Tube Bus Box

The Iron Age Bus Box is a flexible stereo signal processing unit for mic or line level use. It features three tubes per channel, a pre- and line-level amplifier, transformer balanced inputs and outputs, and low or high passive shelf filter.

Block Diagram

The input section features a single 12AU7 tube preamplifier fed by three different modes. In mic input mode it is a classic tube mic pre using a Jensen input transformer and providing up to 70 dB of clear, warm gain. The mic preamp features switchable phantom power and a -20 dB pad. In line input mode a 35 dB pad is used for a balanced +4 dBu nominal input into the input transformer. The DI mode uses a front-panel accessible high impedance instrument input.

The preamplifier stage drives a passive equalizer that offers +2 and +4 gain at low or high frequencies, followed by a volume control pot.

The line amplifier uses a 12AU7 and a 6922 to drive a transformer balanced output. An interstage output volume control is available.

Finally a negative feedback switch is available to open the feedback loop on the preamplifier stage. This raises the level of distortion for a pleasantly driven sound.

The combination of the two volume pots and the NFB switch allow for a wide range of level control and sonic possibilities, from clean to warm to gnarly.