124
6.
HIGH VACUUM, LOWVOLTAGE TUBES
'
6—7.
T etrodes.—The tetrode is a four—electrode vacuum tube.
_
One type contains a second grid, in addition to the usual lament,
plate and control grid. This space charge grid is mounted close
to the lament in the region of large space charge, as indicated
;
'
gure 6—12. As shown, this grid is positively charged by the potential drop in R and serves to reduce the space charge, thus permit_
ting a comparatively large current to ow through the control
.
'
grid to the anode P.
Conversely,
,
_
CG- P
'
With the reduced space charge, the
plate current can be maintained at a
'
normal value by only a small voltage
_
……
n
_
\ smc:
r0
on the plate.
_
.
R
CËÊÊ>Œ
The operating conditions for one
_
'
Ec
Es
tube of this type, the FP 54, are:
-
+-l+
-
l+——°
E, = +6 volts,
=
+4 volts,
F
Fm. 6—12.
A space—charge-gri d
Ec = -—4 volts, lament voltage = 2.5, «
tetrode.
,u = 1; r,, = 40,000 ohms and g… =
25 micromhos. The tube is intended
as, a current amplier and is not useful for voltage gain. It has
been constructed in such a manner as to have an unusually high
-
resistance between the control grid and the lament; as high
as
1016 ohms.
For example, insulator leakage is reduced by
‘
bringing the control grid lead wire out of the tube at the top,
'
away
from all other leads, and by using quartz supports. Also,
photoelectrie emission from the grid, due to light from the
_
lament, is reduced to about 10“15 ampere by the use of a low
temperature lamehtof the thoriated type. The positive
ions
by the
which are of the order of 10—11 ampere,
.
back to the lament by the positive space—charge gr1d
'
_
and,
do not reach the control grid, thus reducing the grid current'
a.d.îçréasig the input resistance.
'
?
space charge type of tetrode can be used to measure small
…Î ?
even to as loW' a value as 10*._17 ampere5 (see also section
!
17—16,m the chapter on Small Current Technique). First, the
changes
current are recorded When known voltages are
Inputterminals
,
the unknown current HS
resisçacègrm' (see gure 6—12), Which is ofhlgh
andkn0WValüe106 —101°ohms fT—'ähÏe,change in plate current
6.
HIGH VACUUM, LOWVOLTAGE TUBES
'
6—7.
T etrodes.—The tetrode is a four—electrode vacuum tube.
_
One type contains a second grid, in addition to the usual lament,
plate and control grid. This space charge grid is mounted close
to the lament in the region of large space charge, as indicated
;
'
gure 6—12. As shown, this grid is positively charged by the potential drop in R and serves to reduce the space charge, thus permit_
ting a comparatively large current to ow through the control
.
'
grid to the anode P.
Conversely,
,
_
CG- P
'
With the reduced space charge, the
plate current can be maintained at a
'
normal value by only a small voltage
_
……
n
_
\ smc:
r0
on the plate.
_
.
R
CËÊÊ>Œ
The operating conditions for one
_
'
Ec
Es
tube of this type, the FP 54, are:
-
+-l+
-
l+——°
E, = +6 volts,
=
+4 volts,
F
Fm. 6—12.
A space—charge-gri d
Ec = -—4 volts, lament voltage = 2.5, «
tetrode.
,u = 1; r,, = 40,000 ohms and g… =
25 micromhos. The tube is intended
as, a current amplier and is not useful for voltage gain. It has
been constructed in such a manner as to have an unusually high
-
resistance between the control grid and the lament; as high
as
1016 ohms.
For example, insulator leakage is reduced by
‘
bringing the control grid lead wire out of the tube at the top,
'
away
from all other leads, and by using quartz supports. Also,
photoelectrie emission from the grid, due to light from the
_
lament, is reduced to about 10“15 ampere by the use of a low
temperature lamehtof the thoriated type. The positive
ions
by the
which are of the order of 10—11 ampere,
.
back to the lament by the positive space—charge gr1d
'
_
and,
do not reach the control grid, thus reducing the grid current'
a.d.îçréasig the input resistance.
'
?
space charge type of tetrode can be used to measure small
…Î ?
even to as loW' a value as 10*._17 ampere5 (see also section
!
17—16,m the chapter on Small Current Technique). First, the
changes
current are recorded When known voltages are
Inputterminals
,
the unknown current HS
resisçacègrm' (see gure 6—12), Which is ofhlgh
andkn0WValüe106 —101°ohms fT—'ähÏe,change in plate current