New-York tribune., 1922-05-14
Grid Function
In 'ube Set
De- scribed
I. Mo, III portant Elcmen. 4.
- Of ac. I'M Tube, as 1t
Co, 20 the Action OF
De, r and Ampliflcr
Grid ak Also Important
Ju,por,an. Grid Condenser
and Leak Are Made of
the Simplest Materials

ri, o.. ,mnortant element in the
cs.uum tube is the grid, and as the
,..,,u tube itself is the most lmpor-
| ..rt instrument. IN receiving set the
I'D naturally assumes major impor-
/ ,.ne. IN the whole receiving apparatus
~ ,bi. reason intend in this weeks
1 -.,.,. 70 describe tho function Of the
/ .,;d and the apparatus that Is neces-
! s.rv for use In conjunction with it.
N the first place, the grid is to the
/ x,coum tube what the trigger is to the
| run. +. actually controls the action of
| n. tube and releases the storcd-up
| .n.rrv OF the "B" batteries through
the operation of the tube. In order
tia. this function should operate sue
cessfully, it Is necessary that the cir-
cult connected to the grid should con-
tain a grid condenser and a grid leak.
The grid condenser Is the smallest
condenser used in radio work. 1t can
consist Of two plates one square inch
| In area, separated by single dielcctric,
! giving it a capacity of .0002o MID.
No Mystery in Apparatus
From the foregoing it will be seen
how simple the grid condenser really
is. There is nothing more mysterious
about the grid leak. 1t is simply a
very high resistance shunted across
the condenser itself. This grid leak,
for instance, can be constructed with
an ordinary soft lead pencil by SIM
ply making mark upon piece of
fiber and then connecting terminal
at each end OF the mark. The graphite
from the pencil will form a pathway
between the two terminals of q very
high resistance, the value Of which, Of
course, will depend upon the quality
Of the graphite in the pencil. As an
average amount, this resistance Will
be in the neighborhood of l megohm;
or. in other words, 100000 ohms.
It IS necessary, Of course, that this
line be continuous throughout, and in
the event Of home made grid leak of
This character. it is possible to expert
ment until the correct value Of re-
sistance is obtained by adding TO the
thickness or width of the pencil line.
until the correct value is attained. Of
course, correspondingly, if too much is
already on, it can be erased, care be-
ing taken to see that the continuity
Of the line is not disturbed
Such is the description of the ap-
paratus used in conjunction with the
grid. You may quite naturally ask,
why is the grid leak necessary, es-
pecially when it consists of such SIM
question is best given by describing
waves is received on the aerial.
Grid Has Negative Charge
The grid in the vacuum tube is
charged to the negative potential, which
is necessary for the successful
functionng OF the vacuum tube. Now,
as each OF these wave trains passes over
the aerial, and then induces current
in the grid circuit, the grid has an &C
emulation of negative charges piled
upon 1t In rapid succession and some
means must be taken to permit these
to leak off.
Without the grid leak the only way
IN which this can be done is through
She tube itself, or over the edges of
the condenser, but, OF course, in this
case the leakage is not rapid enough
20 return the valve TO its normal oper-
alton. The addition of the grid leak
takes care of this situation and permits
thoSa Sci oh
easily and at the same time prevent
any accumulation that would reduce
the efficiency of the tube or choke it
up.
OF course. this grid leak and grid
condenser are only used in connection
with the vacuum tube where the tube
itself Is being operated as detector
OF incoming electric oscillations. When
vacuum tube is being employed as
an amplifier the tube naturally is per-
forming an entirely different function
and, OF course, operates in an entirely
different manner. Consequently, the
grid leak and grid condenser are not
necessary.
Standard Equipment Is Trend
The modern practice IN the design of
radio receiving equipment is to pro-
NuN7 SIAiiudio dppdiatuS LQaL Will
operate at certain average perform.
ancc under every and all conditions
and the consequence is that all manu-
featured grid leaks ere of standa I'd
value OF resistance. For the expert.
! enter. however, better results can
'bc obtained with receiving set where
ali of the parts are adjustable, and this
Of course applies TO the grid leak and
the grid condenser. 1t is perfectly
feasible to get good results, absolutely
good results, with fixed grid condenser
and fixed grid leak: but Oh the othe,
iand absolute maximum results car
be obtained by having both Of these in.
struments variable, the drawback being
cnat It adds two more adjustments
necessary to make in series of cir
cuts that are ali eady largely adjust
able. For the experimenter, Of course
!Als is practically no drawback, but
Tor the average radio fan it is serious
handicap and the trend of all the
manufacturers is 10 devise a receiving
apparatus that will have the minimum
number of adjustments.