[AMRadio] Properly tuning a linear amplifier on AM


Gary Schafer garyschafer at comcast.net
Mon Nov 6 21:37:02 EST 2006


I should have re-titled the subject for this.

Using a ranger or other AM transmitter with a linear amp requires the
amplifier be tuned up with full modulation on the ranger with reduced power
out of the ranger watching a scope at the output of the amplifier. It does
not have enough carrier at full power to properly drive the SB200 and tune
it properly.

The easiest/best way to find out how much drive the amplifier needs is to
use your rice box first. If you want to run 125 watts carrier out of the
SB200 on AM that means you need to tune the amplifier up for 500 watts or a
little more output with the load and plate tune controls on the amp peaked
for maximum output with a given amount of drive. That drive will be around
80 to 100 watts. Try 80 watts of carrier from the rice box and peak the
output of the amp for maximum output. If you can't quite get the 500 watts
out increase the drive slightly and re-peak the amplifier tune and load.

NOW reduce the drive until you have 125 watts out of the SB200 and MEASURE
the drive power required to produce the 125 watts out of the amplifier and
DO NOT TOUCH THE TUNING ON THE AMP!

The amount of drive that you measured is the amount of carrier you will need
from a ranger or other AM rig to drive the amplifier properly. No more no
less!


If you want to tune up the amp with the ranger driving the amp it needs to
be done with full modulation of the ranger and a scope connected to the
output of the amplifier. 
The first thing you need to do is find out how many peak volts on the scope
represents 500 watts out of the amplifier. This is done easier with a rice
box used as a driver first to establish the 500 watt level on the scope.
Once you have that you can connect the ranger as a driver.

Tune the amplifier up for maximum peak output, full modulation on the ranger
with a tone or voice, watching the scope. Adjust the carrier output of the
ranger so that you do not get much over 500 watts peak out of the amplifier.
Readjust the tune and load controls on the amplifier for maximum output
after each adjustment of the carrier out of the ranger. Always be sure that
you are near 100% modulation and not going over on the negative side.

Reducing the modulation to zero should yield a carrier right around 125
watts if you tuned everything right. That will allow the modulation peaks to
go to 500 watts peak and the amp will be loaded properly. If you don't get
125 watts or near out you haven't tuned up properly.

It is very difficult to tune this combination properly as drive is hard to
adjust easily. It is much easier to use the rice box first to establish the
parameters.

Watching the scope on the output of the amplifier should show the modulation
peak voltage double what the carrier voltage is on the scope. If it doesn't
you haven't tuned up properly.

73
Gary  K4FMX





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