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Categories | QRP Accessories | HF Projects HFPA Ver 4 QRP 35W HF Packer Amplifier Kit Help

Show all reviews of the HF Projects HFPA Ver 4 QRP 35W HF Packer Amplifier Kit

You can write your own review of the HF Projects HFPA Ver 4 QRP 35W HF Packer Amplifier Kit.

W9IQ  Rating: 5/5 Jun 8, 2011 06:03  Send this review to a friend!
A top quality kit to give your QRP signal a boost  Time owned: 3 to 6 months
I relish operating QRP from the mountaintops here in Germany and Switzerland. Band conditions do not always share the timing of my enthusiasm so I began looking for a compact HF linear that would fit in my backpack and operate from a small 12 volt battery. I finally settled on the HF Packer Amp V4 from HF Projects run by Virgil Stamps, K5OOR . This terrific, high quality kit takes 0.1-5 watts in and produces 35-40 watts out on all HF bands, 160 – 10 meters, in all modes. There is an option to have a fan on the heat sink for high duty operation.

My detailed review follows. If you are a moderate to advanced level kit builder looking to give your QRP rig an occasional boost in the field or at home, I know you will enjoy this quality kit.

- Glenn DJ0IQ and W9IQ


• Drive: 0.1 - 5W RF 160-10M
• Power Out: nominal 30-35W 100% modulation (2-tone test)
• Input: 50 ohms, BNC Output 50 ohms, SO-239
• Control: Carrier operated or PTT (RCA jack)
• DC Input: 12VDC, 10A max (9-16V range), standby current: 23-29mA
• Chassis mount Power Pole connector w/ 30A contacts
• Power Switch: Rocker switch
• Operate / Bypass: Rocker switch w/ LED indication of operation
• Weight: 1 lb 15 oz
• Case Size: 6.5 x 3 x 2.25 inch (7.75 x 3 x 2.25 with knobs and connectors)


I contacted Virgil to order the kit and he put me on the list for the next run of HFPA V4 kits. About a month later, he confirmed that he was ready to start shipping so I sent him payment via PayPal. The kit arrived about ten days later. I had already read through the on-line manual so I was anxious to see the quality of the kit first hand and to curl some rosin smoke.

About two weeks later, Virgil circulated an email to recent kit recipients noting that he had discovered he had short shipped two pins for a connector. To my surprise, the additional parts arrived in the mail a few days later with no prompting from me – I hadn’t even gotten that far in the assembly. This is just one example of the great customer service I have come to appreciate from Virgil.

HF Projects ships out of Houston, Texas in the United States and offers global shipping for an additional charge.


It is very clear that Virgil takes the pride of a craftsman in his products – the assembled kit rivals any commercially produced amateur radio product that I own. The cabinet is a two part, sturdy gauge steel assembly with all connections, controls, and indicators clearly labeled with white legends. Small rubber feet on the bottom of the cabinet provide slip resistance. The internals of the amplifier consists of two printed circuit boards which are professional quality, double sided, plated through PCBs with silk screened solder masks and component legends. All connections between the boards, as well as to the chassis, are made with removable connectors for easy assembly and maintenance.

The manual reflects the quality of the kit with step by step instructions and an abundance of quality color photographs. Virgil even includes pictures of all wound toroids and a resistor color code chart. The manual contains an introduction with an overview and suggestions for a successful project, detailed assembly instructions, initial testing and adjustments, troubleshooting hints, a comprehensive theory of operation, schematics, and a full bill of material with cross references to common catalog part numbers.

All components came bagged and labeled for the appropriate part of the kit – main PCB, filter PCB, or chassis. Included in the kit are the two hex wrenches needed to install the front knob and to fasten the MOSFETs to the heat sink. Virgil kindly included two extra MOSFET finals along with their retaining screws in the event I ever need to replace them. The kit even includes a mating set of Power Pole connectors and the associated roll spring pin.


I am an experienced kit builder with a well equipped lab. It took me about twelve hours of leisurely paced work spaced out over several days to build up the kit to the point to be ready for testing and adjustment. All parts are thru-hole components with one exception which comes pre-soldered onto the board. The single largest chunk of time was spent winding the various toroids for the multi-band low pass filter. Virgil now offers pre-wound toroids for those who would like to purchase this option to save time.

I experienced no problems with any aspect of assembly. All components were included in the kit with no shortages. There are two tacked on components to correct issues discovered after the release of the PCBs but these are clean modifications.

The testing and adjustment went very well taking me about one hour to complete. It operated exactly as specified the first time I tried it into a dummy load. The finally assembly was completed about fifteen minutes later.

