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Homebrew 100 Watt Six Meter Brick Amplifier

This is a homebrew 100 watt six meter linear amplifier. The project was designed by Pat Bunn - N4LTA, and was featured in the October 1990 issue QST magazine. It is built around the inexpensive Motorola MRF 492 power VHF RF transistor. It operates from 13.8 volts DC and consumes approximately 13 amperes while producing 100 watts RF output. Here are some photos of the completed amp.

Interior View
Printed Circuit Board
Rear View of Chassis
Another View of the Amp

The amplifier easily produces 100 watts when driven by a low power exciter such as the Ten Tec 1208 transverter, which produces 8 watts of drive power. The amplifier can be operated in either class AB linear or class C modes, I installed a front panel switch to do this. The amp features a sensitive RF sensing circuit to key the antenna change over relays from receive to transmit. The massive heatsink I used barely gets warm even when operating at rated power for extended periods of time.

Take a Sneek Peek at the Amplifier's Schematic

Schematic : 100 W 6 M Brick Amp

Download Plans Here for the 100 W Brick Amp : ARRL "Members Only" Web Site

ARRL : Project Plans Six Meter 100 W Brick Amp

The Motorola MRF 492 transistor, heat sink, and the ceramic trimmer capacitors were obtained from RF Parts Company. The chassis, relays, and other small parts were obtained from Digi Key and Mouser Electronics. A circuit board can be obtained from Far Electronics. The heat sink I used is overkill, as I was thinking of using the amp in continuous duty as part of a beacon transmitter. You won't need one that large.

The plans give little information on the RF choke connected to the power transistor's collector, this is RFC1. Through experimentation I found that this can be made by winding 15 close-spaced turns of number 14 enameled wire wound on a 1/4 inch drill bit shank. Originally I wound this choke on a small ferrite rod that would become warm to the touch. Through experimentation I found that it works better air-wound, which stays cool to the touch.

I was unable to obtain trimmer capacitors with capacitance values and physical sizes exactly as specified in the QST article. Never fear, order the nearest "ball park" value tending to perhaps lean toward a little larger maximum capacitance. This is what I did and they work great. The trimmers I used are medium sized, about 3/4 inch square, meant for use in transmitting circuits. The amplifier tunes up easily and mine is unconditionally stable.

So what are you waiting for? Build this or another project...let's get the fun rolling!

Here are the Links to Parts Suppliers for this Project

RF Parts Company
Digikey Electronics
Mouser Electronics

Link for the FAR Circuit Board : Listed as the QST Oct90 project "Compact Brick 100W Amplifier for 6 Meters Using MRF492."

Compact Brick : FAR PCB

Questions or Comments About the Amplifier
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© 2002 - 2005 Rex Greenwell K0KP