For linear amps, the general method is to first determine the highest
possible power output by loading the amp on CW for maximum. Lets say the
wattmeter shows 1200 watts for your SB-220. For AM, this 1200 watts would
represent the PEP conditions under modulation, so the drive should be
reduced until the AM carrier is 1/4 of the maximum power or 300 watts.
Generally the efficiency will be between 30 and 40 percent. Since the male
voice has higher peaks than a sine wave, the carrier should be downgraded
even more if you expect to modulate over 100 percent on positive peaks.
Since a controlled carrier driver has a much lower idling carrier, as
compared to standard screen or plate modulation, tuning for 300 watts of
carrier would produce serious flattopping and distortion, therefore the
SB-220 must be tuned under the highest PEP conditions, not to exceed the
1200 watts PEP as used in our example. A scope is mandatory for tuning a
linear under AM conditions, especially a controlled carrier driver. The
linear amplifier efficiency will be higher with controlled carrier, but
because of the high ratio of PEP to carrier under modulation, controlled
carrier systems generally do not sound quite as good as straight screen
modulation because of the high receiver AGC swings.
Without a scope it's almost a given that you won't have a real clean signal
and you will probably flattop, unless you seriously degrade the output
The long drawn out discussions of tuning up AM linears, with controlled
carrier drivers, are usually by those who do not have a scope and try
their favorite "sure thing" method which varies for as many guys as enter
> This question comes up regularly on the AM forums. In the latest query the
> guy wants to use a Heath DX 60. Using a SB-220 as the example how should
> it be run for maximum clean signal?
> What is the amp efficiency at that point?
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