Distilled wisdom with respect to HF linear amplifier kits

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”

Richie notes:

“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.

Reference