If the design provides
insufficient output within the limits of dissipation
and distortion, or
the output is much greater than is
required, the
wrong tube type
probably has been selected and a redesign
should be
made using a different tube.
Fig.3-3 Load
lines for static and dynamic design.
The element voltages
used with the tube should be as small as possible,
consistent with the following
restrictions:
( 1 ) The screen
voltage should be larger than 20 to 50 volts to keep operation
out of the
area where tube behavior may be erratic.
( 2 ) The
minimum plate voltage, at zero control grid bias,
should be greater
than one-half the
screen voltage for class-B amplifiers, and three-quarters the
screen voltage for
voltage amplifiers, to keep the plate current relatively
independent of plate voltage.
( 3 ) The screen
voltage should be high enough to ensure that the
static
bias is not in the contact
bias area. ( A static bias more negative
than
-1 volt is normally required.
)*
As the screen
voltage is usually constant, it is designated as Ec2,
and is
represented on
the pentode G-Curve by a vertical line at the appropriate
voltage. Data
may be read along this load line in exactly the same
manner
as with the
more conventional triode load line.
*Note - The bias should be
sufficiently large that grid current will not flow
during the static or signal conditions. This will make certain
that
the bias is not altered by grid current flow - a flow causes increased
distortion.
15
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Copyright 2008 for Phyllis
K. Pullen, M.D.,
by Robert J. Legg