Ive been using a fender mini tone master amp for a practise one (not the greatest amp i know) and its been fine for a few months. But now after I use it for 5 or 10 minutes the sound becomes distorted even when the gains on 0, and after that the sound just cuts out. The power light stays on so I know the amp isnt turning off and I dont think Ive mistreated it in any way. Does anyone know what the problem might be?