This belongs to a friend of mine .It's a Sonax 550-b (Traynor 1973)solid state amp. For a 30+ year old amp it looks like new. Problem is when you power it on it has a nasty loud hum. This is with no cord in the amp at all. Control knobs do nothing to effect the tone or volume of the hum. It sounds like that noise you get when you have one end of your cord plugged into your amp and you place your thumb on the end of the cord. Any ideas where to look or what to check?