One should note that the alignment of the low pass filters for the various bands does require some test equipment to do it effectively. I used my MFJ antenna analyzer along with a 50 ohm dummy load. This made quick work of adjusting the spacing on the toroids to get the right band spread. A VNA is another possibility. The manual and support site offer other ideas as well.

Some additional gear that makes the project proceed smoothly is a Power Pole crimper, a heat shrink gun, a capacitance meter, a “Dremel” tool to remove cabinet paint around RF connectors as instructed, a 50 watt dummy load, a DVM with 10 amp DC capability, an HF power meter, and PCB de-flux spray.


The amplifier is an updated version of the WA2EBY “A Broadband HF Amplifier Using Low-Cost Power MOSFETs” that first appeared in the March 1999 edition of QST. The design makes use of two cost effective IRF-510 MOSFETs setup as a class AB push-pull amplifier with separate bias controls for each MOSFET.

Virgil has enhanced the design by including a DC to DC converter that steps up the MOSFET supply voltage to around 30 volts when operating. This not only improves the output performance, it gives the amplifier the ability to operate over a wider range of input voltages including a sagging 12 volt battery. This was one of the primary features that attracted me to this kit. I can get hours of operating using a gel cell or Li-Ion pack and can extend this further with my portable solar panel.

The new design also sports a PIC microprocessor to control timing sequences and supervisory functions. At key up, the microprocessor shuts down the DC to DC converter and then goes to sleep in order to reduce power consumption and eliminate circuit generated noise.

Keying of the amplifier can be accomplished through its internal, automatic RF sensing or by using the external PTT input. I find the automatic sensing to be more than adequate for my modes of operation. There may be a very short burst of receiver hash in this mode when the transceiver is unkeyed before the amplifier shuts off its DC to DC converter. There is also a CW jumper on the main PCB to optimize the timing sequences if this is your preferred mode of operation.

The allowable input power range is controlled by a pi attenuator. The builder must select the desired attenuation for their situation during the time of construction. The maximum RF input to the MOSFET drive circuit is normally 0.63 watts so Virgil includes the necessary resistors to construct an on-board 6 or 9 dB attenuator allowing power input levels up to 5 watts.

The two power MOSFETs are mounted on a heat sink that is located on the top of the cabinet. The kit can be ordered with a standard heat sink or an optional forced air version. I chose the forced air option but found for normal CW and SSB operation that it is overkill. Other than a bit more space consumed in my back pack, the only other downside is a slight increase in current draw although the fan is controlled by the microprocessor timing sequence.

The low pass filter for the amplifier meets FCC harmonic suppression requirements through the use of rotary switch selected Chebychev and Cauer filters. High quality silver mica capacitors are used throughout and all inductors are toroids. I ran the components values from the schematic into Elsie and found that they could be better optimized to allow more power output but the amplifier meets the specifications without tweaking. I didn’t take the time to measure the harmonic suppression of the kit as built but considering the class AB design and the skirts of the filters chosen, it should be well within the FCC requirements.

While the overall design is very solid, it is unforgiving if you present it too much of an output mismatch, leave the band switch in the wrong setting, or you substantially overdrive the input. These conditions will likely cause failure of the MOSFETs. The good news is that the MOSFETs are very inexpensive – less than 2 US Dollars a pair. The hassle is having to change them out and recalibrate the biases to get the amp back in operation. I estimate this would take about 30 minutes since you will need to completely dismantle the amplifier to unsolder the MOSFETs – not a likely scenario for repair on top of a mountain. Virgil and I have exchanged some ideas to minimize blowing the MOSFETs. I wouldn’t be surprised to see Virgil address this in a future version.


Nothing is more frustrating than buying a new gadget and not being able to get proper customer support. The industry should take lessons from Virgil. While fortunately I did not need to raise any customer support issues, I was able to listen into several “help” calls from other kit builders via Virgil’s Yahoo Group. Without fail, by the time I saw the original email, Virgil has already responded to the inquiry. Sometimes the inquiry showed that the manual could be more explicit to avoid a particular problem so Virgil would post an updated manual within a few days once he got the problem resolved for the inquiring builder. It is also noteworthy that his responses were coaching, kind, and considerate in this age of terse, if not insulting, internet responses.

Virgil has found a few opportunities for minor improvements to the design and has kept the group informed and offered very cost effective migration components when needed.

I did have a pleasant email conversation with Virgil when I suggested a few potential improvements to the kit. Virgil took the ideas to heart and we bantered about some pleasant and professional what-if and how-about concepts.

I would have no hesitation to order another kit from HF Projects with K5OOR at the helm.

Product is in production.
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