Tube audio oscillator troubleshooting

I have a Heathkit A0-1 Audio Oscillator that my father built in the '50s, I replaced the caps and cleaned up the switch contacts and it seems to work ok except the sine wave shows some distortion on a scope. I am a beginner and have no idea about how to troubleshoot this circuit to find the trouble. I've searched the internet and can only find info on how to use the audio oscillator as a troubleshooting tool, nothing on how to trouble shoot a tube audio oscillator. Thanks in advance for any help or direction you can share.
6stringer

Define what the distortion looks like.

Many inexpensive generators have a less then perfect looking sine wave envelope, especially on the high frequency bands.
Even better if you can display a picture of it.