Conductance.
7
’
(Two ranges
are provided,
the 15
mA/V
or
3 mA/V
positions being selected
according to
the
anticipated
result.)
Turn
knob A
clockwise
until
pointer
reaches
the XI mark.
Turn
knob
B
clockwise
until
pointer
returns
to
zero.
Press
meter
switch
and
read
mA/V
directly.
(Note a)
:
With a
few
valve types
with
low
emission it may
not
be possible
to reach
the
XI
mark
even
with knob A fully
clockwise.
In these cases
use the
X2 mark and
multiply
the
mA/V reading by
2.
(Note
b)
:
If
main
selector is moved to
Mutual
Conductance position before ABC
selectors
are
correctly
set, voltage
may
be
applied to the wrong electrodes.
This may
damage
the
valve or meter
or
blow
the
fuse.
(See
Section 1/3 for fuse
replace-
ment.)
It
is best to leave the
main
selector on Cathode Leakage whilst
ABC
selectors
are altered,
as this
keeps
the valve heater at
operating temperature.
(Note
c)
:
If the
meter
pointer
is seen to be
vibrating
this
is
due to unrectified A.C. across
the
meter
circuit and
is
probably
caused
by
an internal short in the valve under
test.
The
instrument should
be
switched
off
immediately
to
avoid damage
to the
meter,
or
the
main selector
can be
turned
back
to
Cathode Leakage.
2/8
GAS BUTTON
After the
meter switch has been used
for
measuring
Mutual
Conductance
as
described
in
the
last
section,
the valve can
be tested
for excessive grid
current
as follows,
leaving
selector
switches,
bias
control and anode
Screen
volts
control
in the same
positions.
Turn
knob
B in fully
clockwise
position.
Turn
knop
A
in
clockwise
position
up to
point
when
meter
pointer
reaches centre scale.
Then
press Gas
Button.
If
meter
pointer
goes
off
scale
in
either
direction the
valve
has excessive
gria current.
The
pointer
deflects
to the right
for positive grid
current,
to the
left for
negative
grid
current. The
latter may
be
due
to
grid
emission.
3. PRINCIPLES
OF OPERATION
(See
circuit
diagrams, centre
pages.)
3/1
RECTIFIERS
In
this
test
100
volts
A.C. is
applied to
the
anode
circuit
via a load
resistor
chosen
so that
the average
good
rectifier
will pass
a current
sufficient
to
give
a meter
reading
in
the
“
Good
”
sector.
The
meter
is
shunted to
about
22
mA.
full
scale.
Screen
voltage
is
not
applied
but
Grid
Volts
are
available
for
tests on
grid
controlled
rectifiers.
3/2
DIODES
This
test is
similar
to that
for
rectifiers except
that
the test
voltage
is
reduced
to 60
and
the
load
resistor
is
much
higher.
The shunt
is
removed from
the
meter
so that its
full
sensitivity
of
250
jxA.
is
available.
Neither
Screen nor
Grid
voltages
are
available.
3/3
FILAMENT
CONTINUITY
Approximately
£
volt
rectified
A.C.
is
applied
to the
filament
through
the
meter and a
limiting
resistor.
The
maximum
current
is
about 200
microamperes and
is
quite
safe
for
all valves
7
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