The LC circuit is shunted by a 2.5K ohm resistor to reduce its Q, and tunes from 7 mHz to 30 mHz, thus eliminating the need for an input band-switch. The coaxial input coil is resonated in parallel by a 1000 pf variable capacitor (C3), which is mounted above ground and tuned with an insulated shaft. The filament voltage is then applied to the “cold” end of the coil, as well as its interior wire, as shown in the schematic drawing below. The coil is made of 3/16″ copper tubing which is connected directly to one of the tube’s filament terminals, and an insulated #12 wire passing through the interior of the coil’s tubing is connected to the other. To best describe this unique circuit the 813’s directly heated cathode (filament) becomes an integral part of the resonant cathode tuning circuit by the use of a coaxial coil (L2). In most respects it is a typical grounded grid linear amplifier, with the exception of a rather unique tuned cathode input circuit that was developed by Pat over 10 years ago. It is only 11″ wide, 4″ high, and 9″ deep. This compact little amplifier is the brain-child of Pat Murdoch, ZL1AXB, in Auckland, New Zealand. A low cost 600 watt ultra-linear amplifier for 40, 20, 17, 15, 12, and 10m
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January 2023
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