If your grid is +29vdc and your cathode is positive +27vdc then your grid bias is still +2vdc. Normaly on cathode bias, you would have about +27vdc on your cathode and 0vdc on your grid. this would give you a grid bias of -27vdc, which would be fine. The goal for cathode bias is to keep the grid referenced to ground. Have as close to 0vdc as possible on your grid. You say you are at +29vdc.