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ALC Adjustment Procedure

Alan Applegate (K0BG) on February 8, 2005

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ALC Adjustment Procedure:

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

Summary of content (12 pages)

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ALC Adjustment Procedure Alan Applegate (K0BG) on February 8, 2005 View comments about this article! ALC Adjustment Procedure: 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.
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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.
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Bravesemo! This article should be COMPULSORY reading for hams getting their 1st Amp. Wayyyy too many hams fall into that old contester credo....."all knobs to the right"! W9WHE RE: Less IS more! by KE1MB on February 8, 2005 Mail this to a friend! Less is more... Try to explane to some people that a watt meter takes into account all of the power comming from your xmiter. And not all of that power is comming from the fundemental either.
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by LNXAUTHOR on February 8, 2005 - who uses ALC anyway? Mail this to a friend! :-) - seriously, tks for the article... RE: ALC Adjustment Procedure by NO9E on February 8, 2005 Overdriving an amplifier by a cheap transceiver INCREASES readability. Here it is why. Mail this to a friend! Cheap radios have AF processor, which is not terribly useful. A radio with speech processor off has an average power of only 5% of peak power, a miserable 5W out of 100W.
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RE: ALC Adjustment Procedure by N6AJR on February 8, 2005 Mail this to a friend! Belive it or not I never use ALC. I set my amplifiers manually. I just sold my 811h, and am using a IC-2KL for most of my stuff I run them behind the IC 756 most of the time and / or the 746 some of the time, but I usually run the 746 to the Alpha. I ran the ts 2000 through the 811H.
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AM RE: ALC Adjustment Procedure by WA2JJH on February 9, 2005 Mail this to a friend! notone-am, your system sounds very good to me. I may employ a similar process to my quad mrf-150 amp. One can get uo to 800W out of 4 mrf-150's(push pull combinerless) You are using a comparator type adjustment. You seem to have only 2 states. Amplify with no adjustment.(no ALC OR ANYTYPE OF CORRECTIVE SIGNAL FEED BACK.
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This adjustment procedure is good, but even if it's followed to the letter, their is a "variable" that is very dificult to account for; Individual transmitter ALC attack and decay time charactristics. To my knowledge, I don't believe any OEM's are implementing a standardized ALC Attack/Decay specification. Instead, I have read and experienced that the opposite is true - and in some cases, the Attack time difference was as much as 10:1! No wonder there is so much inconsistency in ALC use.
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the biblical code and ther types of psuedocryptology! Be warned M&M lyrics played backwards and translated to 1000 B.C. arameic are where it is at!!!!!!! RE: ...ALC and the ZEN of CALBERTS flatuosis error by K4JSR on February 10, 2005 Mike-A-Zoid, Mail this to a friend! Cheesy dolphins give our lives porpoise. Sorry you could not join all of us at Ruth's Crisp Mouse Steak House for some victuals and potable liquids last Friday. We had a ball.
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procedure in the PW1 manual, but is similar in concept to the method described here. I have my PW1 set as described in this article and run my exciter at about the 25 watts level which gives me a full 1 KW out. Keeping an eye on the PW1's ALC meter is an easy way to ensure that I have the exciter's output power set correctly. I look for roughly a 1/4 scale deflection on voice peaks. On the air reports have been very complimentary.
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by WA1RNE on February 11, 2005 Mail this to a friend! K2WH, any idea what happened?? I was trying to create another post and my computer hung, but I don't think it had anything to do with these posts.... ALC Adjustment Procedure by KB8ELK on February 11, 2005 Mail this to a friend! I think it is important to point out to the casual observer that all amplifiers are not created equal, and have their own tuning chracteristics and requirements.
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**But tranceiver ALC control characteristics are not standardized; each model has its own unique gain and dynamic characteristics. (Internal ALC in tranceivers works well because the ALC source and controlled stages are designed together as a system.) ALC feedback from Alpha 87A to tranceiver normally isn't necessary because the power output of most modern tranceivers can be quite accurately controlled ....and because the Alpha 87A's grid current limiting system easily absorbs occasional drive peaks.
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