How far is the tuner from the "ground stakes"? How long is the wire from of the balun to the ground stakes? I agree with 'WIK; if you want to feed the existing Lwire on 80m, get a transformer like this. That will allow use of your existing Lwire on 40m and 20m, likely while bypassing the MFJ tuner. When using 160m, then switch the Lwire direct to the "50 Ohm" Output of the tuner (bypass the built-in balun; it just gets in the way). Your Lwire, referenced against earth (if the stakes are close-by and the ground wire from tuner to stakes is less than 2m) will present an impedance of ~45 Ohms at resonance on 160m. That is why the balun is not needed. Your Lwire will present a very high impedance at twice its resonant frequency, so that is why you need to switch in the 1:49 transformer.