You have to be methodical, look at one sub section or stage at a time.
In this case, I would start with a basic service. Clean the inside the chassis if necessary, clean the jacks, pots, tube sockets, tube pins, etc. Then look for issues starting with the power supply.
Remove the tubes. Then take voltage readings in the power supply and compare them to what is listed on the schematic. If the electrolytic caps are old, they may need to be replaced. This can be the source of the hum. A low voltage can also account fot the saturated sound. But there are a lot of possibilities.
If the power supply checks out, start with the first stage and examine each subsequent stage in tern.
Official Ampeg Portaflex Club #89
Last edited by beans-on-toast : 02-12-2014 at 08:53 AM.