What kind of an adapter?
If it's the "usual" 1/8" stereo jack -> 1/4" stereo plug, You'll only get left channel if the jack in the amp is mono (or stereo without R-S switching contacts).
Some (stereo) jacks can short the right channel to the ground.
If You have the rarer adapter 1/8" stereo jack to 1/4" mono plug, it's possible to have phase issues when the l & R are just connected together.
The "right" way is to sum the channels with a mixer, either a passive or active.
OTOH, a 1/8" to 2*RCA (and a RCA -> 1/4" adapter) or 2*1/4" will allow You to listen either the R or the L channel separately.