I found on my FT-847 that the strength of the sub audibles got weak as frequency decreased. I normally work all linear sats, but I had a request to work another ham through SO-50. I could not bring SO-50 up with my FT-847. I listened to the 847's output with a handy talkie and sure enough when I got down to 67 Hz, the tone was not there. The high frequencies worked OK but started loosing strength as I programmed the 847 downward. I used the handy talkie to make the QSO.