It seems that main job for electronic engineers is to operate CAD system and test various functions in the virtual world, not with real materials these days. It helps them a lot, but sometimes it is very stressful and never makes them feel satisfied. I think it is due to a lot of components and functions so called "black box" which they cannot touch and have to give up understanding. On the contrary, when I made a classical radio with big vacuum tubes, old type of variable capacitors, high voltage transformer, heavy chassis and --- etc, I got feeling of accomplishment. I can get almost every information of each components and know how they work. Learning, designing, making and testing on my linear amplifier gave me a lot of fun.
The figure below is the diagram of my linear amplifier. It handles 40m and 20m band, not all HF bands. I referred various papers and books about a linear amplifier. One of them is "How to Build the DX-811A All-Band Linear Amplifier CQ Magazine September 1982 by F.T. MARCELLINO, W3BYM"
I started to think of my homebrew linear amplifier in 2008, then finally I had it inspected by the authority almost three years later. I used a signal generator by PC free software, an oscilloscope and SDR-IQ to test how it works. Particularly SDR-IQ is helpful to monitor of FFT of the signal and hear my voice saved in the PC.
