This is a 12U7, a valve made for high-end American cars in the late 1950s. Engineers took a standard 12AU7 and re-engineered it to run at just 10 to 30 volts.
It’s a fascinating piece of history from the late 50s and early 60s, right when transistors were taking over.
These were called “Space Charge” tubes, built specifically to run at voltages low enough to work alongside the new transistors of the era.
A standard high-voltage 12AU7 usually runs at 250V. At low voltages, a standard tube works great as a clean buffer, but once you wire it for actual amplification (voltage gain), it runs out of headroom and distorts.
Because of how this 12U7 was redesigned internally, I was able to configure it for real voltage gain—and what surprised me most was feeding a hot, loud signal into the tube with zero clipping. 🎶