacuum Tube Preamp & Direct Box Project

First appeared in the January’97 EQ   Original ©1996 by Eddie Ciletti  Most recent update June 2004

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Part-1: The Sound of Tubes Exploring the Altec 1566     Figure-1: Three overlayed scope traces show clean (green) and "soft" distortion (violet and red)

Part-2: The Sound of Tubes Modern and NOS Eval

There are all sorts of tube circuits, some of which actually deliver The Sound people associate with Vacuum Tubes.  Did you know, for example, that asymmetrical distortion generates even-order harmonics while symmetrical distortion produces odd-order harmonics?  Neither vacuum tubes nor transistors are exclusively responsible for either type of distortion, it’s really about circuit design.  Figure 1 shows distortion variations in red and violet.  Notice how the violet wave follows the green wave perfectly during its positive excursion but falls short of the mark during the negative swing. The red wave is more symmetrical, but does not "clip" like an overdriven IC op-amp. This is what I like to call "The Sonic Airbag," or Electric Crayon effect.  Op-amps are linear-clean right up until the point of clipping, some discrete transistor and vacuum tube designs have a non-linear window between clean and clipped.  More...