This is my tuner: http://www.bosscorp.co.jp/products/en/TU-80/

It doesnt tune my guitar properly (gives wrong readings) when:

1) I connect it directly to the guitar using 1 cable, without an amplifier, whether I plug it into the input or output bit

2) When I play the guitar with or without an amp using its acoustic microphone included

Am I supposed to have 2 cables - 1 going from the tuner to the amp, the other going fro the tuner to the guitar - to tune my electric guitar properly?

Thank you.