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RF Tank Circuit

The tank circuit both places a load on the tube(s) and transforms the output impedance to match the line. Prior to the popularity of the Pi and PiL networks, and their cohort, coaxial feedline, amateurs utilized air wound transformers to match the output impedance of their tubes to then popular balanced feedlines such as twin-lead and ladder line. Air wound transformers work well, however they are bulky and most often single banded. Hams often used plug-in sections that required changing out when ever they opted to go from one band to another. In addition, neither twin-lead, nor ladder line "jumpered" very well.

With the advent of lower loss coaxial cable and advancements in feeding balanced antennas with unbalance line, the Pi and PiL networks became the tank circuit of choice. Both use C1 in conjunction with L1 to properly load the tube. From there C2 works with L1 (and L2 in the PiL) to match the load to the line. The values of C1, C2, and L1 should be selected for the lowest band of intended operation. Remember that the caps are adjustable from near zero to their maximum value. L1 becomes adjustable with the introduction of a rotary switch used to tap and short portions of L1, reducing its operational inductance in the process.

The rating of the blocking capacitor should be at least 2.5 times the voltage applied to the plate. Likewise, the air space and insulation of C1 should be rated the same. In the case of a Pi network, C2 can be simply a broadcast type in good condition. The switch used to short out portions of L1 should be a ceramic wafer switch, although needn't be particularly heavy provided it shorts from the feedline side in, rather than from the plate side.


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SELECTING A TUBE

RF INPUT CIRCUIT

RF TANK CIRCUIT

TRANSFORMER POWER CAPABILITIES

PARTS IS PARTS

ROLLING YOUR OWN - TRANSFORMERS AND CHOKES

POWER SUPPLY PROJECTS

LEGAL LIMIT AMP PROJECT