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Pride DX300 CHAPTER 3
3.0 Circuit Description:
The DX-300 power amplifier uses a 4CX-250B Tetrode ceramic/metal construction, and utilizes forced air-
cooling. It is in a grounded cathode, driven grid configuration that provides very high power gain, high peak
power output, and excellent linearity and harmonic suppression. The use of tuned input circuits and broadband
matching transformers permits low drive operation combined with additional rejection of exciter harmonics. Class
AB operation of the tube permits high peak powers with low distortion products and harmonics. The amplifier has
a tune function that restricts the power gain of the amplifier for easy, safe tune-ups. The use of a single tube in
the design avoids the problem of matching tubes and it also simplifies repairs. RF actuated switching eliminates
the need for switching contacts on the exciter, which are not found on some of the lower powered rigs on the
market. An internal receiver preamp provides approximately 10db of gain on the higher frequencies.
Power supplies:
3.1 High Voltage Supply PCB:
The high voltage supply consists of the plate supply transformer located on the top of the chassis, and the two
rectifier filter boards below chassis directly under the plate transformer. A bridge rectifier circuit is used, which
allows good utilization of the transformer. Both circuit boards are the same, with the exception of the jumpers
(JP1 and JP2), which determine whether the filter capacitors are on the high side of the series filter string, or on
the low side. The low side board also contains two 6.8ohm, 2W metering resistors (R24, R25).
Each board contains one half of the bridge rectifier circuit, with appropriate jumpers between the boards. The
filter string consists of six 100µf electrolytic capacitors (C11, C12, C13, C22, C23, C24) in series, with voltage
equalizing resistors (R9, R10, R21, R22, R23) across each one. The time constant of the filter when no current is
being drawn is approximately 20 seconds, so that supply will be largely discharged 60 seconds after turn off
(ALWAYS CHECK). The high voltage developed by the supply is approximately 2200 VDC no load, and 1800
VDC at 500mA.
Figure 3-1 High Voltage Power Supply
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