Heathkit SB-401 - METERING CIRCUITS

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BIAS SUPPLY
GRID
Figure 6-16
The
AC
voltage
obtained
from
the
low
voltage
secondary
winding
(brown
lead)
is
approximately
125
volts
(rms),
A
half-wave
rectifier
circuit
consisting
of
diode
D2,
capacitors
C404
and
C405,
and
resistor
R412,
removes
the
60
Hz
ripple
from
the
bias
voltage,
With
suitable
divider
networks
(see
the
Schematic
Diagram),
variable
bias
(-45
to
-70
volts)
and
various
fixed
bias
levels
are
obtained,
All tube filaments receive voltage from a sep-
arate power transformer filament winding (yel-
low, green, and yellow-green leads),
The primary winding of the power transformer
is fused and a TV type quick-disconnect line
cord is used,
METERING CIRCUITS
Grid (Figure 6-16)
Current in the grid circuits of final amplifiers
V10 and V11 is shunted through the meter, which
has a1 milliampere full-scale, 100 8% move-
ment, This provides a 0-1 ma range in this
position of the Meter switch,
Plate (Figure 6-17)
Plate current is indicated by measuring the
amount of current in the cathode circuit of the
final amplifiers, For 500 milliamperes of
cathode current in the cathode circuit (com-
bined resistance-capacitance cathode circuits),
there will be 1 milliampere of current in the
meter,
Page 137
* REPRESENT COMBINED ~
RESISTANCE AND CAPACITANCE
Figure 6-17
Automatic Level Control (Figure 6-18)
The meter is balanced for no-signal conditions
(zero current) in the cathode circuit of isolation
amplifier V3 by ALC Adjust control R120, When
a sideband signal is transmitted and ALC (nega-
tive) voltage is applied to the grid of V3, the tube
draws less current, causing the cathode to be-
come less positive and the screen to become
more positive, The voltage difference developed
across the meter causes a current through the
meter, which indicates ALC action inthe Trans-
mitter,
APPLIED
ALC
CH6
ALC ADJUST
Ol
Figure 6-18