Tube Headphone Amplifier
(12AU7 (ECC82) Tube IRF510 MOSFET Headphone Amp)

Breadboarded Circuit

DIY Audio Project Link

http://diyaudioprojects.com/Solid/12AU7-IRF510-LM317-Headamp/

Circuit Schematic

The circuit has two stages. First, a common cathode tube volt
amp stage. The second stage is a MOSFET source follower for
current gain. The LM317 voltage regulator provides a constant
current source. Since most headphones are rated below 50 ohms
only a small amount of current is needed.

PCB Designs

Revised printed circuit board design will appear soon (28.12.2017).

Circuit Operation

A Bipolar Junction Transistor (BJT) can be used in place of the MOSFET, but MOSFETs tend to be more stable with temperature and current shifts. Use caution when handling MOSFETs as they are very static sensitive.
The signal enters the grid via a coupling capacitor, it leaves the plate (anode) amplified and 180 degrees out of phase, it is then directly coupled to the gate of the MOSFET and leaves the source. The signal is then coupled to the headphones via a electrolytic capacitor, which blocks the DC from your headphones. Since the sleeve of your headphones is common it completes the circuit.
The MOSFET is biased into class-A operation and will be constantly conducting at approximately 125 mA. The LM317 regulator is configured as a constant current source and regulates at 125 mA in the given configuration. You can use the online LM317 current regulator calculator to determine the current through the regulator by adjusting the program resistor. It is suggested that a 10 ohm 1/2W program resistors is used for R4 (you can use two 1/4W 20 ohm resistors in parallel). Note that the regulator and MOSFET devices will heat up and radiate heat. There are some real in depth calculations for heat, but know that the MOSFET can dissipate at least 1.6 Watts and the LM317 voltage regulator IC 2 Watts to air, at room temp. I tested a prototype over a continuous 24 hours period in a 150 cubic-centimeter (about 9 cubic-inch) enclosure and there were no thermal stability issues. You can add heatsinks to the devices, just ensure that if you gang the FETs together to use mica and silicone washers to prevent the 12V supply from transferring to the heat sinks.
The schematic above shows only one channel, you will have to wire the tube for both channels using different pins. The only wiring that is common for the 12AU7 tube between the two channels is the heater.