21
10.2.3 Hints on good linearity and efficiency
Operating with good efficiency
: If you are watching the plate current indicator, you must be aware that the total
power the amplifier is drawing from the mains is proportional to the plate current. Power that is not sent to
antenna is dissipated by the tube. As explained in the Tuning section, the best tuning achieves a maximum
power output at the same time with a minimum plate current, drastically minimizing the tube dissipation. Be
aware that at a low power level, slightly different tuning is required than is for full power.
Achieving the best linearity
: The amplifier must be tuned and operated all the time in the linear mode. This is
important because a non-linear operation will generate harmonics which will disturb RF communications in
other bands and frequencies.
We must distinguish two situations:
Less than full power.
With less then full power (say 3000W or less), correct tuning and no input overdrive indication, you will
always
have your DX-3SP operating with a good linearity and clean signal.
Full power operation.
At full power, there are two things that must be watched to see when you reach the limit of linear operation: the
screen current, Ig2, and the grid current, indicated by the "Over-drive" LED.
In this case a good linearity is maintained up to the moment when the screen current is approaching the full
scale value of the Ig2 indicator and it starts flashing, or when the overdrive indicator fully turns on.
A high screen current, indicated by 4 LED's on in the Ig2 display, will create a moderate level of distortion.
Even a small amount of grid current will cause severe distortion. When the "Over-drive" indicator just begins to
turn on, there is a very small amount of distortion, created by a grid current of about 150 microamperes.
With this indicator flashing, you are at the limit of the RF carrier clipping. This is probably more acceptable in a
voice mode of operation. In CW or digital modes, the best is to slightly reduce the drive, just below this level. A
severe level of over-driving is not possible anyway, due to the cut-off protection. The cut-off will happen at
about 200 microamperes of grid current. See
Appendix 2: Waveforms
- and
Section 12: Troubleshooting
for
oscilloscope displays showing examples of a two tone generator driving the DX-3SP in linear mode.
The Ig2 indicator should have three to four LED's flashing ON at peak of modulation. This (combined with a
correct plate tuning), indicates an optimum loading, good efficiency and good linearity. A lower screen current,
due to excessive loading, will reduce the output power, with an equal amount of extra power dissipated by the
tube. A higher screen current, making all four LED's in the Ig2 flashing, will increase the distortion.
11 SWITCHING THE POWER OFF
When you have finished using the amplifier and you want to switch it off, the following procedure is
recommended: Stop transmitting. Put the switch on STBY. Leave the DX-3SP running idle for a few minutes, to
cool the tubes, and then switch the power off.
12 TROUBLESHOOTING - If something goes wrong
12.1
No power
When you switch on a DX-3SP, it takes about 3 - 5 seconds for the lights on the front panel display to come on.
This is due to the "soft start" circuit. If nothing happens after turning the power on, switch off at the front panel,
then check the following:
make sure the power point has power
make sure the power plug is fully pushed in into the power outlet
