The Little Chickadee 6U8A QRP Transmitter

Published:2013/3/7 3:11:00 Author:Ecco | Keyword: Little Chickadee , QRP Transmitter | From:SeekIC

The triode section of the 6U8A is used as a tuned crystal oscillator stage in the crystal version of the circuit and a tuned amplifier in the VFO version. The triode's tuned circuit consists of L1, a fixed-value silver mica capacitor and a ceramic compressions trimmer capacitor. For 80 meter operation, the silver mica capacitor should be 130pf and for 40 meter operation, the capacitor should be 22pF. L1 is made with 47 turns of #24 insulated wire on a 3/8 air core cardboard form, this works for both 40 and 80 meters. The trimmer capacitor should be adjusted so that the circuit resonates at the crystal frequency. Resonance can be observed with an oscilloscope, the probe should be lightly coupled to the oscillator plate, connect the scope probe to the plate via a small capacitor such as 5pF. The crystal version of this transmitter is rock bound to a particular frequency. Common 3.58 Mhz crystals can be used, but this frequency tends to be occupied by digital-mode signals. It is a good idea to use the crystal oscillator (VXO) variable capacitor to shift the frequency up a few kilohertz to a quiet spot on the band. Other frequencies can be used by switching different crystals into the circuit or using a crystal socket. The oscillator's tank circuit should be re-peaked for different crystals if they are more than a few tens of kiloherz apart in frequency.



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