The Hughes & Kettner Red Box Classic converts Line, Out and Speaker Out signals into balanced, frequen- cy-compensated signals, enabling a direct feed to a mixer. The Red Box Classic features two voicing options, a 4x12" cabinet and a 2x12" combo.
You can find the original manual PDF here (must read!)
Original schematic is available here.
You can use an adapter or the 48V Phantom power via Balanced Output jack.
...and without Ground Lift switch