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Number 15 0n your FHdbkk e.rd Six Meter QRP Station Who says we don't use tubes anymore? by Tima Popovich ex-YUlFR T he unit described in th is article proved the theory that it is possible to establish contacts at quite res pectable distances with very low power. Designed primarily for mobile use . the sta tion runs fro m a 6 volt battery. For 12 volt operation. the healers will have to be rewired and changes made in the power supply. An AC supply is included, mak ing it a statio n for all-a round use . Before going into the details of construction, here is a briefdescription of the various pans. T he T rans mitter The oscillator is a Jones ci rcuit which, compared with other circuits. furn ishes a high range of potent harmonics. The crystal is of the highest possible frequency so that enough drive is available for the final. The final uses push-pull 6AK5s whic h. at low input, give I to 1% Walts of output. I used 6A K5s because of their low heater drain (1 75 rnA) and good high frequency cfficrency. the final amplifie r. The microphone is coupled to the 6AQ5 through a carbon microphone transformer. The quality is good and there is plenty of modulation . Power Su pplies There are two independent power supplies in the transceiver. The mobile supply is a conve ntiona l vi brato r supply delivering about 200 volts at 80 rnA . The AC supply is also convention al and delivers the same voltages in addition to rec tified and filtered low voltage DC for the operation of thc relay and microphone . Warm Up The Iron! Now that I've given you a description ofthe transceiver, the next step is to drag ocr the soldering iron and begin the construction. 1be schematic for the transmitter and modulator is shown in Figure I . The Jones oscillato r is of the cathode feedback variety . The feedback is caused by the RF voltage drop across the RFC in the cathode. A small RF choke in parallel with a 3- 30 pF trimmer is used. The trimmer adjusts the amount of feedback to compensate for the lack ofactivity of some crystals. To adj ust this. use an inactive crystal and set the trimmer so that the oscillator cuts in smoothly and reliably. This setting will be good for all other crystals. The cho ke in the cathode consists of #28 wire close-wound I N on a slug-tuned *N form . Thc crystal can eithe r be 12.5 or 16.5 MHz. The screen supply of the oscillator has a form of voltage regulation caused by feed ing voltage through a 10 k resisto r with an N E2 o r similar neon connected from the screen to ground. The plate ofthe oscillator is connected to a transformer consisting of Ll and L2. The final amplifier is a conventional push -pull circuit with L2 as the grid coil and L3 as the tank circuit. Modulator layou t is not critical . the only T he Receiver The receiver is a superhet with a regenerative detector. This gives the best compromise between battery d rai n and pe r formance . W hen possible. I used 6AK5s to reduce battery drain. The line-up uses a 6A K5 oscillator. 12An cascode RF amplifie r . 6AK4 oscillator. 12AT7 cascode RF am plifie r . 6A K5 I F amplifie r, 6AK5 regenerative detector. and 6A K5 audio output. T he output is e no ug h to drive a pair of headphones or a small. sensitive speaker. If you want more out put. you could use a 6AQ5 at the cost ofhigher battery d rai n. The ~ Iodulato r The modulator is extremely simple , consist ing of one tube. A carbon microphone provides enough output to d rive a 6AQ5, plate and screen modulating 50 73 Amateur Radio • June, 1989 -r 6AK$ • • ~... . T ~ II ".. .. --, r ..~ ,., Figure 1. SChematic for the ORP transmitter and modulafor. , f--h"T. ' •••,..... PE::~=: : : , ---' To • • to....
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