Nate,
Thanks for your answer. About the 20M band I already finely tuned the Audio gain (db) in Putty in order to get on each band a +30dB signal strength in JS8Call and for that I have set 30M to 76, 20M to 58, 17M to 58, 15M to 59, 12M to 71 and 10M to 71.
I will see if I can tweak L3 and L8 to get more power on 15M.
73 - Pierre - FK8IH