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THE TUBE CURVES

The curves in the following section represent a compilation of tube
data organized to facilitate circuit design. They have been obtained in
several ways, among them:

     1- Replotting of manufacturers' data
     2 - Measurement, followed by coordination with published data
     3 - A combination of  1 and 2

At the same time that they have been prepared, an effort has been made
to evaluate the importance of the various parameters in practical design
and to prepare the curves in a way which takes best advantage of the
important factors. In this way the curves themselves tend to help the
user become an experienced designer - they rapidly show him the range
of characteristics available in a tube as well as showing him a great deal
about the relative linearity of the device.

Interestingly enough, the data which prove to be most critical are
the small-signal, or conductance data. The static contours can vary in
position over an appreciable range without introducing serious accuracy
problems, whereas considerable difficulty may be encountered if the
conductance contours are incorrectly positioned. As a result, the in-
cluded curves, because of the conductance information, both speed up
the design process and make it more accurate.

Because of the ways in which the curves have been obtained, the
reader may find that some variations exist between manufacturers' data
and the curves. These differences are most pronounced in the static
contours, and usually indicate either poor control of g_p or G_m2 on the
tubes themselves, particularly from producer to producer, or indicate
that possibly the tube is being used outside the normally controlled
area. For these reasons, and because new tubes are being issued at fre-
quent intervals, it is planned to reissue this Manual as need arises so
that the user can be kept up to date.

As the dissipation of a tube is such an important factor, the plotting
of contours corresponding to critical values of dissipation on the G-
Curves can be useful. For this reason, contours marking the positions
of the half-rated and full-rated plate dissipations are marked by red
curves, a broken curve indicating half-of-rated, and a solid curve rep-
resenting full-rated dissipation. This method of marking the power
contours is used to prevent confusion with the bias and conductance
contours. Note that where two power ratings are carried on one sheet
(6C4-12AU7, for example) the lower one is plotted.

A convenient technique in using the power contours is to locate the
point corresponding to either 1/2 or 3/4 of rated dissipation on the zero-
bias contour, calling the coordinates of this point (E_bp, I_bp). Then, for
a single-tube amplifier, the static operating point may be defined by
the equations:,

 E_bp = 0.6 E_b Z_L = E_b / I_bp = 5 E_bp / 3 I_bp
    I_b = 0.6 I_bp

The static supply voltage is 5/3 of the zero-bias voltage, and the plate

                                                                                                   39.

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Copyright 2008 for Phyllis K. Pullen, M.D.,
by Robert J. Legg