OTL Tube Amplifier

My father has a great passion for listening to high quality music and building his own tube audio equipment. Since I also like old-school components, I built a vacuum tube headphone amp as a present for my dad. I did not have much experience with building vaccum tube circits, so I started with an existing design and modified it to my liking. Because vaccum tubes normally feature a relatively high output impedance, an output transformer is commonly employed for driving the speakers. In this design, a low impedance triode tube (6AS7) is being used, allowing to drive the headphone speakers directly. This should (in theory) provide a much more linear frequency response. DC bias offset is removed via decoupling capacitors.

Mechanical

The chassis was constructed from folded aluminium sheet metal. This amplifier is very inefficient, thus a lot of heat is being generated. I added various vents to allow free air convection and improve heat dissipation.

Power Supplies

The triode tubes require a relatively low voltage of roughly 150V at a current 150mA each. I use a 2x115V toroidal isolation transformer to generate two independent 150V supply rails. The triodes require a heating supply of 2A at 6.3V each. Both triodes are wired in series to reduce the required heater current. The heaters are powered with DC current. I unwound a standard 12v toroidal transformer until the resulting DC voltage matched 12.6V.

Amplifier

The amplifier was hard-wired.