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How to Tune a Tube Linear Amplifier

Vacuum tubes have been largely supplanted by transistors in modern electronics. There is one area where the vacuum tube is still king, however, and that is in high-power radio frequency amplifiers. Although medium to high power solid-state amplifiers do exist, the classic vacuum tube linear amplifier does its job so well that few users have any desire to replace it. Amateur radio operators who have mastered the task of loading and tuning the tube tend to view it as part of the fun of ham radio.

Things You'll Need

Dummy load
Antenna switch
Amplifier owner's manual
Morse key

Instructions

Preparation
- 1
  Connect the dummy load to the amplifier via the antenna switch. Switch the amplifier power on. Read the plate voltage meter on the amplifier and make sure the voltage is within the limits specified in the amplifier manual.
- 2
  Power up the transceiver. Select the band and frequency you wish to operate on. Set the mode switch on CW, and turn the transmitter drive control all the way down.
- 3
  Set the amplifier bandswitch to the desired band. Set the "Load" control at approximately 25 percent for 160, 80 or 40 Meters, or 50 percent for higher bands. Set the "Tune" or "Plate" control in the middle of the range indicated on the dial for the desired band.
- 4
  Set the amplifier meter to relative power. Set the "CW/Tune - SSB" control to "CW/Tune," if the amplifier has this feature.
Tuning
- 5
  Plug the Morse key into the transceiver. Key the transmitter, and advance the drive control until the relative power meter on the amplifier begins to move.
- 6
  Adjust the Tune control for maximum indicated power.
- 7
  Adjust the Load control for maximum indicated power. Switch the meter to grid current, and check that the reading is within the limits specified in the manual.
- 8
  Advance the transmitter drive control to full power. Readjust the Load and Tune controls for maximum output power. Observe the plate amperes meter and check that it does not exceed the maximum value specified in the manual. Release the key.
- 9
  Switch from the dummy load to a resonant antenna for the band in operation. Key the amplifier and re-peak the Tune and Load controls. Select the desired operating mode on the transceiver.

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