Last year, I made this hybrid vacuum tube headphones amplifier project. The basic idea is based on the pictures on the internet that a headphone amplifier is using a vacuum tube with only 2 pieces of AA batteries. It is visually (from the internet photos) can drive the full size tube on it. I curious because there is no proven or any review about this vacuum tube amplifier in a working condition yet. Then comes to the conclusion when I built one here , it simply doesn't works at all. The power from 2 AA batteries cannot heat the tube heater, thus increasing the voltage doesn't help it at all. Because it facing with power supply current issue, not the voltage issue. I changed the layout by simply using 12 volt 2 ampere external switch mode power supply. Then the final layout is using Nichicon UKA as upgraded capacitors here and I also using the 3 batteries 18650 in series with total 12,6 volt 2.200 mAh here . It is now a working hybrid vacuum tube headphone a...