A Tube Stereo Amplifier, From Scratch [Video]

A conventional tube amplifier has a circuit whose fundamentals were well in place around a hundred years ago, so there are few surprises to be found in building one today. Nevertheless, building one is still a challenge, as [Mike Freda shows us with a stereo amplifier in the video below the break.

The tubes in question are the 12AU7 double triode and 6L6 tetrode, in this case brand new PSVANE parts from China. The design is a very conventional single-ended class A circuit, with both side of the double triode being used for extra gain driving the tetrode. The output uses a tapped transformer with the tap going to the other grid in the tertode, something we dimly remember as being an “ultra-linear” circuit.

There’s an element of workshop entertainment in the video, but aside from that we think it’s the process of characterising the amp and getting its voltages right which is the take-away here. It’s not something many …