Taylor 45C - 3;5 Cathode Leakage; 3;6 Mutual Conductance

3/4
ELEMENT SHORTS
The
same arrangement
is
used as on
Filament
Continuity except
that the test
points
are
applied to
all
electrodes
in turn by rotating the selectors. Should
there
be a short
it
must be
shown on
at least one
position.
(With
a
few valves
spurious results may be
obtained. Such
are
those
having
heater
centre
taps or internal
connections on pins
which
would
otherwise
be
11
blank.”)
3/5
CATHODE
LEAKAGE
In this test
all
the
elements except cathode and
heater
are
joined
together
and a
con-
denser is connected
between
heater
and cathode. An
A.C. voltage
is
applied
so that the
valve
acts as a
rectifier
and generates about 50
volts
D.C.
between
heater and
cathode.
The
meter
then
reads any
leakage current and this
can
be interpreted in
terms of
megohms
leakage
on
the
lowest
meter
scale.
3/6
MUTUAL
CONDUCTANCE
In this
test
a
choice
of
anode, screen and grid voltages is
provided.
These
are A.C.
except
that
the
grid
voltage
is
prevented
from going
positive
by a
small
rectifier.
The
transformer windings
are
phased so that the anode and
screen
voltages are
in
phase
whilst
the
grid voltage is
out of
phase
.
When
the anode
and
screen
voltages are
positive
,
therefore
,
the grid voltage is
negative
and
meter
current
passes.
When
the anode and
screen
voltages
are
negative, the
grid
voltage
is zero
and no meter current
passes.
The inertia of
the
meter
causes
it
to
read
steadily
although
the
current
through
it
is
intermittent.
Knob A
comprises
an
adjustable
meter
shunt. When
it is
fully anti -clockwise
the
meter
is shorted and
reads
zero.
Knob B
controls a
bucking
voltage derived from
a separate transformer
winding and
small rectifier.
When it
is
fully
anti -clockwise the extra
bucking
voltage is
zero.
On
the Mutual
Conductance
test
the
screen voltage is
applied between
cathode and
screen, the grid
voltage
between
grid and the lower end
of
a cathode resistor, and
the anode
voltage between the
anode
and the lower end of the
cathode
resistor. The cathode
resistor
therefore carries
anode
current
only. The meter reads
anode
current and is
initially shunted
to
zero.
As
soon as
the
valve
warms
up
pulsating
anode current
flows
and
the
meter reads
when
knob
A
is turned
clockwise,
The XI mark is at
3/5
of
full
scale.
Next
knob
B is turned
so that sufficient
bucking
voltage
is
applied
to
the
meter
to
reduce
its reading to
zero.
Pressing
the meter
switch
then shorts
out the cathode resistor
and
the resultant
anode
current
increase is
shown
on
the
meter
on
the
Mutual Conductance
scale.
The
cathode
resistor
has
a value
of
555
ohms on
the
3 mA
/V
range and
111
ohms
on
the 15 mA/V
range.
The theory
of
operation
is
as follows
:
—
Suppose
the
meter
is
shunted
so that it reads
x mA.
full
scale. Since
the
XI
mark
is at
3/5
full
scale
the
anode
current is 3x/5
mA.
=
3x/5000
amps.
On
the 3
mA/V
range
the cathode
resistor
is
555
ohms
so
the
cathode bias
voltage
=
555
x
3x/5000
=
x/3 volts.
Suppose
the
valve
on
test
has
a mutual
conductance
of
3
mA/V
under
the
specified test
conditions.
Then
its
anode
current
increase
when
the
cathode resistor
is
shorted is
3
x
x/3
=
x mA.
The
meter
therefore
reads full
scale
corresponding
to
3
mA/V.
Similarly
for
lower
values of
mutual
conductance
on this
range
.
Operation
on
the
15
mA/V
range is the
same
except that
the cathode
resistor is
111
ohms
and the
meter
is
read
on
the
0-15
mA/V scale.
10