Roly,
That's a nice little integrated amp there I would recommend keeping it, and sounds like you have a bad tube or tubes, at least that's most likely. When did your father in law last use it, or re-tube the unit? If you can't get that info or can't remember and you don't have access to a tube tester I would simply re-tube the whole unit. Luckily the tubes it uses, 12ax7, 12au7, 6sn7, and EL34 are all easy to find plentiful and can be found relatively cheaply. For power tubes Genalex Gold Lion KT-77's are a great choice, a drop in replacement for an EL34 its anything but a bright sounding tube. I haven't had any gear that uses 12ax7's or 12au7's so I won't make a recommendation there. For a solid basic 6sn7 something like Electro-Harmonix, or Tung-Sol reproductions should work fine. Good luck and if the problem persists that's a very easy unit to get work done on if you're in the USA.
That's a nice little integrated amp there I would recommend keeping it, and sounds like you have a bad tube or tubes, at least that's most likely. When did your father in law last use it, or re-tube the unit? If you can't get that info or can't remember and you don't have access to a tube tester I would simply re-tube the whole unit. Luckily the tubes it uses, 12ax7, 12au7, 6sn7, and EL34 are all easy to find plentiful and can be found relatively cheaply. For power tubes Genalex Gold Lion KT-77's are a great choice, a drop in replacement for an EL34 its anything but a bright sounding tube. I haven't had any gear that uses 12ax7's or 12au7's so I won't make a recommendation there. For a solid basic 6sn7 something like Electro-Harmonix, or Tung-Sol reproductions should work fine. Good luck and if the problem persists that's a very easy unit to get work done on if you're in the USA.