Erhart DF3FY, as well as providing a fix for the audio pop problem, has upgraded his PA output stage with RD16HHF1s. Erhart suggests the IRF 510s are not ideal for portable field operation. The small output transformer represents a mismatch for power outputs greater than 5 W so he has incorporated a new output transformer. He is now achieving 18w output on 20m and 12w on 10m, with an average of 15w output on all bands.
For details see his PDF file: fixes PA.