50W Power Amplifier for the QCX. Includes PCB, all components (through-hole), connectors and two large aluminium heatsinks. Can be built for 80m, 40m, 30m or 20m bands. Fast solid-state transmit/receive switching allows full break-in operation (QSK).

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This 50W Power Amplifier kit has no Surface Mount Components (SMD) to solder. A suitable power supply e.g. 20V 5A is required to be able to achieve the full 50W output - refer to the manual. It is designed to be driven by a standard QCX+ CW transceiver kit (QCX+ powered by a 12V supply) or any member of the QCX-series transceivers, including QCX, QCX+ and QCX-mini. A key feature of the kit is the solid state fast transmit/receive switching which provides full break-in (QSK) operation (hearing the band between your own dit and dah keying). This amplifier is not intended to be a Linear (for SSB use) - it is designed for single-band CW operation only. 

The design uses only low-cost, commonly available components such as the pair of IRF510 in push-pull as the power amplifier transistors, and 1N4007 as the PIN diodes for transmit/receive switching. If you damage any components such as the IRF510 transistors, they are easy to source and replace. 

Please refer to the FAQ below for any questions. 


  • Designed for the QCX+ CW transceiver kit and QCX CW transceiver kit and QCX-mini CW transceiver kit
  • Up to 50W power output on 40m with 20V supply, falling to around 25W at 13.8V supply
  • 50-ohm input and output
  • Fast clean solid-state Tx/Rx switching provides full break-in operation (QSK)
  • Can be built for one of 80, 40, 30m or 20m bands with the supplied components (or other bands with suitable choice of Low Pass Filter components)
  • Uses two low-cost IRF510 transistors as the power amplifier in push-pull configuration
  • Recommended amplifier operation in Class C
  • On-board 7-element Low Pass Filter for harmonic attenuation
  • Two black anodized heatsinks sized 130 x 28mm with 25mm fins typically limit the temperature rise to < 20C over ambient in normal CW operation
  • Through-hole plated PCB, all through-hole components (no Surface Mount Devices)
  • All connectors board-mounted: two BNC, one 2.1mm barrel power connector, one 3.5mm jack connector for keying
  • PCB size 130mm x 59.5mm (2.744 x 1.338 inches)
  • Standard inexpensive components throughout (easy to replace)
  • Optional black anodized aluminium enclosure (63w x 25h x 130d mm plus another 25mm for the heatsinks installed on top)

The enclosure is custom-designed and manufactured specifically for this 50W PA kit. It has top and bottom sections made from black anodized extruded aluminium, with front and rear panels drilled, cut and printed for the PA kit LEDs and connectors. The top half of the enclosure is drilled to fit the heatsinks supplied with the kit. See photos section below for some pictures of the enclosure. 

Kit Contents:

  • Professional quality, double-sided through-hole-plated silk-screen-printed 1.6mm FR4 PCB, size 130 x 59.5mm
  • Two black anodized aluminium heatsinks, each sized 130 x 28mm and 25mm fins
  • All board-mounted components and connectors
  • Mounting hardware to bolt the PCB to the heatsinks

OPTIONAL enclosure kit contents:

  • High quality black anodized aluminium extruded 2-part enclosure (tongue-and-groove fit), top drilled to fit the heatsinks. Size 63w x 25h x 130d mm. 
  • Drilled and printed front and rear panels, to match the 50W PA PCB LEDs and connectors
  • 8pcs screw to bolt the front and rear panels to the extruded top and bottom pieces
  • 4pcs self-adhesive rubber cabinet feet

Comment from designer Hans G0UPL:

"During the summer of 2019 the 40m version of this amplifier was tested extensively here in QTH KM46, with well over 500 QSOs in the log. Despite the poor band conditions and my very mediocre antenna and location, QSOs ranged from New Zealand and Japan in the East, to US in the West; Finland in the North, down to Saudi Arabia and everywhere in between. (500+ real QSOs averaging 10 minutes, not contest exchanges). During QSOs I estimate around 90% of correspondents are using power in the 50-100W range, the remaining 10% operate with higher power than that or a few of them, with QRP (5W and below). This 50W PA kit puts you and your QCX on a level playing field with the majority of CW operators on the bands."

Assembly and operating instructions

The assembly document includes circuit diagram and detailed steps for the kit assembly. Please follow them carefully. There is also a large section detailing the design theory. The document includes the installation into the optional enclosureMAKE SURE YOU DOWNLOAD THE RIGHT MANUAL FOR YOUR PCB REVISION! Revision 2 PCBs have "Rev 2" written in the bottom-left corner.

