From: dnewkirk@arrl.org (Dave Newkirk)
Newsgroups: local.tech
Subject: Solid TIS Info on Using Screen-Grid Tubes as Triodes
Date: 30 Jul 93 11:37:23 est
Organization: American Radio Relay League
 
*Notes on Operating Screen-Grid Power Tubes Grounded-Grid, as
Triodes or Otherwise* (David Newkirk, WJ1Z, July 1993)
 
(Asked to comment on a possible response to a TIS letter, I 
collated the results of my reading on this subject into the 
following. It can serve as general answer to further TIS traffic 
on this topic.)
 
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. (For example, too
much plate voltage on a zero-bias tube 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* generally 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.
 
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; the control grid closer to the cathode than the
screen and often 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 RF *and dc*. In this
situation, the screen operates at the same dc potential as the
grid and therefore doesn't draw electrons past and away from the
grid as it does when it's significantly more positive than the
grid. *This is especially so if zero-bias operation is
attempted.* With no bias to overcome, applying almost any drive
at all immediately drives the control grid positive and makes it
draw current.
 
November/December 1959 *GE Ham News* carries 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 text: "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 the tube's
element currents individually, at least until the circuit is
finalized, certainly when operating a screen-grid tube as a
high-mu triode, and preferably when the tube is operated as a
grounded-grids tetrode.
 
Calculating the operating conditions for a tube operating in
grounded-grid is relatively straightforward for 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 as a grounded-grids tetrode (with normal screen
voltage applied). Experiment will likely be necessary to
determine particulars in this case.
 
Regards/WJ1Z
 
David Newkirk, Senior Asst Tech Editor  |  voice: 203-666-1541 X280
American Radio Relay League             |  fax: 203-665-7531
225 Main St, Newington CT 06111 USA     |  net: dnewkirk@arrl.org
 

