The Ultimate Guide to Homebrewing a VU2EVQ Design HF Linear Amplifier -1

VU2EVQ design HF linear amplifier has a nominal power rating of 100 W, and is meant mainly for 40m, though it can give same output on lower bands and a lower output on 20m. But you will have to change the low pass filter circuit for each band. It is not recommended for higher bands. Earlier the full amplifier was available from VU2EVQ. Recently I bought a PCB from VU2EVQ with all the SMD components premounted and homebrewed the rest of it successfully. All components are locally available. Toroids can be purchased from VU2PTI and the transformers hand wound by us. Contact information of VU2EVQ and VU2PTI are available from QRZ.COM. Good support is available from VU2EVQ in case you have difficulty in homebrewing, which I found very helpful. VU2TDD is another person well versed in the assembly of VU2EVQ linear amplifier from whom I could get support, in addition to a lot of support from many more local amateur radio friends on the air.

This is the PCB which I purchased from VU2EVQ. It is meant mainly for beginners who want to have a 100W signal on 40m using the popular BITX radios. BITX radios have about 10W output on 40m. We need only about 5W input drive for this linear amplifier which will work on 13.8V, the standard voltage of power supplies and battery. Though power output can be higher at slightly higher input voltages, that is not recommended. A good heatsink the size of the PCB is needed to prevent overheating of the final IRFP150 MOSFETs.

This is the heatsink which I used, meant for audio amplifiers. I have used a couple of them in the RF linear amplifier. The final power MOSFETs were screwed on to these heatsinks.

An insulator silicone sheet and heatsink compound were used for that. The silicone sheet electrically insulates the MOSFET from the heatsink while permitting heat transfer. Here you can see the final MOSFETs and the silicone insulator sheets. Though both of them are kept on same heatsink for the initial demo, they were mounted on two heatsinks separately.

Picture of heatsink compound which came in a syringe and it was easy to use by spreading over the MOSFET and the insulator sheet.

Voltage regulator ICs 7805 and 7812 with heat sinks are shown here. 7805 supplies the regulated bias voltage to the final MOSFETs. 7812 provides the regulated supply to the transmit/receive DPDT relay. Power supply to the final MOSFETs is directly from the power supply, through an RF choke and RF output transformer.

Here are the 6A reverse polarity protection diodes, relays and 2kV ceramic capacitors. You can see two of each as my initial plan was to homebrew two boards and combine the outputs, which I have deferred for time being. Please note that 6A diode is not connected in series with the power supply, but to the ground. Hence power supply should have protection to prevent high current damage in case of accidental reverse polarity usage. That is, the fuse in the power supply should blow off in case of inadvertent connection in reverse polarity. The diode is meant to protect the linear amplifier and not the power supply!

I had inadvertently purchased through the hole components of some of the SMD components which were already mounted on the PCB like the transistors and diodes shown here! Thick resistors are for the input attenuator. Similar ones are needed for the feedback circuit at the output of power MOSFETs as well, which I had not purchased initially.

Usual type of enamelled copper wire is used for winding RF choke and low pass filter inductor coils. White wire is the winding wire meant for submersible pumps, to be used for winding transformers. The coating of these wires are less likely to be damaged while threading through the binocular toroids of input and output transformers.

A good 12V DC fan has to be running directly over the final MOSFETs for adequate cooling. VU3GZR has used an additional fan which starts running when the temperature of the MOSFET reaches a threshold value. He is using a temperature probe on the MOSFET with a circuit board and relay for that. I had also purchased that type of board later, and demonstrated its working some time back. I have not added it to my linear amplifier.

Coils L2 and L3 for the LPC were wound on toroids and mounted, along with the ceramic capacitors for the Low Pass Filter.