NOTE: PCB Rev 3 is now being supplied. Rev 3 boards have many components now implemented as pre-mounted SMDs. There is as yet no assembly manual for the Rev 3 board. The manual is pendiing... meanwhile you can use the Rev 2 manual, all the steps involving the filter, the power transistors, power resistors, 1N4007, inductors, transformers and connectors are the same; but skip the steps involving low wattage resistors and 0.1uF capacitors, small MOSFETs etc.

Rev 2 Assembly instructions (PDF document revision 1.00t, 12-Sep-2022)
Assembly instructions (PDF document revision 1.00k, 01-Jun-2020)

This is an A4-size document, to print on letter size paper select "Fit to page" option in your print dialog box. 


Connection to QCX+ and QCX-mini

QCX+ and QCX-mini have a 3.5mm PTT output connector, and all that is required is a standard stereo audio cable with 3.5mm stereo jack plug at each end. 

Note that the first batch QCX+ kits had a reversed tip/ring connection. Therefore a modification is needed to the board, or swap the trip/ring connections in the audio cable - refer to the QCX+ manual for details on this. Later PCB revisions are not affected. 

Connection to QCX 

The PA requires a "Push To Talk" (PTT) signal from QCX, which is a positive-going signal to indicate to the 50W PA when to switch over to transmit mode. This is taken from IC3 pin 11 the "TX" signal; the same signal is also found at one end of R60. 

A recommended modification is the addition of a pull-up resistor between IC3 pins 13 and 14. This is documented here

Gregg W7GRM provided two photographs, showing the PTT connection and the resistor modification. Thanks Gregg!

Frequently Asked Questions (FAQ)

1. Can the 50W amp be used with other rigs, not just QCX-series transceivers?
2. Can it be used as a Linear for SSB and other modes?
3. Does it produce an FCC (etc) compliant output?
4. Why do you need 20V supply for 50W? Why can't you get 50W output with a 12V or 13.8V supply?
5. Why is it single band only?
6. Could it be modified for multi-band operation?
7. Why can it be built only for 80, 40, 30 or 20m?

1. Can the 50W amp be used with other rigs, not just QCX-series transceivers? 

Yes! Why not? You need an input power of around 2-4W to get 50W output; you also need an active-high (+5V) "PTT" signal to cause the transmit/receive switchover. 

2. Can it be used as a Linear for SSB and other modes? 

NOT SSB as it is - it is designed as a Class-C amplifier, for CW operation (or FSK modes). Single Sideband (SSB) requires a Linear. If you want to experiment using this amplifier as a Linear, you can try setting the bias trimmer potentiometer to achieve a higher "idle" current during transmit, of around 250mA. This will dissipate some watts of heat even when there is no signal, and is normal for Linears. The efficiency will also be lower. Accordingly, heat dissipation may be higher too (depending on the average power output and other factors). So proceed with caution if you want to modify the amplifier for LINEAR operation - you will be in experimental territory. 

3. Does it produce an FCC (etc) compliant output? 

Yes - the amplifier includes a 7-element Low Pass Filter for harmonic attenuation. 

4. Why do you need 20V supply for 50W? Why can't you get 50W output with a 12V or 13.8V supply? 

Because this amplifier is designed as a low cost amplifier - using IRF510 transistors as the amplifiers. If you want 50W with 13.8V supply, sure, it's possible - but using a more complex design and different components - and accordingly, a higher cost too. 

5. Why is it single band only? 

Because the necessary Low Pass Filter is included on the board, and to keep costs low. It is also designed as an accessory to the QCX transceiver which is a single-band CW transceiver. 

6. Could it be modified for multi-band operation? 

Certainly. Just bypass the Low Pass Filter section of the design, and use an external switched Low Pass Filter for the bands of interest. There are no other band-specific components except for the Low Pass Filter components and it will work across all HF. As usual... everything gets harder at higher frequencies so expect the power output to start to fall off (see also the measurements section of the manual). 

7. Why can it be built only for 80, 40, 30 or 20m? 

It's all about COST. To cover 80, 40, 30 and 20m bands, we need to include eight 250V NP0 capacitors. There are four capacitors per band but by combining the capacitors we am able to cover 80, 40, 30 and 20m bands with just these eight capacitors, with two or four capacitors left over, depending on which band you build it for. To cover more bands would need a larger number of capacitors included, which would mean more capacitors left over; OR, it would mean supplying a band-specific additional kit depending on the choice. BOTH these options would increase the kit cost. Since 80, 40, 30 and 20m covers well over 90% of QCX kits sold, this seems like a reasonable compromise. However - if you DO want to build it for a band other than 80, 40, 30 or 20m - there is nothing to stop you! Just obtain and install appropriate Low Pass Filter components, the LPF design is the same as the QRP Labs LPF Kit - you just need larger inductors (T50 size) and higher voltage capacitors (at least 100V).