I have a tip for anyone that has, or is considering assembling the FRB 40 watt no-tune FET amp.
I had the opportunity to run some tests on that amp that a friend had built. He assembled the kit, and placed the circuit board on a massive heat sink.
After bolting the heatsink/circuit board assembly together, he enclosed the circuit board in an aluminum enclosure from Mouser electronics. Input and output BNC ports were installed in the enclosure. Inside the box, a 9 pole filter was installed on one side of the enclosure w/ a shield over the filter. The DC input leads inside the box pass through a large double-holed ferrite bead before hooking to the amp.
I hooked up his 1 watt exciter into the amp and fired the amp up. The output of the amp was coupled into a high dollar service monitor for checking the amplifier output.
I was looking at the amp output on the analyzer for output power level, stability, "cleaness" of carrier. Adjusting the bias, I had noted that the unit didn't deliver much more than about 30 watts at max. I also saw a "spur" quite close to the lower freq side of the carrier. Obviously, there was something amiss with the amp operation.
While I had the cover of the amp open, I noticed that the heavy wire coil inductor used to choke off RF and also supply DC to the FET was only four turns of wire. I thought to myself that this choke wasn't of sufficient inductance to keep RF from finding its way back into the DC supply. I decided to remove that component, and replace it with a choke made from winding a heavy wire through a small double holed ferrite bead. I soldered this component in place of the four turned coil.
Firing the exciter and amp up, the carrier now had a very clean pattern on the scope... the spur had disappeared. The amp was also capable of delivering full output, as seen on the monitor.
The amp now is very clean and stable, and was given back to its owner. He tells me that because of the massive heatsink he put on the amp, he is able to run "key down" for several hours with the amp without needing a heatsink fan.
I would say that the FRB kit is a good buy, but I would recommend anyone purchasing the kit to replace that choke as I did.
The inductor was small, and it wouldn't have photographed well because it had black shrink tubing around it.
There's really not much to making one yourself. Since the amplifier draws about 5 amps, I would just use some 16 guage wire, or whatever you had that would carry the needed current without heating. I would wind the wire through a small ferrite toroid a few times, or make a few passes of turns through a small double-holed ferrite toroid. I don't think it's too critical on exact number of turns. The idea is to increase the inductance over the stock coil used. You have to keep RF from getting into the DC supply components of the amp.
Whatever you make, you have to keep it of reasonable size to fit like the previous coil did on the board.
