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The following tables list tubes based on power conductance, in their order of power-
handling ability. The triode table lists the tubes in ascending order of plate
conductance; the Pentode table (including tetrodes) lists the tubes in ascending order
of screen-to-plate transconductance. The conductance values measure the
approximate amount of current which may be passed by the tube for a given value of
screen or plate voltage, indicating the amount of power which can be developed for a
given dissipation.Since good design is obtained by plotting the load contour roughly parallel to the
constant-dissipation contour in the neighborhood of zero bias, it has been found
convenient to list the approximate values of conductance on the zero-bias contour at
the specified dissipation. In addition to these data, the approximate values of
transconductance at the same point are included, as are the nominal power
dissipations for the significant electrodes - plate, or screen and plate, as required.
These latter data are convenient in that they give the user an idea of the types of
applications for which the tube may be used, for example, audio or video amplifiers,
etc.The data may be accumulated at any set of conditions which will give an
indication of the behavior of the tube in its area of high dissipation, as in any case an
adjustment factor is required for numerical design.* For this reason, the zero bias
condition at three-quarters peak dissipation has been chosen for triodes; a correction
factor or gamma of two is convenient in adjusting the dissipation levels. With
pentodes, the zero bias condition with the plate dissipation one-half the peak has
been chosen to allow an additional margin for the variation of screen and plate
dissipations. The value of the gamma factor again is near two.* Pullen, K. A., "Guides to Tube Selection," Electronic Design, Nov. 1, 1956.
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Copyright 2008 for Phyllis K. Pullen, M.D.,
by Robert J. Legg