A linear amplifier sits between the transceiver and the antenna and raises the radio's output, typically from 100 W or less to several hundred watts, up to the legal limit. "Linear" means the output is a faithful, larger copy of the input, which SSB and CW need. (An amplifier that is not linear distorts and splatters on SSB.)
What the extra power buys
Power is judged in decibels, and one S unit is 6 dB, a factor of 4 in power (on a meter that is calibrated that way). So the gain is smaller than the watts suggest.
Power on a logarithmic scale: equal distances are equal ratios (equal decibels).
Power ratio10.0 ×
Gain10.0 dB
From
To
Gain
At the far end
100 W
500 W
7.0 dB
about 1.2 S units
100 W
1000 W
10 dB
about 1.7 S units
100 W
1500 W
11.8 dB
about 2.0 S units
1000 W
1500 W
1.8 dB
about 0.3 S unit
An amplifier is a real help on a weak path, but a better antenna is usually the cheaper improvement. Going from 1000 W to 1500 W is barely noticeable.
Tubes and transistors
Vacuum tube
Solid state
Supply voltage
thousands of volts, lethal
typically about 50 V DC
Tuning
tune and load controls
usually none: band-switched filters
Warm-up
needed
instant
Tolerance of mismatch
fairly tolerant
needs fast protection circuits
Size and weight
large and heavy
compact
Either type needs an output low-pass filter to keep harmonics off the air, a fan, and a relay to switch the antenna between radio and amplifier. In a tube amplifier TUNE resonates the output for a dip in plate current, and LOAD sets the power:
TUNE: find the plate-current dip (output peaks). LOAD: raise output without crossing the plate-current limit.
Plate currentAt the dip
Connecting it properly
ALC protects against too much drive; the key-line delay lets the relay finish switching.
Drive: set the radio's power to what the manual requires. Too much drive overloads the amplifier and creates distortion that wrecks your signal on the band.
ALC and keying lines: the amplifier sends a level back to the radio's ALC to limit drive, and a keying line makes the amplifier's relay switch before RF arrives, so the relay is never hot-switched.
Test into a dummy load first, then check the antenna's SWR; a poorly matched antenna stresses the amplifier. See Power output and SWR protection.
Power it from a circuit rated for the load; high-power amplifiers often need their own 240 V circuit. See Wire gauge and fusing.
Safety
RF burns: at high power an antenna, tuner or open connector can burn skin on touch. Keep them out of reach while transmitting.
Ground the chassis to the station ground with a short, wide strap and use a grounded outlet.
More power raises RF exposure, so recheck your distances; see RF exposure.
In the US, external amplifiers that can operate below 144 MHz generally must be FCC-certified for amateur use before they are sold, though an amateur may build one for personal use. See Part 97 for the details.