I have a Marshall RCD-50 or something, bought it from my sister's boyfriend about a year ago. Lately its been making this fading sound and when it fades out it will get really quiet and if i have distortion on it will mostly go away. At first I thought it was my guitar but I tried it on my friend's amp and it didn't do it, and I tried my other guitar on my amp and it did it with that one too. What are the possible problems and will they be expensive?