Checking Signal Generator Accuracy With A Frequency Counter and Scope

[Notimetolooz]

The lowest range does have the worst wave shape. The oscillator section has almost 100V peak-to-peak on the plate at some settings. I think the distortion is mostly due to the large signal there hitting the limits of the tube operating range with the supply voltage. I am puzzled why they chose a audio tube (12AU7) rather than something that works better at high frequencies (12AT7,12AV7).