AL-800H Instruction Manual
4
ALC
The AL-800H 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. The 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 50-0800H-3 (behind the
"MULTIMETER"
switch). IC301 (pin 9) samples 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 (pin 10).
Pin 8 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.
Dynamic Bias
Conventional bias circuits allow 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-800H 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-800H has a full resting period of very low
dissipation between dots and dashes on CW and between words on SSB. The lower idling current
dramatically reduces component temperatures 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's conduction angle is never reduced below class AB.
The dynamic bias circuit is located on the Power Supply Board P/N 50-0800-H1. 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 is, in effect, a current buffer for zener diode
D103.
