OTL Tube Amplifier

I built this vacuum tube headphone amplifier as a gift for my dad. Vacuum tubes typically have a relatively high output impedance, which is why an output transformer is often used to drive speakers. However, in this design, I used a low-impedance triode tube (6AS7), which allows the headphones to be driven directly. This approach should, in theory, result in a much more linear frequency response. DC bias offset is removed using decoupling capacitors. I mostly followed an existing design I found online and modified it to my liking.

Mechanical Design

The chassis is made from folded aluminum sheet metal. Given the inherent inefficiency of this amplifier, it generates a significant amount of heat. To address this, I incorporated multiple vents to allow for free air convection and improve heat dissipation.

Power Supplies

The triode tubes require a relatively low voltage of approximately 150V at 150mA each. To generate two independent 150V supply rails, I used a 2x115V toroidal isolation transformer. The triodes also need a heating supply of 2A at 6.3V each. I reduced the required heater current by wiring both triodes in series. The heaters are powered by DC current, which was achieved by unwinding a standard 12V toroidal transformer until the resulting DC voltage matched 12.6V.

Amplifier

The amplifier section was hard-wired. It helps in maintaining the classic "vintage" aesthetic and charm of a tube amplifier, giving the amp a more personalized feel.