AMERITRON AL-80B, Инструкция по эксплуатации

Страница: 8 / 38

AL-80B Instruction Manual
4
Circuit Descriptions
ALC
The AL-80B is one of the few amplifiers using a "true" automatic level control (ALC) circuit. In
this amplifier, the ALC actually samples grid current and power supply loading to determine the
ALC voltage. The front panel "ALC SET" knob adjusts the level of grid current where ALC action
begins. ALC action begins softly over a range of a few milliamperes to minimize distortion. This
soft-touch ALC is an Ameritron exclusive.
The ALC circuit is located on the METER BOARD P/N 50-0080-3 (behind the "MULTIMETER"
switch). IC301 (pins 9 and 10) compares the voltage drop (caused by grid current) across R106 in
the main power supply board. ALC voltage is derived by comparing the grid shunt voltage to the
voltage from the power supply.
The output of IC1 (pin 8) will go negative if the grid shunt voltage exceeds the voltage from divider
R311 and R2 (chassis mounted control), or if the supply voltage sags from excessive line voltage
drop. Pin 8 is followed by current buffer Q303 that sources the negative ALC voltage to the ALC
output jack.
This amplifier includes an ALC Limit control on the rear panel. This control reduces overshoot and
eliminates "motor-boating" (a slow rhythmic fluctuation) in the transmitted signal. These problems
are caused by excessive transceiver ALC attack time delay. This control sets the maximum voltage
level available from the ALC circuit. Adjustment details are given on page 15.
Dynamic Bias
Conventional bias circuits force high power linear amplifiers to dissipate hundreds of watts during
low or no signal periods. This creates needless heat, since virtually no dissipation is required unless
the amplifier is being driven with large signal levels.
The AL-80B contains an exclusive bias circuit that reduces the idling (quiescent) current very close
to the tube's cut-off region. The power amplifier tube in the AL-80B has a full resting period of
very low dissipation between dots and dashes on CW and between words on SSB. The lower idling
current reduces component temperature on both CW and SSB. If only a few milliwatts of RF power
are applied to the amplifier, the quiescent current will increase. Linearity remains excellent with
this circuit because the tube can remain biased for class AB operation without unnecessary standing
dissipation.
The dynamic bias circuit is located on the Power Supply Board P/N 50-0080-1. Diodes D101 and
D102 rectify a small sample of the RF drive voltage. This voltage is applied to the base of dc switch
Q101. If Q101's base is driven with a few microamperes of current from the RF sampling circuit,
Q101's collector will pull the base of PNP transistor Q102 low. This turns dc switch Q102 on.
When Q102 is on, zener diode D103 is connected between the collector and base of Q103.
D103 sets the operating bias. This zener applies forward bias to Q103's base whenever the collector
voltage of Q103 exceeds the voltage of D103. This forward bias will turn Q103 on harder and will
reduce the collector voltage. If the collector voltage is less than the breakdown voltage of D103,
Q103 will move towards cut-off and the collector voltage will increase. Q103 functions as a current
buffer for zener diode D103.