[Amps] re mismatch occuring in my Henry 3K Premiere amp

[Fern Rivard]

    I the past few days, I noticed that I was getting some mismatch on twenty meters with my FT-1000D running thru the Henry 3K Premiere but the amp was not on but by-passed. At first, I thought that I was having a problem with my TH-11DXX beam  but I was getting similar results when feeding to my Bird dummy load. So I completely by-passed the henry amp and it all seemed to be fine. I then tried another approach by adding about 10/12 feet of RG-213 coax in the input line to the Henry amp and the bad swr disappeared on the twenty meter band but still showing some swr on 18 mhz but more  acceptable. All of the other bands from 160 to 10 meters don't appear to be affected by this syndrome except slightly on 10 meters. Before I loose too many hairs, has anyone experienced this weird problem especially with the Henry amp and if so, what was your cure to rectify this please.
    Thanks from Fern   VE7GZ