QDX PA MOD KIT

Acknowledgment: Thanks to Barb WB2CBA for sharing this mod and making it public to the ham community.

QDX PA MOD KIT – High Power MOSFET Power Amplifier Upgrade (WB2CBA Design)

The QDX PA MOD KIT is a high-performance power amplifier upgrade module designed to enhance the output stage of the QRP Labs QDX transceiver. Developed by WB2CBA (Barb), this modification replaces the original BS170-based final amplifier with a more robust and efficient MOSFET-based PA design, significantly improving durability and RF performance.

This upgrade kit is intended for experienced builders looking to increase reliability, improve thermal handling, and achieve more consistent RF output under real-world operating conditions.

The PA MOD uses modern low RDS(on) RF MOSFET devices, providing improved efficiency and higher power handling capability compared to the original BS170 configuration. The design has been tested to deliver approximately 5–6W output on HF bands, including stable operation on 20m and higher bands under proper supply conditions.

The module is supplied as a PCB-based daughterboard kit, designed to integrate directly into the QDX transceiver layout with minimal mechanical modification. It also includes improvements in protection, including better tolerance to high SWR conditions and more robust thermal performance.

This modification has been widely discussed and tested within the amateur radio community and is known for improving long-term reliability, especially during digital mode operation such as FT8, FT4, JS8Call, and WSPR, where duty cycles are high.

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Key Features

• Designed by WB2CBA (Barb) QDX PA modification project
• Replaces BS170 final amplifier stage
• High-efficiency MOSFET RF power stage
• Improved thermal stability and ruggedness
• Typical output: ~5–6W HF (band and supply dependent)
• Better tolerance to high SWR conditions
• PCB daughterboard integration design
• Suitable for continuous digital mode operation
• Optimized for QRP Labs QDX transceiver platform

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Performance Benefits

• Increased robustness over original QDX PA stage
• Reduced risk of BS170 failure under stress
• Improved efficiency at higher supply voltages (within limits)
• More stable RF output during long FT8/WSPR transmissions
• Better thermal distribution under continuous duty cycles

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Applications

• QDX transceiver PA upgrade
• HF digital mode operation (FT8 / FT4 / JS8 / WSPR)
• QRP station reliability improvement
• Experimental RF power amplifier development
• Amateur radio transceiver enhancement projects

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Installation Notes

This is a modification-level kit intended for experienced builders. Proper RF assembly practices are required. Thermal management and correct supply voltage selection are essential for reliable operation.

It is strongly recommended to test using a dummy load and regulated power supply before on-air operation.

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What’s Included

• QDX PA MOD PCB (WB2CBA design)
• MOSFET RF power stage layout
• Installation reference documentation