Subject: Re: Synthesized Triode from Tetrode or Pentode

             By Dave Newkirk/ July 28, 1993

  Connecting  together the screen and control grid of  a  tetrode 
(or  the suppressor, screen and control grid of a  true  pentode) 
turns the tube into a high-mu triode. The resulting tube may  be, 
but  is  not necessarily, a *zero-bias*  triode.  True  zero-bias 
tubes  are designed to be so, and must be operated  according  to 
their  designer's  specs to work well with zero  bias.  Too  much 
plate  voltage  on  a zero-bias triode can  overcome  its  grid's 
control and make it draw too much plate current when idling. So a 
high-mu  triode synthesized by connecting together  the  multiple 
grids  of  a screen-grid tube *may or may not* be operated  as  a 
zero-bias    tube,   depending   on   its    resulting    control 
characteristics and how you power the tube.

  Relatedly, a screen-grid tube connected as a high-mu triode may 
not  necessarily exhibit the low-IMD characteristics of  a  good, 
designed-for-low-IMD  zero-bias  triode.  In  other  words,   you 
wouldn't  necessarily  synthesize a 3-400Z  by  high-mu-  triode-
connnecting a 4-400. (Even a 3-400Z is old hat. There are  newer, 
better   zero-bias  tubes,  where  *better*  equates  to   "lower 
intermodulation distortion.")

  I haven't mentioned so far what is probably obvious: Connecting 
a  well-screened screen-grid tube as a triode means that it's  no 
longer  a  screen-grid  tube.  An amplifier  built  with  a  tube 
connected  in  this way must be (A) neutralized if  the  tube  is 
operated grounded-cathode or (B) operated in grounded-grid so its 
connected-together grids can act as a screen or (C) configured as 
a  cathode  follower (which is largely impractical  if  you  want 
significant power output).

This sets us up for the next question:

  What would happen to a beam-power tube, such as a 6DQ5, if  its 
screen is connected to its control grid? It would act as a  high-
mu  triode. *But*--and this is very important--it  *couldn't*  be 
operated grounded-grid (without neutralization, that is)! This is 
so  because, like many beam power tubes, the 6DQ5's  beam-forming 
elements  are internally hardwired to its cathode. This  bypasses 
the shielding afforded by grounding its control and screen grids. 
Beam power tubes that cannot be operated in grounded-grid without 
neutralization  include  the 807, 1625, 6146 and  many  TV  sweep 
tubes. *Some* TV sweep tubes and RF beam power tubes (6KD6, 5763) 
bring their beam-forming elements out to a separate pin.  *These* 
can  be operated in grounded-grid without  neutralization;  you'd 
ground  their control grids, screens and beam-forming plates  for 
RF. You have the option of tying all the grids together for dc or 
feeding them separately. This gives on one last issue:




  There's one more important issue in using screen-grid tubes  as 
high-mu triodes, grounded-grid or not. By construction, a  tube's 
control  grid exerts more control over its electron  stream  than 
its  screen; it's closer to the cathode than the screen and  made 
of  finer  wire.  Further, screen-grid  tubes  are  intended  for 
applications in which the screen is *considerably more  positive* 
than  the control grid. *Because of this, severe  grid  overdrive 
will  likely  result* if the control grid and screen  are  merely 
tied  together (for dc *and RF) during  high-mu-triode  operation 
because the screen operates at the same potential as the grid and 
therefore  doesn't  draw them past and away from the grid  as  it 
does when it's significantly more positive than the grid. *GE Ham 
Notes*  (see  enclosed) goes into detail on this, as does  V.  S. 
Campbell  and  W. S. Skeen, "Grounded Screen-Grid  Operation  for 
Tetrodes,"  *QST*, Nov 1959, pp 37-39. (The introductory box  for 
this  article  goes like this: "A tetrode with control  grid  and 
screen tied together to form a high-mu triode for a grounded-grid 
circuit  makes a very simple arrangement. However, this  type  of 
operation   invariably   results   in   excessive    control-grid 
dissipation. This article shows a simple method of avoiding  this 
difficulty.")   It's  therefore  important  to  monitor   element 
currents  individually, at least until the circuit is  finalized, 
when operating a screen-grid tube as a high-mu triode.

  Calculating  the operating conditions for a tube  operating  in 
grounded-grid is relatively straightforward for true triodes,  as 
explained  in G. Grammer, "Input Impedance and Fed-Through  Power 
in Grounded-Grid Amplifiers," Technical Topics, *QST*, Dec  1958, 
pp    32-35,   184.   Calculating   these   parameters    *isn't* 
straightforward if you want to run a screen-grid tube in grounded 
grid  and  apply  normal  screen  voltage  to  the  screen  while 
grounding  it  for  RF; experiment will likely  be  necessary  to 
determine particulars in this case.
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