ALC Adjustment Procedure: By: Alan Applegate, K0BG
Contrary to popular belief, modern HF amplifiers do not require a full 100 watts to drive them to full output. The truth is, most tube amplifiers only need about 65 to 80 watts for full rated output, and some as little as 40 watts. Depending on their input circuitry, some solid state designs require just 25 watts! Regardless of their design (tube or solid state), over driving any amplifier produces a lot of negative results. Intermodulation distortion (IMD) is increased, tube (transistor) life is shortened, power supplies are over stressed, and the resulting splatter is anger inducing.
With the exception of most solid state mobile amplifiers, almost every modern amplifier has an ALC output. ALC, which stands for Automatic Level (Linearity) Control, provides an adjustable negative-going DC voltage which is fed back to the transceiver to control the amount of drive to the amplifier. If it is connected, and properly adjusted, ALC can limit or eliminate the aforementioned negatives. The problem is a lot amateurs never connect the ALC believing that the 100 watts PEP their transceiver delivers can’t overdrive their amplifiers. This is an erroneous notion. Even when it is connected, all too often it is not adjusted properly.
The proper level is easy to arrive at and requires just two pieces of test gear; a dummy load and a peak-reading wattmeter. The dummy load is necessary for obvious reasons. The need for a peak reading wattmeter is less obvious. Without getting into a deep technical discussion about power supply dynamics and other esoteric data, let’s just say it is best to adjust the ALC by transmitting via SSB while monitoring the PEP.
Before we start let’s go over what we’re going to do. We’re going to set the ALC just at the level where the power out is just below the point where the amplifier transverses into non linearity (the chart at right was made for a fictitious solid state amplifier, but clearly shows the transverse point). Then we’re going to increase the ALC slightly so we’re well into the linear portion of the amplifier’s power curve. So here we go.
First, set the transceiver to put out its standard 100 watts PEP (into a dummy load). Your speech patterns are important here, and rather than say “test, test”, recite your address, city and state.
Next, tune up your amplifier like you normally do, but with the ALC disconnected. WATCH YOUR DRIVE LEVEL! 30 to 50 watts is typically enough for tune up purposes. When you’re finished put the amp in standby.
Next, hook up the ALC and turn the amplifier’s ALC adjustment pot fully on (usually clockwise). Turn on the amplifier and transmit in SSB again with full drive. If the ALC is working as it should, the peak power out will be well below what it was without the ALC connected. Slowly decrease the ALC level while continuing to talk. At some level, the peak output power will stop increasing. The control should be turned back up until the power again drops. A good rule of thumb is 10% down from the peak power out. This keeps the amplifier well within its linear curve. If you’re not going to use any built in speech compression, this is where you want to stop (more on this in a minute).
The last item is to readjust the transceiver’s drive level. It should be reduced just to the point where the peak power starts to drop off. Next, turn the amplifier to standby and measure the peak power out of the transceiver. Increase it by about 10%. There’s a good reason for this. Using a little more ALC than is required, and with just a little more drive than is required, will provide a moderate amount of RF compression. If you intend to use speech compression, you’d be well advised to increase the ALC level an additional 10%. Remember, compression puts heavy demands on power supplies, no matter how well they are built. Whether excessive IMD is caused by overdrive or power supply dynamics, splatter is splatter!
The aforementioned is applicable to tube or solid state amplifiers alike. However, if you’re using an amplifier without an ALC output (SGC SG500, Ameritron ALS-500, etc.) then I suggest you read the article on my web site geared toward those units. Incidentally, most of the new series of solid state amplifiers hitting the market these days, require the ALC to be connected as it is an integral part of their built in self-protection scheme. What’s more, solid state amplifiers are much less forgiving of overdrive than most tube ones, and extra caution is warranted.
To close, allow me to add a few bits of wisdom. It is indeed possible to drive any amplifier with more power and get a little more power out of it. What you have to do is ask yourself, is it worth it? Not only does over-driving cause excessive IMD (splatter), it taxes every other part of your station, and shortens the lives of every part in it. And for what purpose? Do you really believe an extra 50 or 100 watts out will make that rare DX station suddenly hear you out of the pile up morass? I seriously doubt it. But I do know this. A good, clean signal with just a hint of RF compression is a prescription for increased contacts, to say nothing of the comments you’ll receive about having a clean, clear signal, and one free of trash and splatter. Moderation is the key. Think about it!
Alan Applegate, K0BG