Heathkit SB-401 - AUTOMATIC LEVEL CONTROL

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DRIVER (Figure 6-12)
Driver stage V9 functions as a voltage ampli-
fier to increase the low level heterodyne mixer
output signal to a level sufficient to drive the
final amplifier tubes, Since the grid and plate
are tuned to the same frequency, the tube socket
is shielded to prevent self-oscillation,
The plate of V9 is tuned by capacitor C4C (Driver
Tune) with the proper tuned circuit (L15 through
L19) selected by the Band switch, On the 3,5
megahertz band, capacitor C4D is added to
increase the tuning capacitance,
Neutralizing energy is coupled from the final
amplifier plates through capacitor C24 and is
adjusted by capacitor C23 to neutralize the
final amplifier grid-to-plate capacitance,
The output signal of the driver stage is coupled
through capacitor C9 to the grids of the final
amplifiers, In this instance, at a frequency of
3,895 megahertz,
FINAL AMPLIFIERS (Figure 6-13)
Final amplifiers V10 and V11 are parallel con-
nected and function as class AB1 linear ampli-
fiers, A fixed operating bias of approximately
-50 volts is maintained on the grids of these
tubes, limiting zero signal plate current to 50
milliamperes, Under Standby conditions, the B+
voltage is removed from the screen grids to
reduce the plate current to zero, thus cutting
the tubes off, RF driving voltage is developed
across RFC6,
Peak driving voltage is continuously variable
in CW, and is set by Level control R16, In
the LSB and USB Modes, the peak driving
voltage is controlled by Microphone Level con-
trol R1 and the limiting action of the ALC, Plate
voltage is shunt fed through RFC9 and by-
passed for RF by feedthrough capacitor C67,
The output of V10 and V11 is coupled through capacitor
C25 to the pi-section plate tank. RF parasitic chokes RFC7
and RFC8 in the plate leads eliminate any tendency toward
parasitic oscillation. The proper tap of the plate tank coils
(L20 and L21) is selected by the Band switch. Capacitors
C27, C28, C31, and C32 (depending on the Band switch
setting), with capacitors C29A, B, and C (Final Loading),
[— $e | HBATHEIT
provide a 50 Q impedance point on each band. Loading is
variable from 40 Q to approximately 150 Q on any band.
Plate tuning resonance points are marked on the front panel
of the Transmitter.
AUTOMATIC LEVEL CONTROL (Figure 6-14)
When operating in the LSB or USB modes, the
ALC circuit is connected to the grid of iso-
lation amplifier V3, A normal fixed bias
condition exists at the grid of V3, providing
maximum gain, When the final amplifier grids
begin to draw current, a voltage occurs at the
junction of resistor R33 and capacitor C308,
This voltage follows the audio voltage peaks and
is coupled through capacitor C308 to diodes
D301 and D302, A portion of this fluctuating
voltage is rectified by the diodes and appears
as a bias voltage at the junction of resistors
R303 and R304,
This bias voltage is fed to the grid of V3,
effectively limiting its output and thereby re-
ducing the drive available to the final ampli-
fiers,
The resistance and capacitance network con-
sisting of resistors R303 and R304, and capac-
itors C306 and C307 is afast-attack, slow-decay
network which effectively prevents overdrive
in all but extreme settings of the Level control,
The ALC Adjust control is used to provide a
zero meter setting when the Meter switch is
in the ALC position,
TO ISOLATION
AMPLIFIER
THROUGH RELAY
RLI
TO
BIAS ADJUST
CONTROL
LSB
D301 6308
TO LINEAR
AMPLIFIER
INPUT
2
Figure 6-14