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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