Per the manual for the Pride DX-300:
2.1 OPERATION:
1) Check to see that the TUNE/STANDBY/OPERATE switch is in the center or STANDBY position. Apply power to the unit by switching the POWER switch to the ON position. You will hear the cooling blower come up to speed. Wait 60 seconds for the tube to come up to operating temperature.
2) During the time that the amplifier is warming up or at any time that the TUNE/STANDBY/OPERATE switch is in the STANDBY position, the exciter may be operated normally "straight through" the amplifier.
3) Set the band switch to the same band as that the exciter is tuned to.
4) After warm up, place the TUNE/STANDBY/OPERATE switch into the TUNE position. The OPERATE indicator will light up, indicating that the unit is ready for operation. If the RECEIVE switch is on, the RECEIVE indicator will light also, indicating operation of the receive preamplifier.
5) Rotate the PLATE LOADING control to the nine o'clock (counter clockwise) position.
6) Apply several watts of drive from the exciter and quickly peak the PLATE TUNE control for maximum output as indicated on the front panel wattmeter. The SCREEN overload indicator may glow at this time, and will peak in brilliance at approximately the same place of tuning that maximum output occurs.
7) The PLATE LOADING control is then turned clockwise 1/2 division or so, and the PLATE TUNING control is peaked again for maximum output. The brilliance of the SCREEN indicator will peak again, but not quite as brightly as before. This process is continued until a further increase in the PLATE LOADING control will give no further increase in power output. This will normally result in approximately 100watts output for three to four watts
of driving power.
8) Un-key the exciter, and place the TUNE/STANDBY/OPERATE switch in the OPERATE position. When the exciter is keyed again, the power output will be somewhat higher, and the SCREEN indicator may glow brightly again. Repeat the procedure as before, tuning the PLATE LOADING and PLATE TUNING controls alternately for maximum output.
The PLATE TUNING control should always be the last control that is adjusted. Power output should be about 250watts for five watts drive at this point in the tune up procedure. When proper loading is accomplished, the SCREEN indicator should glow only dimly. The plate current, as read on an external meter, should be in the neighborhood of 200-250ma.
9) For
SSB operation, drive power should now be gradually increased to 10 or 12 watts, and the unit peaked for maximum output. This can be done using the carrier insertion control on some exciters, or if this is not available, a steady tone may be applied to the microphone input. When maximum power has been achieved, operation consists of simply keying the microphone and speaking. The
SSB delay switch should be in the on position,
which will give the R.F. actuated relays a time delay, so that they won't chatter during transmission. During the
SSB transmission, power output peaks will occur so rapidly that the output meter will not be able to follow the signal. If a monitor scope is available, it can be seen that the peak power is considerably better than the 400 watts or so that was obtained during tune up. The SCREEN indicator will flicker with the transmission level, and will reach full brilliance at high peak power levels. This is a faster indicator of peak drive conditions than is the power output meter.
10) For CW operation, tune up as in step nine above. Power output will be nominal 250 watts for 5 watts of driving power. The
SSB delay switch may be left in the on position to prevent relay chatter during keying.
11) The 4CX-250B transmitting tube has been proven is commercial and military uses where long life and reliability are major factors. Although it is designed to work a little harder in amateur service, it will still be extremely reliable if a few common sense rules are applied.
a) Make sure that the tube always receives a good airflow. Remember that a lot of watts are being packed into that rather small package, and that the only way to keep it cool is to get air through the tube fins. Do not block the top of the cabinet, or stack other equipment on the amplifier. Make sure that the rear of the chassis (where the blower gets its air) is open to cool room air. If the bottom cover of the unit has to be removed for service, see that it is replaced properly prior to plugging in the unit (for safety also). The bottom chassis must be pressurized
in order to force air through the tube fins.
b) Wait 60 seconds before applying drive to the tube. Allowing the tube to properly warm up will help you get the longest service from it.
c) After a long period on the air, let the amplifier run in the STANDBY mode for a minute or so before turning off the power switch. This allows the blower to bring the tube down to a cooler temperature fairly rapidly.
d) Avoid excessive R. F. drive to the amplifier.
e) Use care in tuning most tube failures are the result of operator error, not equipment failure. These transmitting
tubes are used for thousands of hours in commercial service, and similar results should be yours with reasonable precautions.
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How to tune a tube amp. 
SO if someone could tell me how to tune it, I would GREATLY appreciate it 
Linear Mode
