Heathkit SB-401 - AUDIO PREAMPLIFIER AND CATHODE FOLLOWER; VOX AMPLIFIER

Page 126
AUDIO PREAMPLIFIER AND CATHODE
FOLLOWER (Figure 6-1)
An audio signal applied through the Microphone
input is coupled by capacitor C101 to the grid of
Audio preamplifier tube V1A, Capacitor C101
limits the low frequency response of the input
signal, while capacitor C102 limits the highfre-
quency response, Any RF signals that may be
picked up by the microphone are bypassed to
ground by capacitor C102,
The amplified signal from V1A is coupled through
capacitor C105 and Mic Level control Ri to the
cathode follower tube, VIB. V1B is used to
match the low impedance input of the balanced
modulator circuit,
The push-to-talk (PTT) and voice operated (VOX)
transmitter circuits are disabled by the Mode
switch in the CW and Tune positions,
VOX AMPLIFIER (Figure 6-2)
When the Mode switch is in either the LSB or
USB positions, the amplified audio signal from
V1iA is also applied through VOX GAIN control
R131 and resistor R132 to the VOX amplifier
tube, V12A, Any distortion that may occur by
grid rectification in V12A is limited by re-
sistor R132,
MIC
LSB
TO RELAY
AMPLIFIER
CATHODE
CONTROL
Gag i ratvuKir
In the CW mode of operation, a signal (approxi-
mately 1000 Hz) from sidetone amplifier V12C
is coupled through capacitor C131 to VOX ampli-
fier V12A, Depending on the mode of operation,
either the audio signal from V1A or the sidetone
signal from V12C is amplified by V12A andthen
is coupled by C133 to diode D105, Diode D105
rectifies this signal to produce a positive DC
voltage,
RELAY AND ANTI-VOX AMPLIFIERS
(Figure 6-3)
The positive DC voltage from diode D105 is
coupled directly to the grid of relay ampli-
fier V12B,
For VOX operation, V12B has its bias level set
by a voltage divider network, consisting of re-
sistors R136 and R135, This fixed value is at
such a level that a small amount of positive
voltage applied to the grid of V12B will cause
current to flow through V12B and close relay
RL1, The closing of RL1 causes the circuit
changes required to place the Transmitter on
the air,
VOX delay is determined by the time constant
of the resistance-capacitance combination of
resistor R149, capacitor C141, and VOX Delay
control R148, The delay time is made variable
vB
2 6EA8
CATHODE
FOLLOWER
TO
1500 [BALANCED
MODUL ATOR
BLK-RED
~D
Figure 6-1