ALC Systems  Ameritron Amplifiers

Ameritron uses two basic ALC systems.

Old Style ALC

Older amplifiers used a simple RF cathode voltage detector. This system was common in virtually all brands and models of grounded grid amplifiers. The circuit was generally a simply half wave rectifier with the negative output voltage driving a potentiometer.

Other systems used a threshold voltage, generally using negative voltage taken from an internal bias supply. This voltage back-biased the ALC detector diode into cut off. As long as RF cathode voltage remained below that threshold,  the ALC rectifier diode would be inactive.

Some systems had an ALC control, some did not. None of the old-style cathode sample methods were particularly effective. The primary reason they were ineffective was cathode voltage does not provide a useful measure of linear operation.

Ameritron or Ameritron based amplifiers using old-style ALC include the AL80, AL80A, AL811 series, SB1000, AL1200, AL82, and AL1500. 

New Style ALC

Newer Ameritron units, those produced during or after the AL80B development period, use grid current to control the ALC system. Grid current is one of the best indicators of proper tube operation.
Amplifiers using this system include the AL80B mainframe, such as the AL572 and AL800 series. (The newer system is NOT used in the AL811, AL1200, or other older layouts.)

Ameritron solid state amplifiers, at least up through the ALS1306, sample reflected power and forward power. As in solid state transceivers, ALC is derived from the SWR directional coupler system.


The newer ALC systems are further broken down by dual control systems that include an adjustable "gain" or "voltage" limiter, and single control systems without a gain or voltage limiter control. 

Systems having the gain limiter include the AL80B and others in that AL80B style mainframe, along with the ALS1306 and 606.  The ALC voltage or gain controls are either side or internal access in newer solid state amplifiers, or rear panel mounted in tube amplifiers. This control is an ALC voltage gain, or maximum ALC voltage limiter. It determines the absolute maximum ALC voltage available for  the radio. This control is set once for a particular radio. This control will generally work with any transceiver, although transceiver manufacturers are all over the place with ALC requirements.

This sample text was extracted from the ALS-600 manual.

The Amplifier's back panel has an ALC limit control. This control is to limit the amount of ALC voltage sent to the transceiver. This is necessary due to there is not a standardize ALC voltage on transceivers. To set the ALC limit control, adjust the ALC set knob on front of the amplifier to 10 (fully clockwise). Apply enough drive into the amplifier until the amplifier's power meter reads about 400 to 500 watts. Note: The amplifier's power meter should never exceed 700 watts at anytime. Plug the ALC into the back of the amplifier (if not already done). Turn the ALC set knob (on front of amplifier) to 0 (counter-clockwise). Key amplifier with RF applied. Adjust the ALC limit control (on back of amplifier) until the ALC light on front of amplifier just comes on. No other adjustment needs to be done to the ALC limit control, unless a different transceiver is used. The only adjustments for ALC will be done on front of the amplifier. Follow the operator's manual for ALC adjustment.

I understand that may be confusing. Here is how I would phrase it for newer amplifiers. This does not completely apply to older amplifiers, or the AL811 series, that use systems similar to old-time amplifiers:



ALC is mostly a safety system for steady operation. ALC cannot prevent leading edge power surges, or "ALC spikes", from poorly designed radios.

The amplifier ALC system supplies negative voltage, sometimes with a slight hysteresis and hang time to prevent motor boating,  to an ALC jack. This ALC jack connects to a radio ALC input, and this is where any problem usually starts. There isn't any standardization in the radio manufacturing industry. Radio pin connections, ALC voltages, and time delays in ALC response are not standardized. Some radios take just 1-2 volts of ALC, while others might take several volts. Many ALC systems are slow, they respond many RF cycles after the power is too high.

