1 5
4-4. Tuning
Tuning is possible only in operate mode, so you may need to press the OPER button in order
to illuminate the LED above it (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
antenna impedance only, and not on the amplifier tuning. If the antenna impedance is
different from 50-Ohm pure resistive (nominal), the REFLECTED POWER reading will always
indicate reflected power presence (even at a perfect tuning). Proper tuning will allow
operation 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 I500W and
REFLECTED 250W, the real OUTPUT POWER will be equal to their difference - I250W (into a
2.4:1 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 < 300W".
Matching is assured for loads with VSWR up to 3:1. Nevertheless, for some loads and bands
matching is possible at even higher VSWR. The maximum usable forward power at VSWR 3:1
is 1200W with 300W reflected. For higher than 300W reflected power you'll get the **
REFLECTED POWER ** soft-fault protection trip. For instance, this can happen at full-scale
I500W forward and 375W reflected power, into an antenna having a VSWR of 3:1 or worse.
C A U T I O N
Using a feeder of coaxial cable at VSWR > 3:1 on HF, and particularly on
the 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 amplifier tuning when you change the band or between CW and SSB segments within
the same band. Update it each time when you change the antenna, even in the same band, as
well as periodically, even though you may have not changed band or antenna. Be on the alert
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 and eventually an increased VSWR.
N O T E
If you use more than one antenna per band, it is necessary that you select the proper
antenna BEFORE the next step. Retune after selecting a different antenna for the same
band, since the impedances may differ substantially (unless their VSWR is excellent,
i.e. below I.2:I for both).
C A U T I O N
Do not switch the BAND switch knob while transmitting with the amplifier! Hot
switching (while transmitting) will eventually destroy the band switch, not covered by
the warranty!
C A U T I O N
When tuning, do not apply continuous drive longer than 3 minutes and after that
pause I-2 minutes for tube cooling.
Summary of Contents for 1500
Page 4: ...4 ...