My first attempt at amplifier building for the new low bands was a disaster. Being low on funds and patience at the time, I tried building a dual band “linear” amplifier that was said to be capable of 25 to 50 watts on both bands. It turned out to be a design plagued by problems which I won’t get into here. I was fretting about what to do next, as there was no budget at all, when Ken K5DNL came to the rescue. He kindly helped with parts and schematic for a modified, low drive version of the popular GW3UEP amplifier. Credit for the design goes to GW3UEP and K5DNL. Where I have made minor changes I will note that in this post.
Referring to the schematic, the 2N2222 provides additional gain to fulfill the low drive objective. This amplifier can be driven to full output with 0 dBm (one milliwatt) input. Mine actually produced full power down to -2 dBm but don’t count on every build being exactly the same. The BC550/BC560 pair forms a squarer to ensure we have a nice clean square wave to drive the FET. The FET in the original GW3UEP was a IRF540. Ken sent a couple of 30NF20 FETs which is what I used in mine. C2 and C4 were not on the schematic I received from Ken. After building it I found the gain and power output peak was around 505 kHz. Looking at the original design on GW3UEP’s web site I noted the the originally specified capacitor values were for that frequency, with a notation to add capacitors for 472-479 kHz operation. After I added C2 and C4, my amplifier peaked at 475 kHz. All capacitors in the FET output circuit should be good quality pulse rated film or silver mica. C1, C3, and C5 in my amplifier are WIMA FKP1. C2 and C4 are CM06 size silver mica with a 500 volt rating. The 1 uF DC blocking capacitor is a WIMA MKS4. Watch the capacitor voltage ratings. Theoretically, 100 volt capacitors should be good enough, though marginal if you intend to run on 24 volts. Some digging into spec sheets on the WIMA capacitors reveals voltage ratings are reduced as frequency increases and we are well down the slope on most of them at 475 kHz. If you are going to buy capacitors I suggest going with the highest voltage rating available. The other change I made was to relocate the blocking capacitor to the location shown. It was at the amplifier output on the schematic I received (and yes, if you are eagle-eyed you will see I have it at the output in the photo below. I relocated it later).
It should be noted that one need not use exactly the parallel combinations of capacitors specified. The important thing is that by whatever method, be it a single capacitor or several in parallel, we arrive at the required total capacitance at each point in the circuit. You will notice my capacitor combination at C3/C4 quite different from the original GW3UEP.
This amplifier should produce 30 watts output at 13 volts, 100 watts on 24 volts. I am running mine on 19 volts and getting about 65 watts out into a 50 ohm load. I have been running this amp on WSPR at 33% duty cycle for several days and it has performed perfectly. One should strive to keep the antenna resonant and matched, but mine has at times wandered off a bit with no ill affect on the amplifier aside from power output variation as the load impedance changes.
Note: This amplifier has a built in low pass filter but it does not meet FCC requirements for spectral purity. If you are subject to FCC regulations, you should use additional low pass filtering after this amplifier.