How to assemble and configure SSB power amplifier 3.5-30 MHz

Today you can buy on the Internet a rather popular kit for assembling a homemade power amplifier for a radio station or a homemade radio transmitter, which operates at frequencies from 3.5 to 30 MHz (SSB + CW, with amplitude modulation I have not tested) and has a maximum output power of 70 W, but such power will not be in the entire frequency range. With this amplifier I conducted radio communication at a distance of 3100 kilometers at a frequency of 7 MHz. I will show you how to assemble and configure this amplifier.

This kit does not have a radiator to remove heat from the transistors, you need to find it yourself. Also required is a power source up to 14 volts and a current of 6-8 amperes. Transistors used IRF530N two pieces. First, you need to wind two high-frequency transformers: input and output.

You need to break off four plates from the board. The input transformer is wound on a ferrite core, similar to a pair of binoculars. Insert two metal tubes into the ferrite and solder the plates to their ends. These two tubes are just the secondary winding with a tap from the middle. Wind two turns of insulated wire through these tubes - this will be the primary winding of the transformer. Solder the finished transformer to the board.

The output transformer is assembled from two ferrite tubes, into which two metal tubes are inserted, to the ends of which two plates are soldered. Everything is exactly the same as in the input transformer. Three turns of insulated wire must be inserted and wound into these tubes.

You can wind the wire after you solder the transformers to the board

Next, we solder the electronic components onto the board; the relay contacts need to be bent

The power choke contains two turns of wire in varnished insulation. Since such a wire is inconvenient to wind, I wound two turns of wire in insulation that I found on my own.Jumpers JP1 and PTT should be plugged. If you have an output filter, then jumper JP1 should not be plugged. You can connect the filter to the LPF in-out contacts.

Here is the aluminum radiator I found. I drilled holes for the transistor flanges. The transistors need to be isolated from the radiator, for this purpose the kit includes thermal pads and plastic bushings into which you need to insert screws and secure the transistors. These plastic bushings are very important and you need to be careful and check whether the transistor drains are isolated from the radiator.

Here is the amplifier assembled

I soldered two pieces of coaxial cable with a wave resistance of 50 Ohm to the input and output of the amplifier. I have not soldered special SMA connectors yet.

Now we set up the amplifier. We supply 13 volts. Just in case, connect a load to the amplifier output - a 50 Ohm resistor. No signal is supplied to the amplifier input. The trimmer resistor must be set and adjusted to a voltage of 3.5 volts on the gate-source of two transistors, the current consumption will be about 70 mA. When switching on for the first time, it is advisable to set the rotor of the trimmer resistor to a position in which the lowest voltage will be supplied to the transistor gates.

This is my 50 ohm resistor rated for 40 watts. It consists of 20 1k ohm resistors in parallel, each dissipating 2 watts. This resistor has the lowest inductance, wirewound resistors are not suitable.

After the voltage on the transistor gates has been set, you can now check the operation of the amplifier. I have a homemade radio transmitter with a power of 200 mW at a frequency of 27 MHz. I connected the output of this radio transmitter to the input of the amplifier, and connected a device measuring power to the output of the amplifier. As you can see, the output of the amplifier will be 7000 mW or 7 W of output power with an input of 200 mW.

Next, I connected the transceiver output to the amplifier input. I received my La-La-La signal at a distance of 3100 kilometers in the city of Oslo, Norway.