ALC is used internally in radio to control the RF power level. Almost every radio controls output power exclusively by use of the radio's ALC, whether we realize that or not. There are many radios that spike well beyond rated power, and well beyond the power they are set at, on leading edges of transmissions. This spike is virtually impossible to see on a standard oscilloscope. Typical overshoot duration can be a few thousandths of a second, or faster. Worse, the unwanted power spikes only repeat after the ALC goes into resting state. Peak reading power meters, while generally better than all but specially triggered storage scopes, still require a very fast rise time to catch these peaks.  

Old amplifier ALC systems usually sample drive voltage at the amplifier tube cathode. Sampling input of cathode voltage is a poor method, even though it was an industry standard method of deriving ALC for many years. Newer Ameritron amplifier ALC systems either sample amplifier output power in solid state amplifiers, or amplifier grid current in tube amplifiers.

Most radios have an input port for external ALC. Most radios are not standard for voltage, the exact voltage being a whim of the particular model and manufacturer. Because the voltage is not standard, Ameritron added a one-time adjustment control in the more advanced amplifiers to set maximum ALC voltage, or ALC system "gain".


Connect the amplifier's ALC jack to the radio manufacturer's specified ALC input port. This is in the radio manual.

The front panel ALC control, generally labeled "ALC Set", is configured like any standard level, volume, or power control. Adjusting the knob clockwise permits higher power or increased grid current. Conversely, adjusting the ALC Set or level knob counterclockwise reduces the current or power before ALC appears. The user should always start new radio setup with the primary (front panel) ALC knob counterclockwise, ideally near fully counterclockwise.

If the amplifier is a tube type, it must be pre-tuned for the band the initial setup is on. Equipment must be in proper working order. The fixed gain or voltage control setup will hold for all other bands, so it is a one time adjustment for any radio model.

 Adjust the main ALC Set on the front panel fully CCW. This ensures active ALC voltage at low drive power. Ameritron has not standardized the ALC limit or ALC gain controls. The most simple way to verify control action is to measure ALC jack resistance to ground, but as far as I know the ALS1300 and newer use CW from adjustment view as minimum ALC voltage to the exciter, and other amplifiers might use CCW as maximum exciter ALC voltage or ALC gain. I find this counter intuitive, but it is what it is. You will quickly learn which way the control functions.

 Set the rear or side panel ALC voltage or ALC gain fully "up" to provide maximum ALC voltage. If you do this, and if you activate the radio, a normally functioning ALC system will hold the radio power either completely off, or hold power at a very low level. Once ALC is connected and the controls are preset, with front panel ALC off (counter clockwise), and with ALC gain fully on, the  following bullet steps must be followed. This is a one time setup for a given radio-amplifier combination. If any step fails, immediately stop ALC setup until the problem is located and corrected:

1.) Place the radio in any mode that will allow a steady carrier at full power. Place the amplifier in operate. Keep the radio power down to a low level.

2.) Advance the radio power control. The radio's power should abruptly limit at some very low power level, and the amplifier ALC LED should light. If the amplifier has an ALC voltage meter, it should register some ALC voltage.

3.) Adjust the amplifier rear panel or amplifier internal ALC limit (or ALC gain) control until about 10-20% power level is achieved. This is not critical. The power increase only has to be a noticeable amount of power increase. It does not have to be anywhere near full power.

4.) After setup, the front panel ALC control adjusts the maximum allowed amplifier power or grid current. The amplifier manual likely gives some guidelines on absolute maximum grid current or power output. You will see ALC light flashing or the ALC voltage increase if the ALC is limiting.

5.) DO NOT use 100% control power with ALC. Bring the radio mic gain or power controls up or down until the ALC is just active on peaks. The ALC's sole function is limiting excessive drive for safety. It does not replace the radio's power adjustments. ALC, despite opinion or advertising, is not a good speech processor. If you want speech processing or talk power, it should be in the system before the radio's SSB filter or ALC system is reached. The further toward the antenna processing occurs, especially after the SSB filter, the more problems occur. A good external audio processor before the microphone jack is actually much more effective than the best RF processing systems (some radio DSP processing systems excepted).

ALC is not a processor, it is a safety device!