To be able to do experiments with vacuum tubes, I bought a cheap DIY 6J1 tube amplifier on AliExpress. Like most vacuum tube circuits, it uses a relatively high plate / anode voltage of 60V. However, a high voltage is not always required for vacuum tubes, sometimes 12V is used. I was curious how far one can go with reducing the power supply. Nowadays the most used power supply voltage for electronics is 3.3V, before this was 5V.

Tube amplifiers are used for nostalgia reasons and to get the characteristic tube sound. For this purpose, this preamp is useful, and you can simply power it from USB. The gain is -3.5 and music sounds undistorted. Just the red light from the filament is dimmed.

With a supply voltage of 5V, resistor R1 better can be 3.9k Ohm.

There are some oddities caused by the low supply voltage of 3.3V

• The grid-cathode voltage is positive instead of negative
• The grid current is very high, 0.3 mA