When I got interested in electronics, vacuum tubes were on the way out and were rapidly being replaced with transistors. I didn't build anything with tubes because transistors were the items that interested me. However, I've always been curious about tubes and recently a friend an I thought it would be fun to make a tube headphone amp.
This project was first simulated using LTSpice IV, a free download from Linear Technologies with the addition of tube models also available online. The SPICE simulation shows a THD of less than 0.12% for a bandwidth of 10Hz to 100KHz and an overall gain of 12dB.
I used a piece of aluminum extrusion that I obtained from a local metal shop for the chassis and milled it to fit the amplifier and power supply boards. The finish is Hammertone to give it the retro look.
Here is a picture of the amplifier board and the high voltage/filament power supply (the high voltage toroid transformer is not shown). The input stage for each channel is an 12AX7. The driver stage for each channel is half of the 12BH7A.
The overall sound quality is excellent, with no audiable hum or noise.
Note: R15 represents the 64 Ohm headphone load needed for the SPICE model to run and is not an additional resistor. Also, only one channel is shown in the schematic.
After operating the unit for a while, I found that the compact chassis design I used required a series of holes to be drilled in the top and sides to provide adequate ventilation. In addition, I found the filament voltage regulator and the high voltage supply pass MOSFET required larger heatsinks than shown in the photo.