Heathkit TT-1 - Page 8

GRID CURRENT TEST CIRCUIT
The
grid
current
test
circuit
will
determine
if,
for
any
reason,
the
tube
under
test
is
drawing
|
—
7
po]
grid
current.
The
most
common
reasons
for
grid
Llanes aus gaa: (9\ laakaga fram ocrid
current
TLOW
are.
(1)
Gas,
(a)
wanage
PO
piu
to
another
element;
(3)
grid
contamination
(sec-
|
MSY
ato
es
Se
active
material
from
the
cathode
is
deposited
on
ondary
emission),
a
condition
existing
when
the
|
oie|
SUPPLY
the
grid,
thus
causing
it
to
emit
electrons.
The
grid
of
the
tube
under
test
is
connected
to
the
grid
current
testing
circuit,
as
shown
in
the
Diagram.
The
12AV6
tube
is
biased
beyond
cutoff
so
that
no
plate
current
flows,
hence
no
meter
reading
is
indicated,
Grid
current
flowing
it
above
cutoff.
Since
the
bias
is
now
above
the
through
the
tube
under
test
will
develop
a
volt-
cutoff
value,
plate
current
will
flow
through
the
age
across
the
18
megohm
resistor.
This
volt-
12AV6
and
the
meter,
thus
indicating
the
grid
age
raises
the
DC
bias
on
the
12AV6,
bringing
current
flow
in
the
tube
under
test.
CONSTANT
CURRENT
HEATER
SUPPLY
The
constant
current
heater
supply
used
in
the
TT_1
onerates
from
the
115
volt
filament
tap.
BSTM™L
VPVLarwwn
44
wees
via
Cyl
ikl
This
tap
is
connected
to
the
tube
heater
through
at
aanacitar
4.3
UF
a
ballast
capacitor.
Since
the
voltage
drop
across
7.1,
10.7,0OR
14.3
U
the
tube
is
small
in
comparison
to
the
voltage
Coal
pmoon
er]
drop
across
the
capacitor,
the
capacitor
imped.
;
TEST
:
iia
;
(o9.10,mty,
ete)}
|
TY
UNDER
ance
will
be
the
current
determining
element
in
\
en
}
|
ataw ta scason
this
series
circuit.
The
tube
heater
is,
essen-
y
Juut
4.7,6.3V,
etc)
€
tially,
a
resistive
impedance.
This
means
that
¢
fot
Al
HH
Hee,
7,
6.3V,
ete.
il
the
impedance
of
the
two
elements
(resistive
and
capacitive)
are
approximately
90
degrees
out
of
phase
with
each
other.
This
fact,
in
addition
to
|
j
the
magnitude
of
each
impedance
relative
to
the
£=VOLTAGE
ACROSS
CAPACITOR
E,=HEATER
VOLTAGE
other,
enables
the
capacitor
toact
as
an
efficient
current
regulating
unit.
The
capacity
value
used
for
each
current
supply
is:
300 ma 7.1 pid
ABN ma
“EUW £2100
10.7 pfd
600 ma 14.3 pfd
Page 8