A number of constructors have eyed up adding an afterburner to their µBITx. The usual cautions apply: make sure your µBITx has clean output with earlier v3 and v4 boards having been upgraded to remove harmonics and spurs, before even considering adding a power amplifier.
There are a range of cheap Chinese and Russian kits available to give you between 40w and 70w output. However, these may be better avoided, since you can build a WA2EBY power amplifier without a lot of effort. This is a very solid design, and well proven.
Some list members suggested it may be hard to source boards for this amplifier design. However, take a look at:
http://www.golddredgervideo.com/kc0wox/wa2ebyamp/ (pointer to availability of boards) and
Allison KB1GMX commented:
Its a good design and allows for getting good performance at higher frequencies. Mine with a little effort does 37W on 10M (1.8W drive) and with the same drive at 40M about 55W. 80 and 20 are about 50. Never though to try 15 or 17m but I’d expect about 44w. The 1.8W is because I use an attenuator at the amp input as most of my HB radios do 4W which is excessive power or the amp. FYI I run it at 28V.
It is a good amp. Mine is now 13 years old and still running the same set of IRF510s. I did use a large heatsink (4×8″ with 1″ fins and the base thickness was .300″). Some call it overkill, but with no fan running and with a brick on the key for 10 minutes, the amp doesn’t fail.
At least one of the “70W DIY AMP” I’ve seen did produce that much power for about 1 minute into a dummy load before it blew up. Failure was likely due to self oscillation or overheating of the supplied heatsink. The heatsink was maybe pentium II vintage with mounting points for
a fan and not at all large or having many fins.
HF Linear Amp 40 Watts Kit
MVS Sarma also points out that Sunil VU3SUA sells a set of black masked PCBs for the WA2EBY amplifier.
Hans G0UPL who operates QRP-Labs has released his 10W HF Linear Power Amplifier. This kit comfortably produces 10W from a 12V supply. It is a compact design with huge heatsink included, which will not overheat even on continuous 100% duty-cycle operation. The amp provides 26dB gain with +/- 1dB gain flatness from 2 to 30MHz.
This 10W HF Linear Power Amplifier kit has no Surface Mount Components (SMD) to solder. There are a number of small transformers to be wound, and assembly requires care and patience.
The push-pull driver stage uses two BS170 transistors in the amplifier design used in the SoftRock transmitter stage. The final uses two IRF510 transistors in push-pull. Yes, this humble low-cost MOSFET really is capable of excellent performance all the way up to 10m band and beyond! Short lead-lengths and PCB layout are extremely important, they are the key to success.
- 10W output from 2 to 30MHz, using 12V Supply
- Generously-sized heatsink, will not overheat even on continuous 100% duty-cycle modes
- 2-stage amplifier provides 26dB of gain
- Push-pull driver and push-pull finals, for high linearity and low harmonic content
- +/- 1dB gain flatness from 2 to 30MHz
- 4dB down at 6m (50MHz) and 8dB down on 4m (70MHz)
- Standard 50-ohm input and output
- Through-hole plated PCB, all through-hole components (no Surface Mount Devices)
- Standard inexpensive components throughout
- Tested for 1 hour at full-power 10W, 100% continuous duty-cycle with no forced air cooling
- Tested for 15 minutes at 20W, 100% continuous duty-cycle with no forced air cooling
- Tested at 20V supply
- Tested into open load, shorted load and various mismatches without instability (oscillation)
As Hans advises on the BITX20 I/O Groups list:
I am not suggesting that this amplifier will solve all problems and without wishing to be offensive, the “garbage in, garbage out” rule always applies – in particular, if you feed it a signal containing spurs, harmonics, or other problems… then you will get the same thing at the other end, just amplified 26dB. It also still applies that all stages leading up to the final amplifier must have flat gain. If the pre-drivers cannot provide a big enough signal then you won’t get full power output.
But, it could be useful or interesting, in the BITX context, to look at the details of this amplifier kit and see the short lead lengths, symmetric PCB and extensively “stitched” groundplanes.
For further information see the product on Hans website.
Many constructors will have thought about adding on to their µBITx one of the cheap linear amplifier kits that can be found on eBay.com or Aliexpress.com.
A thread covering these options was started by Arvo KD9HLC. This article attempts to succinctly cover this ground for those exploring amplifier kits in future.
Arvo asked about the kit illustrated above that costs US$36. There is another cheaper version available and Lee N9LO says:
“I’ve read some reviews on these and it seem the big difference between the $36 100w and the $18 70w is the 100w is on 16v and the 70w is 13.8v. The heat sink for either is $8. They both need a low pass filter for the band you use it on”
“The 45 watt and 70 watt amps use the IRF530, and I own both. They do work, but not for long at the advertised power. It’s very easy to push too hard on the drive and blow the FETs. The 45 watt version only takes 10 milliwatts of drive to get full power, so it can be connected directly to the BitX 40 T/R relay with a 3 to 6 dB pad (bypassing the PA). You can get about 25 watts out on 13.8 volts. The 70 watt amp needs about 1 to 3 watts of drive to work, but can produce 40 to 50 watts from 80 to 20 meters. The output falls off sharply after that. Either way, but some extra IRF530s… you will need them!”
Howard Fidel says:
“I built the 70 watt model. I changed the 2 IRF530s to 4 IRF520s. The IRF530s oscillated and failed quickly. They also run very hot. The IRF520s run much cooler (2x as many junctions) and are stable putting out 50 watts on 20 meters. The output on 15 and 10 is less, but the uBitx puts out less on those frequencies. I am working on getting it working on all bands, but for now I just use it on 20.”
David, N8DAH, says:
“Just my 2 cents. After getting two versions of the “cheapbay” amp I can say the best thing I did. I bought a proper kit (like a WA2EBY) and built it. The Hardrock 50 was also a great amp but the cost is pretty high. I have not used a HFpacker but know a few QRP guys that swear by them as well.
If you really want 100W look at the HLA-150. I have had much better luck with building a known kit then trying to save a few bones on the cheaper kits with next to no info on them”
Jerry KE7ER says:
“Any amateur amp should be followed by a low pass filter suitable for knocking out the second harmonic and beyond. That means separate filters for 160m, 80m, 40m, 20m, 10m. You may piggyback some of the other non-harmonic bands into that set of filters, but that makes the filters much more difficult to design and build. A rig with only a 30mhz LP filter is likely aimed mostly at CB’rs.
“QRP is good for experimenting. But with a 50W+ eBay amp, you really should be testing for compliance with FCC regs. It likely fails, and all the nasties will scale up with the output power.”
Two articles to check out
4Z1NT suggests checking out this article on how to make the 70w unit work well.
Andrew Kasurak suggests checking out this article if you want to upgrade a cheap Chinese eBay amp to a working 55w unit.