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                                         Chapter 1
 
 

THE CURVES

TRIODE DATA

 Triode  G-Curves  are prepared from large scale  sets  of  standard  plate
characteristic curves. This is done by adding  contours  which  define  the
positions of contours for several values of the transconductance and  plate
conductance  parameters.  The  method   chosen   for   distinguishing   the
various contours is to use solid lines for the bias contour curves,  dashed
lines for the transconductance contours, and  dotted  lines  for  the  plate
conductance contours.  The  designation  of  the  specific  values  of  the
parameters applying to  any  contour  is  indicated  by  a  number  placed
beside the contour.  The grid bias values are expressed in  volts,  and  the
small-signal parameter values are expressed in micromhos.

 A value of plate conductance is also  tabulated  along  with  the  corres-
ponding transconductance,  in   the   Table   of   Triode   Power-Handling
Abilities.  These values are determined along the  zero-bias  contour  at  a
point corresponding, with most tubes, to 75%  of  rated  dissipation.  As
a result, the selection of a tube for a modified design is accomplished
by finding one having either  twice  the  nominal  g_p,  or  half,  depending
on whether the design was  overloaded  or  overly  conservative  initially.
Usually two or three trials will lead to a satisfactory tube.
 

Example 1.  Find  the  transconductance  and  the  plate  conductance  for
the 6J5 tube with e_b = 100 volts and e_c = -2 volts.

 On the  G-Curve  for  the  6J5  tube,  examination  of  the  area  around
e_b = 100 volts and e_c = -2 volts  shows  the  following:
                                                                             .
    e_c          g_m          i_b               e_c       g_p           i_b  .
     -         3500       8 ma            -        175        9.7 ma
    -2           -          6.8 ma         -2          -         6.8 ma
     -         3000       5.8 ma          -        150        6.3 ma
                                                                             .

Interpolating with these data gives an approximate  g_m  of  3200  micromhos
and a g_p of 153 micromhos.

  The accuracy required of  the  interpolation  is  fortunately  very  low.
Because of normal manufacturing  variations,  the  positions  of  the  con-
tours may vary from tube to tube by as  much  as  20%.  As  a  result,  a
linear approximation actually  gives results that  are  as  close  as  can  be
justified by both the data and the devices themselves.
 
 

                                                                                                            1

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