11
10.3 USING YOUR DX-2
While tuning and adjusting the amplifier, a CW mode is normally used. When these operations are finished, you can change to the
operating mode you wish to use and start operating.
10.3.1 CW / RTTY and all digital modes
In CW, the setting up of the amplifier is similar to that of the tuning mode. After adjusting the input power level to the amplifier,
according to the desired output level, and tuning the plate and load capacitors, you can simply start operating by applying the same
carrier level as during tuning. If the amplifier is set to deliver full output power, or close to, then screen current will be present. As in
the case of tuning, the best operation is achieved when there is a certain amount of screen current flowing, corresponding to two or
three red LED's turning on in the Ig2 display.
10.3.2 Voice modes (SSB)
When a voice modulation is applied to the amplifier, the drive level is not precisely defined, as in the case of tuning with continuous
carrier. If there is screen current flowing, or an input overdrive, they will be indicated only at peak of modulation, by the respective
indicators flashing. You will have to adjust the microphone gain, or the RF drive, or both, in an attempt to achieve input driving
conditions similar to those you have used while tuning. Occasional flashing of the overdrive indicator is acceptable. If the over-driving
is severe, a momentary cut-off will occur.
10.3.3 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
minimising 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 1000W or less), correct tuning, no screen current and no input overdrive indication, you will always
have your DX-2 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 begins to turn 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
already a small amount of distortion, created by a grid current of about 100 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 400 microamperes of grid current.
See the Appendix 2 and the Troubleshooting section for oscilloscope displays showing examples of a two tone generator driving the
DX-2 in linear mode, and an overdrive situation, when a DX-2 with the overdrive protection disabled
is being overdriven by the two-
tone generator.
The Ig2 indicator should have two 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-2 running idle for a few minutes, to cool the tube, then switch the power off.
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