I picked up a '50s HP 120B oscilloscope for $15 on Craigslist. I wonder if it works?
Whole lotta tubey goodness inside
It works! Here's a sine wave running through the Cow Pi amp, with volume controls set to a clean, undistorted level
As I turn up the master volume, the power tube clipping becomes evident
Turning it up more
Maximum volume. Not really a sine wave anymore. Distortion!
Examining a square wave coming from a signal generator app on my phone. Not very square
Viewing the built-in calibration signal. Also not very square