More time is spent on designing the RF power amplifier module than any other part of "PIXET" project. I started with a design based on single ended topology, but later gave up after cooking several IRF510s. Then I moved on to the present design topology for its inherent immunity to harmonics with added benefits of lesser heat and added stability.
Seperate biasing networks for both IRF510s are used. All tranformers are wound over common T50-43 toroids except the PA output transformer. This is wound with three turns each on BN61-202 twin hole large balun core. Mind phasing for the primary winding only. Winding is with 24 SWG copper enamelled wire.
L2 is wound on a small pignose balun core having 10 turns of 30 SWG. I made it as a choke and a fuse as well but if you wish to use an extra fuse than you can do so.
Though HEXFETS of IRF series are commonly used in HF power amplifiers but their gate capacitance is always to large and shunts down a considerable drive, especially on 20 meter and above. Thus the performance of HEXFET power amplifiers is usually, noticeably poorer on the 20 meter and above. L3 and L4 serves the purpose to tune this capacitance on desired band of operation. I used about seventeen turns of 26 SWG wound over a lead pencil and then removed as a self standing air core coil. You can either spread or compress the turns to make refinement in performance, or may add or remove turns as required on the band of operation. This just serves a starting point. These coils tunes the gate capacitance on the operting frequency of the band and the effect of input capacitance is thus nulls out, making the amplifier perform better on higher frequency bands.