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MOBILE

A simple S-tube rig for 75 meters which operates from a
260 volt 100-ma supply.

Ten Watts Mobile for Twenty Bucks

PAUL L. WHITLOCK, K4ANU


IN THESE DAYS when the average mobile power on the low-frequencey bands is 50 watts or more, it may seem foolish to suggest anything as lttle as 10 watts. But don't underestimate the capabilities of a little rig shown in the phothographs. I have worked hams in Montgemery with Q5 reports, and Montgemery is 100 miles away. Granted that this may be the exception rather than the rule, there is still a great deal of fun to be had working over much shorter distances. There are many hams whose opportunity bulky installation would hardly be warranted. And remember. power economy in the transmitter is carried down the line, through the power supply, to the demand on the car battery.

Circuit

The circuit,shown in Fig.1 could be less complicated. A 6C4 Pierce oscillator is capacitively coupled to a 6AQ5. T2 is a rather unique item made by Triad. The primary is in the form of an autotransformer with an impedance ratio of 5000 to 6750 ohms and rated at a total of 100 ma. d.c. It matches the required load of 5000 ohms for the Class A 6AQ5 modulator and a Class C load of 6750 ohms (about 40 ma at 250 volts for 10 watts input).


From QST. February, 1967
 

The secondary is designed to feed 4-ohm speaker voice coil so that the modulator can be used for p.a. work should the occasion arise. This could be a handy arrangement in an emergency. Microphone voltage is obtained from the drop across a portion of the modulator cathode resistor. The Triad A-5X microphone transformer was chosen not only for its small dimensions, but also because its frequencey response is limited chiefly to the most useful speech frequencies 300 to 3000 cycles.

Constructstion

Most of the construtional details will be clear from the photographs. The unit uses a 3X4X5-inch utility box as a foundation. The 2 &marc12;-inch milliammeter, amplifier plate tank tuning capacitor, filament switch and microphone jack occupy the front panel. Thethree tubes, crystal socket, coax output connector, and modulator transformer are mounted off the back of the box. The output tank coil form. The + prong socket which has been mounted with a machine screw through a hole drilled at its center. A spacer elevates the socket to clear the terminals. The socket isn't really necessary; a similar coil form without pins could be mounted

 

This view shows the change over relay mounted along side the output tank and capacitor. The resistors and capacitors at the upper left ar R5, R6, C10 and C11 in yhe modulator circuit. RFC2 is wired between an insulated tie point and the 6C4 socket. The resistor to the left of the choke is R2.


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