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Since 3/23/99

  This month's issue tackles ultra-linear mode, but without an output transformer. The design principle is applied to line stages, headphone amplifiers, and OTL power amplifiers.
  In November, 1951 issue of Audio Engineering, by David Hafler and Herbert Keroes outlined a new way of using tetrodes and pentodes, which they labeled "ultra-linear." This article is out of copyright and is reproduced in our new section Classic Magazine Articles. Expect to see new (old) articles with up coming issues of the Tube CAD Journal.
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                                 Editor

   Devised in 1951 by David Hafler and Herbert Keroes, the ultra-linear mode was created to cure the ills of both triode and pentode amplifiers. Each amplifier type had its adherents. The triode amplifier boasted a smooth, low distortion sound and a low output impedance, but yielded only limited watts. The pentode amplifier produced many more watts and a more aggressive, exciting sound, but at the cost of greater distortion and much higher output impedance. A middle ground was sought. If a new tube could be designed, it would have to yield the average of the pentode and the triode sound, argued Hafler and Keroes. But rather than create a new tube, their solution was to create a new transformer that would allow the pentode to be used in a new way.

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