If you’ve ever worked with vacuum tubes, you’ll probably have a healthy appreciation for high voltage power supplies. These components require higher potentials to get those electrons moving, or so we’re told. It’s not the whole truth though, as [Albert van Dalen] demonstrates with his tube preamplifier running from only 3.3 V. If your first thought is that he must have made a flyback converter to step that voltage up to something more useful then you’re in for a surprise, because the single 6J6 pentode really does run from just 3.3 volts. Even its heater, normally supplied with 6.3 V, takes the lower voltage.
The circuit appears at first sight to be a conventional single-ended design, but closer examination reveals a grid bias circuit more reminiscent of a bipolar transistor. This results in a positive grid voltage rather than the more usual negative, and an unusually high 0.3 mA grid current. The cathode current is only 0.15 mA, but the preamplifier delivers a 3.5x gain. There is more detail on his website.
It would be interesting to subject this circuit to a full audio analysis and comparison with a more conventional design. As with so much in the world of audio there’s some smoke and mirrors around what constitutes the so-called “valve sound”, and it’s a question whether the satisfaction comes through the sound itself or the bragging rights of having a unit with a vacuum tube on show. Still, this is a simple enough design which takes few resources to build, so we look forward to seeing further experimentation. Careful though – down the vacuum audio route can lie folly.