Made for high gain amplifiers.
Bob Hutchinson, N5CNN  

What is ALC?
I have never used ALC and never really knew anyone that did. I understood that it was an automatic load or level control to keep from over driving an amplifier back when exciters had tubes and power output in the two hundred watt arena. A feed back voltage from the amplifier kept the drive from being excessive to the amplifier. I guess this was important a while back, but it must not be important now, huh?

But, let's see, the amplifiers today are capable of two or three times the power of the old days utilizing high gain external anode tubes, 3CX800, 4CX800, 4CX1600, GU84B, GU78B. Wow! These babes have a hell-of-a-lota gain and need very little drive. These amplifiers have over-current crowbar circuits to shut down the machine in case of excessive grid or screen current caused by too much drive and/or when mis-tuned.

Recently, after finishing the eleventh amplifier review in 12 months, I came to realize that the crowbar over-current circuits were necessary because of the tendency to overdrive these high gain amplifiers.

Negative grid or screen current
Yes, some of these fine amplifiers are so efficient that the screen grid current stays negative until the amplifier is outputting maybe 40% of its total power. They almost don't need any drive. This is like keeping a three year old thoroughbred race horse from running, it just wants to run. Well, when tuning these thoroughbred high gain amplifiers the crowbar or over-current protection circuit trips when the grid or screen gets to about 125ma. It is very easy to trip when tuning for high power.

The makers are protecting your amplifier and their profits with the grid or screen trip circuits and I understand that, but I decided to learn more about ALC and how it really effects things from an operational point of view.

Ameritron AL-800H
This amplifier has a good grid crowbar circuit. When trying to tune for high power it can be tripped many times. Of course the numbers can be recorded for repeating later, but even if touching up the tune the circuit can be tripped. I read the manual about ALC and set it just like the book says and, guess what? I can no longer trip the crowbar circuit. Nope, can't do it. The ALC circuit limits the grid current to what ever you set it to. I used a maximum of 60ma.

Now tune for max. smoke
But, high power can't be achieved with just 60ma of grid, right? Huh? Oh, but it can! Not only can it be achieved, but it can be achieved with the simplest of tuning techniques - The Maximum Smoke tuning method, just like tuning an amplifier with a pair of 3-500Z Eimac tubes. Twist knobs, tweak for maximum power. Since the 800H ALC circuit prohibits over current for the grid, the whole crowbar circuit trip inconvenience thing is no longer. I can increase exciter drive to 100 watts and tune for maximum power, into the bench dummy load, quick and easy, no tripping. Legal limit CW power of 1,500 watts output requires only 45 watts from the exciter and 25ma of grid current.

This first ALC experiment opened my eyes and produced surprising results. For these amplifiers with sensitive crowbar grid and screen current protections circuits, the good ALC circuit can be a blessing. I'm converted for the AL-800H. From now on I will test the ALC in every amplifier that has it.

OK, Now the techno-nerd stuff
If you really want more detail, here's some stuff I copied from the Internet.


Most amplifiers will require a drive level between 30 and 75 Watts to achieve full output power. If external ALC is not used, the output of the transmitter must be reduced using the power output control to avoid damage to the tubes.

Applying External ALC.

External ALC voltage may be applied to control the output power of the combined transceiver/amplifier system. The "sense" of this voltage must be positive; i.e., the voltage increases as the square root of the output power. Further, the voltage value applied depends on the drive requirements of the external amplifier. The external detector's output should be adjustable for precise setting of drive power. Transmitter output power is related to ALC voltage by the following equation:

V alc ~ SQRT ( P output / 25 W )

E.g., for a required drive level of 50 Watts, drive cutback begins at around 1.4 Volts.

Very little filtering should be used at the external detector's output. Enough to eliminate RF is adequate, as further peak detection and loop lag compensation are performed inside the transceiver.

External ALC must be coupled to the transceiver through a diode so that no current is drawn from the transceiver pin when the external amplifier is off. This prevents the internal ALC from being affected.

Automatic Drive Level Circuit(ALC)
The ALC circuit in EMTRON DX-3 amplifier is not of a conventional design like in all other amplifiers. The DX-3's ALC Circuit operates only for one reason - to prevent overdrive and keep the signal clean.  The ALC in DX-3 senses any slight overdrive (50 ma of grid current), warns you, and if you don't do anything about it, will cut you off for 2 seconds. The 2 second cut-off is then repeated until the drive level is reduced. This ALC circuit also produces a 1V - 10V negative going voltage, proportional to the grid current, but only when overdrive is present. The EMTRON DX-3 can be operated without connecting your transceiver to the ALC output of the DX-3 and you are still guaranteed never to overdrive it. However, connecting the ALC will prevent the overdrive condition from occurring in the first place.


I will add information here about ALC from time-to-time. Please send any information you believe we should publish.

Bob Hutchinson, N5CNN
President and Founder
Wireless Industry Association
713 467-0077

If you would like to publish an article here contact Bob Hutchinson, N5CNN.



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