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4-2. Changing Operate and Standby Modes
With the Auto-Operate function disabled (see S.5-2), the OPER button changes two modes alternatively.
When Auto-Operate is ON, the amplifier will be maintained to operate mode by default, unless you
use the OPER button manually to go to standby (then Auto-Operate is suppressed temporarily). Pressing
OPER again would restore the Auto-Operate.
4-3. Tuning
Tuning is possible only in operate mode, so press the OPER button in order to illuminate its LED (unless
Auto-Operate is active).
a) Preliminary information.
Tuning the amplifier is a procedure of matching the impedance of the currently used antenna to the
optimum tube load resistance. This will ensure maximum plate efficiency and RF gain at nominal output
power, with minimum IMD at that.
Please note, that the REFLECTED POWER readings and the measured VSWR depend on the load
impedance only, and not on the amplifier tuning. If the load impedance is different from 50-Ohm pure
resistive (nominal), the REFLECTED POWER reading will always indicate reflected power presence
(even at excellent tuning). The proper tuning will allow you to operate at greater power without distortion
or danger to the amplifier.
Note also that the real OUTPUT POWER in the load is equal to the difference between the FORWARD-
and REFLECTED- readings. For instance, at a reading of FORWARD 1200W, and of REFLECTED 200W,
the real OUTPUT POWER is 1000W (into a 2.6:1 VSWR load). At very high VSWR (no antenna or badly
mismatched antenna), the FORWARD and REFLECTED readings will be almost equal, while the real
OUTPUT POWER (the difference between them) will be practically zero.
The amplifier can operate safely if the following rule is obeyed: REFLECTED POWER < 500W.
Matching is assured for loads with VSWR up to 3:1. Nevertheless, for some loads and bands matching is
possible at even higher VSWR. For instance, youll get a
** REFLECTED POWER ** soft-fault protection trip at full-scale (1200W) forward, with more than
500W reflected (700W output power), when antenna VSWR exceeds 4.5:1.
C A U T I O N
Using a feeder of coaxial cable at VSWR > 3:1 on HF, and particularly on the 10 and
6 meters bands, is not recommended. At such high values of VSWR, the high voltages,
high currents, and heat associated with line losses, risk to permanently damage your
coaxial cable or antenna switch.
Update tuning periodically, even if you have not changed band or antenna, in particular when a significant
change in the environment occurs (snow, ice, newly appeared or removed massive objects, alien wires
nearby etc.) that would cause significant changes in the antenna impedance.
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