Specifications
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 overdriving 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
http://www.k0bg.com/
Member Comments:
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ALC Adjustment Procedure
by AP2WF on February 8, 2005 Mail this to a friend!
A very comprehensive and nicely written Article. I really chewed every word of it, visualised and enjoyed it as long time
back I faced this problem and worked on ALC circuits. I would like to highlight another malfunction of the power supplies
(mostly home brewed power supplies) that comes very apparent with over driving is the tunable hum. I would further add
in favour that even with a relatively low drive to keep the amp output within limits and ALC to a level not reducing output
will act as a limit
er for safety. Ofcourse there will be no RF Compression. I have experienced it improves intelligibility and
overall signal quality. Lastly the ALC connection also needs to check and guard against stray pickups if necessary. DX
station will, however, recognise your signal further through your Audio quality.
....the obligatory FD pitch
by KA4KOE on February 8, 2005 Mail this to a friend!
...and of course, coupled with a fan dipole, you can't go wrong, dude!!!
Less IS more!
by KT8K on February 8, 2005 Mail this to a friend!
People need to realize that, in this case, less is definitely more.
Splatter puts enough energy outside the receiver passband that the signal strength at the receiver may actually go down,
not up, when the amplifier is overdriven. A clean signal means more signal IN the passband of the receiver, and you can
have a stronger signal with a lower output power if you don't splatter.
Distortion and splatter have been around a long time (witness the Old Man's diatribes on "rotten QRM"), but, here again,
less is more, i.e., less splatter equals more fun for everyone.
Good reception & 73 de kt8k - Tim
RE: Less IS more!
by W9WHE-II on February 8, 2005 Mail this to a friend!