The MODEL 500TDI is a tube direct box for the 500 series format. It uses the same TrueTube™ technology that is in the popular 583S Mk2. TrueTube™ provides proper operating voltage to the 12DW7 tube so that you get rich tube harmonics and incredible detail. Two knobs allow the user to drive the tube input separate from the output for a wide array of tonal possibilities. The top input is a high impedance input while the bottom input can be used as a through OR a lower impedance input for more sound options.