The Heath Company made and sold electronic testing and communications kits between 1947 and 1992. Their amateur radio Heathkit SB-220 linear power amplifier is still a favorite of ham radio operators worldwide. This amplifier is capable of providing maximum legal radio power of 1,500 watts or more on all the original amateur radio wavelength bands between 80 and 10 meters, using less than 100 watts of transmitter exciter power. The key to SB-220 operation is a pair of costly Eimac 3-500Z triode amplifier vacuum tubes. Carefully load and tune these sensitive final amplifier tubes according to a specific technique to optimize performance and longevity.
Connect the Heathkit SB-220 to a resonant antenna and connect a transmitter exciter to the SB-220 via the short coaxial cable. Plug the SB-220 into a 220 volt receptacle. Some units have been modified and rewired for 110 volts; but the manufacturer and manual both recommend a 220 volt power source. Tune up your transmitter as normal to produce about 90-100 watts. Tune up on an unoccupied frequency, or use a dummy load antenna to prevent on-the-air interference.
Reduce transmitter exciter power to zero. Power the SB-220 unit on via the power switch on the front panel. Key your transmitter or place it in tune mode. The plate meter on the amplifier should read 0.12 amperes (amps), which is the resting plate current for the 3-500Z tubes. Advance the level control until the plate meter shows 0.3 amps.
Alternately adjust the tune and load controls for maximum relative power on the meter. This is called peaking the meter. Advance the drive control until the plate meter reads a plate current of 0.4 amps. The meter readings should be a plate current of 0.35 amps, high voltage of 2,100 volts and a grid current of 110 milliamps (mA).
Continue to alternately adjust the tune, load and transmitter drive controls to obtain your desired input power. At 1,000 watts, plate current should be 0.5 amps; high voltage should be 2,000 volts; and grid current should read 100-200 mA.
Adjust the sensitivity control for your desired relative power meter reading. Unkey the transmitter exciter or place its function switch in standby mode until you are ready to transmit. Your station is now ready to transmit continuous wave (CW) and digital signals. Do not adjust the drive any higher than 0.33 amps on the plate current meter for voice peaks in single sideband voice operation and to avoid flat-topping or over-driving your voice output signal.
Things You'll Need
- Amateur radio transmitter exciter
- Resonant antenna and cable
- Short coaxial cable
- Dummy load antenna
Follow the original Heathkit SB-220 manual for additional information on troubleshooting, loading and tuning your SB-220 linear amplifier.
Dangerous voltages and currents are reached in the Heathkit SB-220 linear amplifier. Do not touch the Eimac 3-500Z tubes, even with power off, until residual capacitance charges have been neutralized by grounding.
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