AMERITRON AL-572, Instruction Manual

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Ameritron AL-572 Instruction Manual
8
CIRCUIT DESCRIPTIONS
ALC
The AL-572 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 pin 8 output of IC1 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 motorboating (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 begin on page 17.
D
YNAMIC
B
IAS
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-572 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-572 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