< Previous - Table of Contents - Next > line and the zero plate current line, and Ibm is the plate current at maxi-
mum power dissipation ( when plate voltage eb = Ebz / 2 ). The dissipa-
tion at any point is:Pp = eb* ib
As the life and reliability of a tube depend on how conservatively it
is operated, a compromise may be required between life and dissipation.
In general, for a-c amplifiers, the plate dissipation calculated at the static
bias condition should not exceed the desired fraction ( e.g., 1/2 to 2/3 )
of the rated dissipation of the tube. For d-c amplifiers it is the maximum
plate dissipation which should not exceed the desired fraction of the
tube rating.Example 6. Calculate the maximum plate dissipations for Cases I
and II for Example 3.
.
Case I: RL = 25,000 ohms Ebb = Ebz = 250 volts Ppm = 0.625 watt
Case II: RL = 50,000 ohms Ebb = Ebz = 250 volts Ppm = 0.313 watt.As the plate dissipation rating for the 6J5 is 2.5 watts, the tube is oper-
ating well within the limits of its rating.
DYNAMIC LOAD LINES
A dynamic rather than a static load line may be required for calculating
characteristics in the design of an amplifier if appreciable loading is
coupled onto the amplifier. If the external load is Rg, then the dynamic
load impedance ( Fig. 3-3 ) is given by the equation:RLD = RL* Rg / ( RL + Rg )
The static load line may be plotted through Ebb in the usual fashion,
a static operating point A chosen along it, and the dynamic load line
then passed through the point A at a slope corresponding to the value
of RLD. The values of the small-signal parameters are then read from
RLD and amplifications calculated in the normal way.
THE PENTODE R-C AMPLIFIER
The following are the steps normally used in the design of pentode R-C
amplifiers using G-Curves:1 - Select a trial tube ( see page 10 ).
2 - Select the bias and screen voltage and determine plate voltage,
supply voltage, and load resistance.3 - Calculate small-signal amplifications at several values of bias.
4 - Calculate output voltage and distortion.
5 - Calculate plate and screen dissipations.
14
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Copyright 2008 for Phyllis K. Pullen, M.D.,
by Robert J. Legg