128
6.
HIGH VACUUM, LOW-VOLTAGE TUBES '
cathode surfaces, the number of electrons is appreciably
‘
plied.
6—10.
Pentodes.—A ve—electrode vacuum tube is called a
pentode. In one form, the additional electrode is called the mp
pressor grid and is located between the screen grid and the plate, as _ ;
in gure 6—16. The suppressor grid is at the potential of the
“cathode and hence serves to repel secondary electrons, driving
them back into the plate from which they were ej ected by primary
electrons.
result, the negative resistance region (ÂB of gure 6—14) is largely eliminated and the tube may be used for greater _ ,
power output With small input grid voltage. The plate resistance
_
of various pentodes in normal operation“ is moderately high
SUPPRESSOR
amo
..
ce
.
,
,
-
+
_
_
…… mr ….
’Ï
'
EB
*”
'
FIG. 6—16.
A pentode.
'
ranges from” 22,000 to. 2,000,000 ohms. The mutual conductance 13
'
‘
high and ranges from 400 to 6000 micromhos and the amplica—
1 tion constant ranges from 70 to 1500. These constants may bei
-
‘
measured With the, circuits of gures 6—6 ahd 6—11.
_
…
pentode, the third grid’s lead wire is brought
…
,
are then, obviously, many comb1natrons
*
the grid nearest the lainent can serve:
Î
la
and the one near€St
PlanfedbythenegaüveSpacecharge producedby electrons which
enr°uœ“"‘h‘î1313Œ,’”°concentratedbetween the screen gr1dand
theplaœAlthüughthe'‘lbôservesthe same purpose asapen—