USB powered valve/tube amplifer
I built this in 2016. A stero pair of 6G6G single-ended pentodes with BC107 transistors as drivers. The 6.3V heaters run directly from a USB port or smartphone charger as each draws just 150mA. For high tension supply a 5V to 35V step-up boost convertor is used - a XL6007 based module from China (via e-bay).