[Amps] SB220 changing SWR in bypass

[Dick NY1E]

    When I arrived back in Florida for the winter, first thing I did was stack my 6 M beams. When I go to operate I find my 1.1:1 match had moved to 51.8 Mhz. after fooling around with the antennas I eventually went back to the original single and the SWR was still at 51.8. It turns out the amplifier is causing it (in bypass mode) I remove the SB-220 (converted to 6M only) and the 1.1:1 is at 50.1 again. Any ideas what could move my resonant point 1.7 Mhz?. It was fine when I left in the spring....de Dick NY1E